ASTAP, Astrometric Stacking Program

ASTAP, the Astrometric STAcking Program ASTAP Forum Documentation Version history Checklist for solving
astrometric (plate) solver, stacking of images, photometry and FITS viewer

Download installers:

Operating system	Program installer	Large star database	Smaller star database	Photometry & colour star database	Large galaxy database
Window 64 bit	Program (0.9.560a, dated 2021-07-18)	H18	H17	V17	Hyperleda
Window 32 bit	Program zipped (0.9.560a)	H18	H17	V17	Hyperleda
Linux 64 bit	Program debian (0.9.560a), Program .tar.gz openSUSE and Fedora support	H18	H17	V17	Hyperleda
Linux 32 bit	Program (0.9.552)
Raspberry PI, 32 bit	Program (0.9.559)
Raspberry PI, 64 bit	Program (0.9.559)
MacOS 64 bit	Program (0.9.560a)	H18	H17	V17	included with star database

You have to install:

1) Program
2) One star database.

You will need only one database, either the H18 (980 mbyte) or the H17 (500 mbyte). Select the smaller star database if your field of view (image height) is larger then one degrees. If you have both the H17 and H18 installed then delete the H17 or select the correct database in tab alignment. The V17 is only required for photometry. Older G17 and G18 star databases are obsolete can be uninstalled/deleted.

Star database usability:

For comment feedback and questions there is the ASTAP Forum. The ASTAP Manual is below

For photometry you could download and install the V17 star database colour up to magnitude 17. A 680 MB file. It contains the calculated Johnson-V magnitude and colour information (GBp-GRp) for star annotations. This one also works best for solving an image with a FOV of more then ten degrees

Hyperleda, a very large galaxy database for deep sky annotation. 2.190.000 objects. Based on extract from leda.univ-lyon1.fr/ Will be placed in the program directory.

Alternative links:

Operating system	Program development version	Alternative star database links	Fits image compression & decompression programs from Nasa HEASARC. Only required if you have files with the .fz extension.	Barebone command-line solver compatible with the GUI version if renamed. No pop-up notifier. Will not accept raw files and will not work with SharpCap since FOV is not stored.
Window 64 bit	ASTAP_installer_(0.9.561), ASTAP executable only	H18 installer H17 installer, G17 installer (obsolete 1)). H18 zipped, H17 zipped, G18 rar ((obsolete 1)), for manual install at \programs files\astap or program directory	Fpack & Funpack	astap_cli (v0.9.560a)
Window 32 bit			Fpack & Funpack	astap_cli (v0.9.560a)
Linux 64 bit	ASTAP_debian_package_(0.9.560a), ASTAP tar.gz	H18 debian H17 debian, G18 debian. H18 zipped H17 zipped V17 zipped , G18 as rar ((obsolete 1)), G17 zipped ((obsolete 1)) for manual install at /opt/astap	install from distribution	astap_cli (v0.9.560a)
Linux 32 bit
Raspberry PI, 32 bit
Raspberry PI, 64 bit
MacOS 64 bit		H18 zipped H17 zipped V17 zipped , for manual install at /usr/local/opt/astap		astap_cli (v0.9.560a)
MacOS on ARM	Any Mac user who could assist in compiling ASTAP for Mac on ARM using Lazarus please contact me.

Questions, feedback: ASTAP Forum
Older versions are available at Sourceforge
Version history
Source code (Object Pascal, Lazarus/FPC)

Donations are welcome:

Introduction
Program installation
Program operation, stacking astronomical images
Stack menu

Images, darks, flats, flat darks tab

How to exclude poor images
Satellite tracks

Alignment menu tab

Results tab
Stack method tab

Blink tab
Photometry tab

DSLR photometry

Inspector tab
Mount analyse tab
Live stacking
Pixel math tab
Background equalization tool
Popup menu

Export FITS data to spreadsheet for analyses

Viewer:

Opening, preview files

Solver, usage as astrometric solver and command line options

Usage as a solver with other programs or as PlateSolve2 substitute

Using with APT, Astro Photography Tool
Using with CCDCIEL
Using with ModelCreator
Using with NINA, Nighttime Imaging ‘N’ Astronomy
Using with SGP, Sequence Generator Pro
Using with SharpCap
Using with Voyager

Installation of the external astrometry.net solver

ASTAP introduction

ASTAP is a free stacking and astrometric solver (plate solver) program for deep sky images. In works with astronomical images in the FITS format, but can import RAW DSLR images or XISF, PGM, PPM, TIF, PNG and JPG images. It has a powerful FITS viewer and the native astrometric solver can be used by CCDCiel, NINA, APT or SGP imaging programs to synchronise the mount based on an image taken.

Main features:

Native astrometric solver, command line compatible with PlateSolve2.
Stacking astronomical images including dark frame and flat field correction.

Filtering of deep sky images based on HFD value and average value.
Alignment using an internal star match routine, internal astrometric solver.
Mosaic building covering large areas using the astrometric linear solution WCS or WCS+SIP polynomial.
Background equalizing.

FITS viewer with swipe functionality, deep sky and star annotation, photometry and CCD inspector.

FITS thumbnail viewer.
Results can be saved to 16 bit or float (-32) FITS files.
Export to JPEG, PNG, TIFF, PFM, PPM, PGM files.
FITS header edit.
FITS crop function.
Automatic photometry calibration against Gaia database, Johnson -V or Gaia Bm
CCD inspector
Deepsky and Hyperleda annotation
Solar object annotation using MPC ephemerides
Read/writes FITS binary and reads ASCII tables.

Some pixel math functions and digital development process
Can display images and tables from a multi-extension FITS.
Blink routine.

Photometry routine
Available for MS-Windows 32 & 64 bit, Linux 32, 64 bit, MacOS 64 bit, Raspberry-Pi Linux 32 and 64 bit.

Stacking of images:

Stacking of astronomical images is done to achieve a greater signal to noise ratio, prevent sensor saturation and correct the images for dark current and flat field. Additional imperfect images due to guiding, focus problems or clouds can be removed.

This is a screen short of the stack menu. It contains several tabs for the file list and settings. File can be sorted on quality and values.The image can be visually inspected in the viewer by a double click on the file or using the pop-up menu.

Program requires FITS images or RAW files as input for stacking, but it can also view 16 bit PGM /PPM files, XISF files or in 8 bit PNG, TIFF or BMP files. For importing DSLR raw images the program DCRAW from David Coffin or LibRaw is used.

Stacking methods: average and sigma-clipping-average.. Internal calculation using floating point numbers.
Simple and intuitive user interface.
Automatic saving of selected options and files.
Can create master files for dark and flat & flat-darks to reduce processing time.
Limited memory use, independent of the number of images stacked.
Bayer algorithm for DSLR/OSC cameras

For stacking the internal routine compares the image star positions to align.

Astrometric Solving:

ASTAP can be used as astrometric solver to synchronise the telescope mount position with center position of an image taken with the telescope. Existing images can be solved to annotate, for photometry or the measure positions of unknown objects.

The ASTAP solver aims at a robust star pattern recognition using the catalog star coordinates in Equinox J2000. The solution is not corrected for optical distortion, refraction, proper motion of stars and other minor effects all to be very minor.

The process astrometric solving is often referred to as a "plate solve". That was a correct description in the past, but in modern times there are no photographic plates involved in the process.

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Program installation:

MS-Windows:

The installer and separate star database will be default installed at c:\Program Files\astap. But they can be placed anywhere as long all files are in the same directory.

Linux installation:

The program and database are provided as a Debian package and will be installed at /opt/astap. The program is also a rpm package is available. In case the executable is manually placed at /usr/..., then the program will look for the database at /usr/share/astap/data/ (version 0.9.412b or higher)

MacOS installation:

The program and star databases are are provide as pkg file. The star database will be installed at /usr/local/opt/astap/

Program operation, stacking astronomical images:

The purpose of the stacking routine is to combine astronomical images to reduced noise and to flatten the image.

Ideally you should have collected

1a) Several light frames. Images of deep sky object unprocessed.
1b) Several dark frames of the same temperature and exposure as the light frames. A dark frame is a frame that represents an exposure done in total darkness. This signal includes the bias signal, but also includes any dark current charge accumulation, and thus any dark current noise that exists within the dark current signal. For darks suburban area (SQM=20.4) you should take about the same amount or more darks as lights. For a light polluted area you could take less darks then lights since the noise in the lights generated by the sky background will be abundant.
2a) Several flat frames. A flat frame is a frame that represents the field flatness and taken from an uniform light source. Ideally with a significant signal level. This to compensate for vignetting and dust particles. Vignetting can greatly darken the corners of your image and have to be compensated.
2b) Flat dark frames or bias frames ideally of the same temperature and exposure duration as the flats. Since flats are taken with very short exposure times, either flat dark of bias image of almost zero seconds will do.

Only light frames are essential.

The automatic stacking process in ASTAP goes through the following steps:

The flats will be combined to an average and the combined average flat-darks will be subtracted to have a near ideal presentation of the vignetting called the master flat frame.
The darks will be combined to an average master dark.
Optional on the master flat a 2x2, 3x3 or 4x4 mean filter is applied to reduce noise.
From each light frame the master dark will be subtracted to extract the pure deep sky signal.
Each light frame will be flattened by dividing it by the master-flat resulting in the corrected light frames.
The corrected light frames are combined to the final image using the average or sigma clip mean (to remove outliers as satellite tracks) method.

Step 3,4,5,6 are done in memory. No intermediate results are stored on disk.

It is possible to mix different exposure times but it is not recommended.

Operation of the stacking program

Start the ASTAP program.

Call up the stack menu window using the ∑ button.

a) Select frames
In tab images select the lights. In tab dark select the corresponding darks. Select in tab flats the flat-field images called flats and in tab flats-darks the flat-darks/bias frames. In most cases you could select all frames in tab images. The program will move the fames to the corresponding tab during analyse. The light and dark should preferable have the same exposure time and temperature. The flats should have the same exposure time and temperature as the flat-darks.

b) Analyse and remove bad frames
In tab images (for the light frames), press analyse and remove manually any poor image. Poor images can be detected by a too high HFD (Half flux diamater stars), low number of stars or high background value( by clouds) . Loss of tracking could result in too low HFD value. If required inspect each image by double on the file name. The list can be sorted by clicking on the corresponding columns. Using the pop-up menu selected bad frames can be renamed to *.bak for deletion later.

c) Set parameters in tab stack method
In tab stack method select the stacking method, average or sigma-clip-average. For OSC camera images select "Convert OSC images to colour". Select the correct Bayer pattern (4 options). Test the required pattern first in the viewer with a single image. The source images should be raw (gray) without colour produced by astronomical camera's.

d) Set parameters in tab alignment
Leave this to the default star alignment.

e) Classify by
Leave all check marks initially unchecked. (This is an option to select automatically a master dark with the correct temperature and exposure time for the lights. Same for master flat selection based on filter used both in the light and flat.)

f) Press the Stack (..) button.
The darks and flats & flat-darks will be combined in a master dark and master flat frame. Then the program will combine the light frames to the final image and save it automatically to FITS. This will take some time.

g) Export
The stack result will be saved as FITS. The program keep a record of all results in tab Results. Stretch the image as required. Crop the sides if required using the pop-up menu. Equalise the back ground if required using the tool in tab pixel math. Export as stretched JPG or 16 bit bit stretched/unstretched PNG / TIFF. The stretched export follows the gamma and stretch setting of the display. For further image processing you could export to 32 bit float TIFF or 32 bit float PFM format.

ASTAP export types:

File formats ASTAP	8 bit	16 bit	32 bit
Import	FITS, JPEG, PNG, TIFF	FITS, PNG, TIFF, PPM, PGM, raw formats	FITS, PFM
Export	FITS, JPEG, PNG, TIFF	FITS, PNG, TIFF, PPM, PGM	FITS, TIFF, PFM

All the program settings and file selections will be save by leaving the program or click on the Stack check marked images button.

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The stack menu:

Images, darks, flats can be added using the Browse button or can be drag dropped on the form.

For a generic description how to stack see Program operation, stacking astronomical images

Poor images can be filtered by either 1) HFD value, 2) Quality, 3) Star-level or 4) Background value.

Stack multiple objects: Several objects can be stacked in one run. If the classify-by-object is check-marked, the program will stack all objects and save the results using the available darks and flats.

Analysing and removing of the bad ones: Before stacking the images can be analysed with the Analyse button. Images with a high HFD value are most likely unsharp and can be inspected in the viewer by double click on the filename. The file can be renamed to *.bak by the pop-up menu to be removed later. Images with a high background indicating clouds or twilight should be removed/renamed to *.bak.

Sorting: Images can be sorted on any of the columns. For example if you click on HFD, the images will be sorted on HFD. You could then remove the images with the highest value or inspect them by double click on the file name.

Dark classification: If classify by exposure time and/or exposure time is check-marked the program will automatically select the correct darks or master dark. So it is possible to keep several dark files check-marked in the darks tab and the correct master dark will be selected automatically.

Colour stack and Colour filter classification: If the classify-by-filter is check-marked, the stack routine will combine the available filters to a RGB image. If only Red + Green +Blue image are available they will be combined in a RGB image. If Luminance images are available it will first stack the RGB colors and then apply a most-common-filter and Gaussian blur on the RGB result. Finally the luminance image is coloured with the RGB result. The filter factor should be set typically near 20.

For nebula you could combine RGB or LRGB. If a luminance filter is detected the LRGB mode is used. First the RGB is combined, stars are removed using a most common filter, the image is then blurred with a Gaussian blur and and the colour is then applied luminance channel. Star colour is lost with this process. The filter names can be set in tab alignment

To record star colours use RGB only.

It could be better to stack in two steps. First prepare the Red, Green, Blue and Luminance stacks. Then run a stack again with the Red, Green, Blue and optional Luminance. Or if you could revisit the produced interim results in the RESULTS tab en copy them to the IMAGES tab using the pop-up menu. You could try different colout factors.

Image file names containing "_stacked" will be un-checked by default to prevent stacks by accident are re-used. If required, just select the file and check-makr it again.

Organising images, darks, flats and flat-darks: Images placed in the first tab will be organised based on the FITS header keyword IMAGETYP. So as soon you click on the image Analyse button, dark and flats and flat darks/bias images will be move to the corresponding tab.

Keyword modification: The pop-up menu has option to update a keyword of multiple files if required.

OSC images (one shot colour images): To import raw files from a digital camera, ASTAP can either use DCRAW or LibRaw for conversion. You can select it in tab "Stack method". Libraw has some advantages since the conversion program convert directly to FITS and exposure time, date of exposure and demosaic pattern are written to the FITS header.

For stacking of OSC images it is best to start with raw images. The raw colour images look mono, but the program will convert them to colour later in the stacking process. There are four different Bayer patterns. The demosaic pattern can be set in the tab "stack method". Try auto or empirical which will result in the correct colours. A terrestrial image could help find the correct demosaic settings. Load a raw image in the viewer and in tab "Stack method" test conversion with button "Test pattern". Try auto or the four demosaic patterns. If the colours are not correct, just hit undo button or type CTRL-Z to recover and try an other demosaic setting.

Power down option after completion: If stacking takes a long time you could activate this option and the program will be power-down the computer after completion.

Clear , button to remove all files from the list.

|| , button to stop blinking cycle.

>> , starts a continuous un-aligned blinking cycle. This is intended to find visually outlier images where guiding has briefly failed.

How to exclude poor images

Sort the images on quality by double clicking on the column quality and inspect visually the images with the lowest quality factor by double click on the row. If poor, rename images with right mouse button popup menu "rename to *.bak" for deletion later. Sort also on background and inspect visually the images with too high values on possible cloud coverage or twilight conditions by double click on the row to open.

HFD is the image median HFD. The lower the better. Depending on focus and guiding. Note that low values could be result of loss of tracking.
Quality of the image. The higher the better. Based on the number of star detections divided by HFD. Depending on sky transparency of the sky and focus. Note that loss of tracking result both in a low HFD and low star count so a low quality factor..
Star level. The higher the better. Depending on transparency of the sky and focus. Note that a high values indicate satellite tracks.
Background. Depending on sky darkness and transparency. The lower the better. A higher value means in most cases a cloud was blocking the sky.
Sharpness. The lower the better.Measures the change between dark and bright pixels. The measurement is very sensitive to satellite tracks. Could be used to detect satellite track when compared with HFD value. Could also be used to sort images of the Moon and Sun on sharpness.(but they can't be stacked) For correct sharpness measurement of OSC images either the FITS header should contain the keyword BAYERPAT or check-mark "Convert OSC to colour" in tab "Stack method".

To uncheck/untick poor images can als be done automatically. First check mark the option "After analyse untick worst images". Then press button Analyse and organise images . The column quality of the images will be analysed statistacally and outliers can be removed using either a standard deviation represented by the Greek lower case sigma σ letter or a percentage.

For a normal distribution you could expect the following:

Confidence interval	Proportion within
1σ	68%
1.5σ	87%
2σ	95 %
2.5σ	98.8%
3σ	99.7%

Satellite tracks

The stack method "sigma clip average" should normally remove any satellite tracks. If after stacking with "sigma clip average" there are still satellite tracks visible, you could lower in tab "Stack Method". the sigma factor from 2.5 to a lower value maybe 2.2 . An other way is the blink/scroll through the images with the >> button. As soon you see an abnormal bright track on the image stop the blinking by esc and inspect visually the image(s) involved by double click on the row. Remove any poor image by using right mouse button popup menu "rename to *.bak"

Results tab.

The stack results are reported in the results tab. By a double click they can be viewed the viewer. The number of files and exposure times are given. With the pop-up menu it is possible to copy the image file path to the clipboard for use in a file explorer.

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Stack method tab

Stack method: Best option is "Sigma clip average". For only 2 or 3 images or when you are in a hurry "average"will do.

Raw one shot colour images (OSC): RAW images of DSLR cameras /One shot color cameras are monochrome and have to be converted into colour images (after applying darks and flats). This converson is called demosaic or debayer. First set Bayer pattern correctly by loading a raw image (grayscale) in the viewer and try one of the bayer patterns till the image colours match in viewer. If not hit CNTRL-Z to undo and try a different Bayer pattern.

There are several methods to convert (demosaic/debayer ) the raw image to colour:

AstroC, colour for saturated stars, as bilinear method but for saturated stars the program tries reconstruct the star colour. Select the range which matches with the value of brightest stars.
AstroM, white stars, as bilinear method but if there is an unbalance between the 4 red, 4 blue or 2 green pixels it uses luminance only. Effective for unsampled images and stacks of a few images only. Star colour is lossed if undersampled but star will become white.
AstroSimple ©, each R,G, G, B pixel colour information is used in a 2x2 pixel range. Simple but very effective for astro images. Works best for a little oversampled images. Stars have very few artifact if any.
Malvar-He-Cutler 1), advanced method for interpolation developed for terrestial images but less suitable for OSC astro images. The background noise is lower.

AstroSimple is © Han Kleijn, www.hnsky.org, 2020. and licensed under a Creative Commons Attribution 4.0 International License.
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

What to select:

In general de-mosaiced OSC astro images are suffering from colour artifacts due to the small size of stars and pixel saturation. If the pixels iluminated by a star are saturated, the red, green and blue values will have the same maximum value and the star centre will appear white. In most case this can be avoid by taking short exposures of 60 seconds or shorter.
Best results are achieved with de-mosaic methods AstroC and Simple.
In most cases the option "Auto level and colour smooth" is required for the correct colour balance and colour smooth. First the three colour channels are adjusted to make the background colour neutral and the stars average white. Secondly the bright stars are smoothed. Both actions can be done manual in the tab pixel math, option "colour correction" and "smart colur smoothing"
If the images are under-sampled and the star colour is random after stacking, use AstroM, white stars. Stars will be whiter. Star colour will be lost.

The principle of the AstroSimple demosaic method:

Auto levels: This is an option to white balance the final colour result. The stars will be average white and the background sky will be gray.

Normalise OSC flat: This option should normally be switched off. Only if your light source for making the flats was very reddish or blueish, you could this option equalise the red, green and blue level. Binning is not recommended for flats since individual sensitive differences between pixels are compensated by the flat.

Colour smooth: This is an option to smooth the de-mosaic artifacts. The colour is smoothed while preserving the luminance signal.

The background colour noise can be removed with option "Background colour removal" in the pixel math tab of the stacking menu.

The DSLR OSC images will be automatic converted to FITS files.

Oversize: This is default set at zero but could be set hunderd pixel or larger. This overlap was introduced to show partly overlapping images. If you don't want a black area around the image set this value to zero.

If your using a monochrome filter like H-alpha with a DSLR/OSC sensro, use the viewer Tool menu to split the images in R, G, G, B before stacking and use the R=red image for future processing.

The program settings will be saved automatically if your either exit the program or start a stack.

Image stiching method (Mosaic)

Astrometric image stiching is possible with the internal astrometric solver. The reference of each pixel is the astronomical position. So stacking is not done against a reference image but against an position array set by the first image. You have to set the in tab alignment the settings for "Image stiching (method)" correctly. If you stitch 4 images, you have set "Mosaic width/height in tiles:" at 2. This will provide enough space to place for the 2x2 mosaic.

Here a suggested work method:

Stack the tiles separately using method "SIGMA-CLIP-average" and use for the alignment the internal STAR alignment method. Inspect the resulting tiles and crop them if required. You can also crop them later automatically with "Mosaic skip outside pixels" Do this for each color separately if you have separate files.
In tab "stack method" select option "IMAGE STICHING METHOD" and select astrometic alignment using either the internal solver.
In tab "stack method", set the "mosaic width/height" correct and check-mark the option "equalise background". If the input images have poor borders, set option crop images larger then 0%.
Select the files. Most likely the files names contain "_stacked, so you have the check-mark the files after selection.
Click on the button Stack check marked images|
Crop the stacked result using the image crop option in the viewer mouse pop-up menu.
Adjusted the stretch range and save as JPEG, 90% quality.

Here an example mosaic x 4 of M31 made with ASTAP:

Here an example of a mosaic build of DSS images:

The size can be reduced by a crop function (right mouse button) later. Making the oversize too large could result in memory overload.

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The alignment menu tab:

For alignment there are four options, internal star alignment, native astrometric solver, manual alignment or ephemeris alignment. For mosaic building you have to use the internal astrometric solver.

Internal star alignment

This internal star matching alignment is the best and fastest option to stack images. It is not suitable for mosaics. No settings, fully automatics alignment for shift in x, y, flipped or any rotation using the stars in the image. It will work for images of different size/camera's with some limitations.

The program combines four close stars into an irregular 2D tetrahedron or kite like figure (and compares the six irregular tetrahedron dimensions with irregular tetrahedrons of the first/reference image. It selects at least the six best matches and uses the centre position of the irregular tetrahedrons in a least square fitting routine for alignment. The four stars are called a quad. The six distances are used to construct a hash code.

There is only two settings relevant but normally you don't have to change them.

Hash code tolerance No need to change it. Leave this at 0.005 unless you have severe optical distortion. If you have false detections, set this lower at 0.003.
Maximum number of stars. Typically set at 500. In some cases like for images with a lot of hot pixels you could set it lower to avoid false detections.

The following image shows the selected quads where the six distances form an irregular tetrahedrons :

The matching process is described here Background info, how does the ASTAP astrometric solving works internally

Astrometric alignment.

Internal astrometric solver (plate solver). The works with the same four star quad as for the Star alignment option. The quads are compared with the G17 star database (to be installed in the program directory). It has the following settings:

Field height: This is the square size of the star field in degrees used for detection and will be set automatically for most FITS files. It should equal to the image height in degrees. If unknown, you could try initaly the option "auto".
Radius search: Search radius in degrees. If there is no match, the program will move the search field around in a square spiral and increasing the distance form the initial position up to the radius specified. A radius of 30 degrees could be searched in a few minutes.
Maximum number of stars to use: No need to change it. This could be set between 100 and 1000.
Hash code tolerance: No need to change it. Leave this at 0.005 unless you have severe optical distortion. If you have false detections, set this lower at 0.003. If you have severe optical distortion or a low resolution image set this at 0.007
Downsample: For large image above 3000 pixels wide down sampling will speed up the solving and increase the signal noise ration of the stars. Also colours are combined to monochromatic so this option is beneficial for DSLR images
Cropping: For very large image above typical 3 degrees height the solver will only look to the center of the image.
Force small search steps. This will force a 50% overlap between search fields. Use this if solving fails. I applied this will slow down blind solvingC
Calibrate prior to solving. If your image is full of hot pixels and soling is less reliable you could try this option. The image will be calibrated with a dark prior to solving removing the hot pixels. Select in the darks tab, a dark or darks with similar exposure duration as the lights you trying to solve. If your select several darks with different exposure length, check-mark the option classify, "Dark exposure time".

The internal plate solver works best with raw unstretched and sharp images of sufficient resolution where stars can be very faint. Exposures 5 to 300 seconds. Heavily stretched or photo shopped images are problematic.

For those are interested: Background info, how does the ASTAP astrometric solving works internally

Manual alignment.

Manual alignment

This option allows alignment of the images based on a single star, asteroid or comet. If this option is activated, the list of images in the image tab turns red. Double click on each image in the list and click on the star/comet of asteroid to be used as reference. This object is then marked with a little purple circle. The position will be auto centered. (and the X,Y position will be added to the list) A poor lock is indicated by a square. If so try again till it is a circle. If all images in the list are turned green, so contain a value, then click on the Stack button .

Options:

Star centering
Comet centering
No alignment

For objects which are moving in the sky, select the stack option "average" and not option "sigma clip".

Ephemeris alignment

Rather then manual selecting the center point it is also possible to use the ephemeris of an asteroid or comet. In ASTAP this is reallised by using the annotation markers written in the header.

To align go through the follow steps:

Preperation:

Configure and test the viewer asteroid annotation menu shortcut CTRL+Q

Stacking:

Select ephemeris alignment
Browse for all the images in the image tab and add if available the dark, flats.
Press the button Analyse the selected file| in tab images .
Select from the combobox the asteroid or comet to align on. See screenshot below.
Hit the Stack check marked images| button.
After the stacking is finished, it is possible to annotate the result.

Only the object selected will be sharp. The stars will form trails

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Blink tab

This tab allows to blink images and to measure variable stars.

Blink comparator

This option allows rapidly cycle (blinking) through the images taken of the same area of the sky at different times. This will allow the user to spot easier moving objects. While blinking the result can be demosaiced (slow) if the "auto demosaic" option in the viewer is activated.

|| , button stops the blink cycle.

> , button starts one blink cycle.

>> , starts a continuous blink cycle.

☑ Align images. With this option check-marked,the images will be aligned using star alignment. The alignment will be refreshed after pressing "clear alignment"

☑ Time stamp. With this option a time stamp from the header will be written to the bottom of the image. If the displayed image is saved as FITS, this time stamp will be written to the saved image.

☑ Filter out stars based on first image, With this option check-marked,the stars will covered with a black circle allowing easier to spot moving objects.. The black cover size can be increased with option "star suppression diameter". The star detection can be optimised with the "σ factor".

Clear, button to remove all files from the list.

Export video This button will export the blink result to an uncompressed .y4m video file (YUV4MPEG2). For OSC images, activate in the viewer the "auto demosaic" option. The menu will ask for a video file name and desired frame rates per seconds. Contrast will be as set in viewer. Compression can be achieved in an external program like VLC or leave it to YouTube. If time-stamp is check marked then the time stamp will be written to the video.

Export aligned This button allow the creation of aligned FITS images. If blinking with alignment works well, stop blinking and hit this button All images will be copied aligned to new files ending _aligned.fit. Alignment will be done against the first image in the list. If time-stamp is check marked then the time stamp will be written to the aligned images.

Photometry tab

This tab allows measuring the magnitude change of one object in a series of images and detects automatically the four most variable objects. The butttons work the same as in the blink tab.

Select the images to analyse. Click on one of the > buttons to cycle trough the images The program will do the following:

Cycle 1, Find an astrometric solution for all selected images and write the solution to the FITS file header.
Cycle 2. In a second cycle, the program will identify stars in the image and measure the star flux against the V17 star database. The mean flux/magnitude factor exlcuding outliers will be used later to measure the magnitude of any object in the image series. So prior to this install and use the V17, Johnson-V version of the star Gaia database provided.
At the end of cycle 2, it will mark the four most variable stars with a yellow circle.
Click on up to three stars. Violet circles labeled Var, Check and 3 will mark the stars. If you click twice on a stars the labels will rotate between the stars. The measured magnitudes of each image will be written to the file list. This complete list can be exported to a spreadsheet using the pop-up menu allowing the creation a magnitude curve over time.
With AAVSO button an extended report can be generated. As comparison star always the Gaia stars are used, so select in ASTAP only the V17 database. You have to enter the designation of the Variable and Check star. The report format is according the AAVSO Extended file format.

For maximum accuracy it is better to first to calibrate the images with darks, flats & flat darks. This can be done using the "calibration only"option in tab "stack method" and then executing the regular stack procedure.

Note that the measured star flux is compared and calibrated with the star database. For most cases you should install the V17 star database containing the Johnson-V magnitudes. After stopping the cycle it is possible to measure any object using the mouse pointer.

Note: ASTAP uses for calibration up to 1000 stars from the Gaia database. So all stars it can find and recognise in the image with an SNR>30. So the Gaia database should be the V17 which contains the calculated Johnson-V magnitudes. The three stars are just measured against the Gaia Johnson-V database. Only two are required for the report. The you just need to select the variable star and a check star. The third star (3) is just a bonus.

About the statistics, from the up to 1000 stars any outlier star is removed if it deviates more then 1.5 sigma from the median factor (Gaia_star_magn - log(flux). Then for the remaining stars the factors are averaged and used for calibration of the measured flux of the variable and check star..

So it is a different setup then usual but there is never lack of calibration stars.

Alignment of the images is done using the astrometric (plate) solution. The astrometric solution is written to the orginal file header. You can refresh the photometric and astrometric calibration using the dedicated buttons for this.

The list contains three dates:

Date/time (start) in YYYY-MM-DDTHH:MM:SS. This is date & time from the header key word DATE-OBS. This is normally the universal time at the start of the exposure.
JD (mid exposure) This is the Julian Day of the exposure midpoint. This time is calculated from the key word DATE-OBS and half of the exposure time is added.
HJD (helio) Heliocentric Julian Day at the exposure midpoint expressed in UTC. This is the event time as seen from the Sun center compensation the maximum ± 500 seconds time difference depending on the positions of the Earth, the object outside the Solar system and Sun. For plotting purposes only the fraction is displayed.

To convert the Julian Day to a date and time in the spreadsheet, subtract the Julian Day by -2415018.5 and format as date or time.

For a series of images, the standard deviation of the measured star magnitudes is typical better then 0.02 magnitudes. The standard deviation of the Check star is used for error estimate if more then 4 images are selected. else an estimate based on the Variable SNR values is used. The star flux values should be below saturation (65500) but reasonable high.

Note that it is beneficial to de-focus an image a little to prevent pixel saturation and spread the flux measurement over more pixels. It also allows longer exposure times. However the image should reasonable focused to allow solving.

Preparation and usage of DSLR images for photometric measurements:

Step 1) and 3) are DSLR camera specific.

Define Bayer pattern: The green pixels of a DSLR camera have a very similar response as a Johnson-V filter and can be reported as filter TG. So to use the green pixels only, it is required to extract the green pixels from the raw. The images should NOT be converted to colour by the de-mosaic routine. To allow to extract the green pixel it is required to define the correct de-mosaic pattern in ASTAP. Load a raw image in ASTAP and in tab "Stack method" check-mark temporary option "convert OSC images to colour". Set Bayer pattern to Auto or one of the other patterns and test the conversion to colour with button "Test pattern". This is best done with a terestial image to be sure. If the correct pattern is select and the colour produced are correct then unselect the option "convert OSC images to colour".
Calibrate: For maximum accuracy it is better to calibrate the images with darks and flats & flat-darks. Select in tab "Stack method" for the stack method option "calibrate". Do not use "calibrate &alignment"and uncheck option "convert OSC image to colour". Load the images in the light tab, darks in the dark tab, flats in the flat tab and flat-darks in the flat-darks tab. Press the large button "Stack.". New calibrated image will be created ending with "_cal.fit". If no darks and flat are available, proceed without calibration.
Extract green pixels. Load the new images in the photometry tab. Press the button "extract raw green". Images will be converted to new images with filename ending "_cal_TG.fit".
V17. Check if V17 star database is selected in tab "Alignment". if it is not available download the V17 and select it.
Astrometric solutions . If the images are not solved yet, press button "Refresh astrometric solutions" This is required to identify the stars for photometric calibration against the V17 star database. If no solution are found, check the image "Field of view (height)" in degrees in tab "alignment" and check initial α,δ
δ
δ
δ
δ
δ
δ del
positon in the viewer. If solving fails, got through the check list for solving.
Mark the Var star and Check star. Double click on the first image in the list and select the Var and Check Star and star 3. If you click a second time on a star the three stars will be swapped. Star 3 is not required for reporting, just an extra star.
Cycle. Press the >| button and check if the magnitudes are recorded. Check if stars are still proper selected. To indentify stars you could select Ännotate variables". The database should be complete up to magnitude 12 .
Aperture, adapt aperture and annulus for maximum accuracy. Use for this the reported standard deviation of the check star in the log.
AAVSO. Use AAVSO report if required.

The lastest version of ASTAP can display and report a light curve directly:

The light curve can be saved to file or with a pop-up menu copied to the clipboard.

Here an example of an exoplanet transit measured using the blink&photometry tab:

A demonstration is available at YouTube:

Measure variable stars

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Inspector tab.

This tab is intended to measure accurately the tilt and curvature of your telescope & camera setup. It's done by calculating the best focus position for each image area. To do this it requires a series of images taken at several focuser positions.The routine will calculate from these images the best focus point of each area. It will measure the median HFD values of each image and area and build a table HFD as function of the focuser position. Form this data, curve fitting will give the transfer fucntion and the best focus position expressed in focuser steps. The focus point differences between the image areas will indicate the tilt of each area. The difference between the centrum and outer areas focus point will indicate the curvature.

The usage is as follows:

Prepare a series of short exposure images with different focuser positions and a lot of stars. Exposure time a few seconds. Move for each image the focuser a small fixed step but only in one way to prevent backlash problems. Images with stars having an hfd above 20 will not be analysed correctly.
Browse with ASTAP inspector tab to the images.
Press curve fitting.
The difference between the focus point of each image area will be reported in focuser steps.

The reported hfd values can be selected and copied for further analysis in a spreadsheet.

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Mount analyse tab.

This tab is intended to study mount pointing accuracy. The table will indicate the mount α, δ position and the image α, δ position in the selected equinox and the offset between them.

Usage: take several images at different location in the sky. Load the images in the tab. Selected the desired equinox. Click on the button add the astrometric solutions. Study the result and if required select all rows, copy and paste the whole result to a spreadsheet for further analysis.

The images shall be of FITS format with the mount α, δ position in the header. This is normal the case. Keywords required RA, DEC or OBJCTRA, OBJCTDEC.

The solution can be written either in the orginal FITS file or in a seperate .wcss file.

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Live stacking tab.

All file(s) in the specified directory will be stacked live. If it is finished it will wait for new file(s). If a file is detected which is 0.2° away from the previous files a new stack will be started automatically. You can save the stack results from the viewer menu .

To identify files which are processed , they are renamed to the extension *.fts. You can rename them back with the button at the bottom.

Note there is no rejection of poor images. All images are added with equal influence:

Assuming the images are A,B,C,D, E... then

Simple serial stacking:

result1:=A
result2:=(result1+B )/2
result3:=(result2*2+C)/3
result4:=(result3*3+D)/4
result5:=(result4*4+E)/5

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Pixel math1 tab.

Several options including background equalising.

Background equalization tool:

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Pixel math2 tab.

Several options.

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Export FITS data to a spreadsheet:

To analyse the relation between the HFD value, focuser position, temperature and altitude it is possible to copy the data from a FITS image list to the operating system clipboard.

Just select a number of images, click on the Analyse button. Then select all relevant files and copy the data with right mouse button. Then copy the data into a spreadsheet for analysis.

Here and example of the result in a spreadheet:

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Viewer:

Viewer, file menu

If the program is associated with FITS image files or any other format, it will show the image as soon you click image file. Only one instance of ASTAP will be allowed. After clicking on the second image it will be show in the first instance of ASTAP. If you want to open a second instance, just start ASTAP. If the program is started without parameters, you can open more instances.

Images can be loaded from the file menu or can be drag dropped on the main form.

Besides all FITS formats, the viewer support most image formats in 8/24 bit of 16/48 bit format. It can export to any FITS format and 16/48 bit PNG and TIFF formats.
ASTAP can display images and tables of MEF, multi-extension FITS. The images of MEF can be saved as a single image. The MEF tables can copied into the clipboard and paste to a spreadsheet. (v0.9.446)

This ASTAP version can import raw images from almost any digital camera, For this ASTAP executes the Libraw tool Unprocessed_raw which is included with the ASTAP Windows edition and can be installed in Linux by the "sudo apt-get install libraw-bin" command.

File formats ASTAP	8 bit	16 bit	32 bit
Import	FITS, JPEG, PNG, TIFF	FITS, PNG, TIFF, PPM, PGM, raw formats	FITS, PFM
Export	FITS, JPEG, PNG, TIFF	FITS, PNG, TIFF, PPM, PGM	FITS, TIFF, PFM

The viewer has a preview function. After opening select "Preview FITS files". The preview is displayed in the ASTAP viewer. The current zoom and position is maintained so you could study the corner of a series images on image quality.

The file open menu with preview selected:

Viewer, file menu, thumbnail viewer for FITS files

ASTAP has a FITS thumbnail viewer (ctrl-T). This could be useful to browse your FITS files. By clicking on the thumbnail it will be opened in the viewer. With a right mouse button click some options are available as changing directory, copy, move, rename or rename to *.bak.

The thumbnail size is depending on the form size. Make it larger, the thumbnails will follow. Thumbnails are organized in 3*X. So the thumbnails are pretty big by purpose. The images are fully loaded in memory so it will consume some memory and time. So don't try get thumbnails of 400 images.

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ASTAP viewer screen shot:

Viewer, tools, Astrometric solving the image in the viewer

With the Solve| button it is possible to find an astrometric solution of the image loaded. For this the estimated celestial center position α, δ should be available. This position is normally retrieved from the FITS header. Secondly the estimated image height in degrees should be specified in the stack menu, tab alignment. In the same tab alignment you can specify the search radius and down sampling. For successful solving see conditions required for solving.

For solving JPG, PNG or RAW files it is possible to add the object position as center position using the deep sky database. Double click on RA position in viewer and enter the object name. The position will be retrieved from the database. This position will be used as a start point for the solver.

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Viewer, Tools, Batch processing:

With the batch routine several FITS image can be "astrometric solved" or converted. This conversion is not required for ASTAP. Automatic conversion is integrated in the menus.

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CCD inspector

The viewer has a CCD inspector under menu tools. By reporting the median HFD value, sensor tilt and curvature the inspector will quickly show any problem of the imaging system.

The image used for testing should be a single raw image with sufficient stars and not containing a disturbing large and bright deep sky object. The exposure length should be long enough to image many stars but not too long preventing star saturation.

The routine will detect and annotate the stars with their HFD value and plot a tilt indicator in the image. A star rich image containing a few hunderd stars or more gives the best accuracy.

The following will be reported:

Image median hfd which is an excellent indicator of the quality of focusing. The lower the value the better the focusing, the sharper the stars are. The value is also depending on the astronomical seeing and the quality of the optics. If the image is solved it will also be indicated in arc seconds.
Sensor tilt as the HFD difference between the best and worst corner median values. In addition as an graphical indication it draws an trapezium in the image based on the four median values.
There can be some variation in images of the same series. Any tilt equal or less then 10% of the range is a good value. Any tilt equal or more then 20% of the range indicates a tilt problem.
Off-axis abberation as the delta between the HFD value in the center and in the outer areas of the image. The stars in the outer area are normally a little larger, oval or comet shaped due to the optics or curvature giving an higher HFD value. The lower this value the better the optics. This value could be a help to adjust the optimal distance between the field flattener and the camera. This measurement is only a valid if the focus for the center area is perfect. A more advanced measuring method is available in tab CCD Inspector.

CCD inspector calculation method explained:

HFD 2D contour.

The star half flux diameters (HFD's) are displayed in a 2D contour. Dark areas indicate a lower and better HFD value. This will quickly visualise sensor tilt or other problems. To avoid false indications by outliers the HFD values are filtered by taking the median HFD value of the three closest stars and allocate the result to all three stars. The HFD values are indicated numerical. They grey levels have no direct linea relation with the HFD.

HFD diagram.

The star HFD values can also be represented by areas of constant HFD. It is in principle a Voronoi diagram, but by taking the median value of each three closest stars and allocate the median to the three stars it looks a little different.White areas indicate a star with an high HFD value. Darker areas indicate a lower value. The grey level is the HFD * 100.

HFD values

This tool will only indicate the median filtered HFD values next to the stars. The same values as in the 2D countour and HFD diagram. To avoid false indications by outliers the HFD values are filtered by taking the median HFD value of the three closest stars and allocate the result to all three stars.

Median background values

This tools writes the median background values as numerical values in the image. For correct measurement the image should only contains stars.

Aberration Inspector

This tool creates a 3x3 mosaic of the images center, the corners and borders. This allows easy an comparison of the star shape at the different sections of the image.

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Viewer, Tools, Calibrate Photometry.

With this option the relation between flux and mangitude is measured. The image should be solved in advance to be able to calibrate the star flux with star database magnitudes. After the calibration is applied, the star magnitude at the mouse cursor is displayed.

Viewer, Tools, SQM report based on an image

With this option the SQM = sky background value relative to the stars is measured and expressed in magnitude per square arc second. The image should be solved in advance to be able to calibrate the star flux with star database magnitudes. The reported value will be equal to a value reported by an Unihedron SQM-L meter. Atmospheric extinction of the stars at lower altitudes will be compensated.

Some background information:

http://www.lightpollution.it/download/sqmreport.pdf

At zenith the measured star flux and sky background flux are defined as comparable and star light extinction is zeroed by subtracting 0.28 magnitudes from the calculated extinction. So at zenith the SQM brightness values are comparable to deepsky object brightness and can be expressed in magn/arcsec². At lower altitudes the star and deepsky flux are lower due to higher air mass and therefore higher extinction minus 0.28 is taking in account.

Pre-conditions
1) Image is astrometrical solved for flux-calibration against the star database.
2) The background value is larger then pedestal value. If not expose longer.
3) Entering a pedestal value increases the accuracy. (mean value of a dark)
4) Apply on single unprocessed raw images only or
calibrate single images and apply zero for the pedestal value.
5) DSLR images could require additional preparations.
6) No large bright nebula should be visible.
7) Extinction plays a role. An atmospheric extinction correction for star light will applied.

Viewer, Tools, Magnitude (measured) annotation.

With this option the stars are annotated with the estimated magnitude. The image should be solved in advance to be able to calibrate the star flux with star database magnitudes.

If the Johnson-V version of the star database is used, the results match very accurate with AAVSO charts as demonstrated below. Camera was an ASI1600 with only an UV-IR block filter:

For best accuracy the image should be monochrome and one of the Gaia Johnson-V star databases should have been installed and selected (V16 or V17). The image should have taken with a Johnson-V filter or none (clear). The measured pixel values should be below saturation.

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Viewer, Tools, Star (database) magnitude.

With this option the stars are annotated with the star database magnitude. This can be done best with V17 containing the calculated Johnson-V magnitudes.

For a G-database the indicated magnitude is Gaia blue. For a V-database the indicated magnitude is Johnson-V and the following the difference between Gaia blue and red, positive for reddish objects. All in 1/10 of a magnitude.

Below, the image is 1) solved, 2) auto calibrated (using the V16) The cursor is at a star and based on the flux of all know stars, the star Johnson-V magnitude is estimated to be 16.1. The stars are marked with the Johnson-V magnitude and Bp-Rp color indication.

See also blink and photometry

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Viewer, Tools, Unknown star magnitude

Any unknown object or a star with an abnormal magnitude is identified. This option is intended to mark nova and minor planets using the star database. Any star like object missing from the star database will be marked. Also if the measured magnitude is one magnitude brighter then the database. The H18 star database up to magnitude 18 is recommended for this option. If the H18 is selected then missing object up to magnitude 17 will be detected. The image should be solved in advance.

Viewer, Tools, Variable star annotation

Variable star are annotated using the variable_stars.csv database.

Viewer, Tools, Asteroid and comet annotation

This option will annotate asteroids using the orbital elements taken from the MPCORB.DAT file and for comets the ComeEls.txt from the Minor Planet Center.

Usage:

- Solve an image.
- Go to to the viewer "Tools" menu, "Asteroid annotation".
- First time download the full MPCORB.DAT from the minor planet center. Link is available from the blue down arrow. Set the path to MPCORB.DAT correct.
- Set the limiting magnitude and maximum number of asteroids to read.
- Press the button Asteroid & comet annotation .

Remarks:

Renew the MPCORB.DAT and CometEls.txt files every few months.

The observation date and time are extracted from the FITS header (date-obs, date-avg) or for other files the file date is taken. If date average is not available it will be calculated from the exposure time and date-obs from the FITS header.

The latitude and longitude of the observation location are also taken from the header. If not available enter them manually or leave them at 0.

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Viewer, Tools, Deep sky annotations

If the image is solved, it is possible to add deep sky annotations. See pull-down menu TOOLS:

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View menu

This menu has the following options:

Image clean up Ctrl+space
Center lost windows Ctrl+F12 Use this if you have multiple screens and once window is out of site for some reasons.
Flip horizontal Ctrl+H
Flip vertical Ctrl+H
Zoom out PgUp
Zoom in PgDn

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FITS tables

The viewer has limited support for displaying FITS binary and ASCII tables. It can read and write binary tables and read ASCII tables only. They are displayed in the memo. Values are separated by a tab #9 and can be selected and copied to a spreadsheet. It can display only one table and will display the first table only. The file can be saved again but all binary values will be all written as 4 byte float.

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Viewer popup menu:

Add annotation, free text label at a x,y position. It can be connected via a line by first holding the right mouse button and moving the mouse away. See sample above. Persistent by annotation keyword in the FITS header. The annotation can be switched off in pulldown menu "View". Remove by removing the annotation line in the header.

Add marker, rectangle marker at x,y position. Draw the rectangle first by holding the right mouse button down and moving the mouse away. See sample above. The Persistent by annotation keyword in the FITS header. The annotation can be switched off in pulldown menu "View". Remove by removing the annotation line in the header.

Add object position, Enabled after astrometric solve. Will add the α, δ at the mouse position. Orange if a lock is possible, see above sample, red if not. Not persistent after image clean up.

Add marker at α, δ position, will place a yellow square on an object for identification by on the entered position. See above sample for orange star. Enabled after astrometric solve. Will add square marker at α, δ position. Persistent. Position will be save with "save settings"

Measure total magnitude within rectangle, Enabled after astrometric solve. The program will measure the flux and try to estimate the magnitude. Hold the right mouse down and pull a rectangle around the deep sky object, release mouse button and then select this menu. For best accuracy the image should be monochrome and one of the Johnson-V star databases should have be installed and selected (V16 or V17). The image could have been taken with a Johnson-V filter for best results or none. The measured pixel values should be below saturation.

A demonstration is available at YouTube: Photometry in the viewer

The measuring principle is as follows:

Use the star database to measure the MEDIAN relation between flux of the detected stars and star magnitude from the database. (That's why solving is required and best result are achieved with the V16 or V17 Johnson-V star databases based on Gaia DR2)
Measure the MEDIAN background 1 to 10 pixels wide outside the rectangle box. This median measurement will ignore stars in the field.
Measure the MEAN flux inside the box.
Calculate the magnitude for the "inside mean flux" minus "median outside box flux" using the relation found for the stars.

You could argue that a Johnson-V filter or green channel is required for the image but in practice the error is limited depending on the spectrum.

Remove all markers and labels. Will remove all none persistent labels. Persistent annotations can be switches off in the viewer pulldown menu VIEW.

Copy image (selection) to the clipboard, copies the displayed image or a selection of the image to the clipboard. The orientation is depending on the selection direction.. (v0.9.417)
Copy position to clipboard, Enabled after astrometric solve. Copies the α, δ position to the clipboard.
Copy position to clipboard in ° , Enabled after astrometric solve. Copies the α, δ position in degrees to the clipboard.
Copy position to clipboard in radians, Enabled after astrometric solve. Copies the α, δ position in radians to the clipboard.

Remove deep sky object. Removes an deep sky object as part of the pixel math tab routine "equalise background tool". Hold the right mouse down and pull a rectangle around the deep sky object, release mouse button and then and then select this menu.

Local colour smooth

Local remove colour.

Local Gaussian blur.

Local equalise tool.

Brighten a small area based on the corner values.

Gradient removal tool

To remove a linear gradient caused by light pollution or twilight. The routine need two empty areas 40x40 pixels in the image to measure the gradient. The area may contain stars but no deepsky object. The area are selected by pressing the right mouse button(first area) and while holding the mouse button move to the second area in the direction of the gradient. Then select in the popup menu "Gradient removal tool". Try to maximise the distance ideally the full image range.

Copy paste tool

Powerful touch-up tool for cosmetic corrections. Small sections of the image can be copied as pasted on hot pixels or artifacts. Select the good part by holding the right mouse button and pulling a rectangle with the mouse and move the copied part to the part to be touched up..

Set area

Set an area for the colour replacement tool in tab pixel math 1.

Remove borders. This menu allows to remove parts of the image near de borders.

Crop fits image. This allows the crop the image. Hold the right mouse down and pull a rectangle, release mouse button and then and the select this menu

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Usage as astrometric solver and command line options:

For images in the viewer:

The simplest way to solve an image is just to load an image in the viewer and hit the solve button. Some settings are available in the ∑ menu under tab alignment.

Diameter field: This is the square size of the star field in degrees used for detection and will be set automatically for most FITS files. It should be in range with the image diameter.
Radius search: Search radius in degrees. If there is no match, the program will move the search field around in a square spiral and increasing the distance form the initial position up to the radius specified. A radius of 30 degrees could be searched in a few minutes.
Maximum number of stars to use: No need to change it. This could be set between 100 and 1000. Default 500.
Hash code tolerance: No need to change it. Leave this at 0.003 unless you have severe optical distortion. If you have severe optical distortion or a low resolution image set this at 0.008
Downsampling: For large image with a height above 3000 pixels select downsampling factor 2 or to speed up the solving and increase the signal noise ration of the stars. Also colours are combined to monochromatic so this option is beneficial for DSLR images

The solution will be added to the fits header and center of the image will be displayed in the log of the ∑ menu. Click on the save button to save the FITS file with the solution. With the solution, the status bar will show the astronomical position of the mouse pointer.

Conditions required for solving:

Quick checklist for solving:

At least 30 stars are visible. For small field-of-view (<1°) and away from the Milky-Way-plane expose longer about 30 to 300 seconds and use the H18 star database.
Reasonable round stars.
Image dimensions at least 1280x960 pixels. For smaller dimensions solving is still possible if the image quality is good.
An approximate position is specified (for a spiral search). This position should be displayed in the left top of the viewer menu. (unless you do a 180 degrees search)
Image is not stretched or heavily photo-shopped.
Correct image height in degrees specified within preferable 5% accuracy. This height (width x height) should be displayed in the status bar of the viewer and in tab alignment under "Field of view (height)" (Check focal length, sensor size settings or try FOV=auto once)" You can set/force a "Field of view (height)" in tab Alignment.

If you still have problems with solving, you could send me a link to the file involved (FITS is preferred) and if I have time I could have a look. Upload it e.g. to http://nova.astrometry.net or https://ufile.io/ and give me the link. For privacy reasons, prior to uploading you could remove your latitude/longitude information from the header using the viewer "Tools", "Batch processing", "Remove longitude latitude information" menu.

The internal astrometric solver works best with raw unstretched and sharp images of sufficient resolution where stars can be very faint. Heavily stretched, saturated, out-of-focus or photo shopped images are problematic. It requires minimum about 30 stars in the image to solve. Images containing of a few hundred stars stars are ideal. For star rich images, the program will reduce the detection limit to limit the number of stars. This will only work for unstretched images where brighter stars have a greater intensity then fainter stars. So ASTAP requires three star dimensions for solving. The star x, y coordinates and star intensity. Oval stars due to tracking errors or severe optical distortion will be ignored and solving could fail.

Check list for successful solving with ASTAP:

Estimated celestial center position of image should be entered/available in the viewer α and δ input or passed by the command line. For FITS file this is normally read from the FITS file header and set automatically. In case you view an an JPEG, TIFF image, double click on α input to search for a know deep sky object position from the database.
Field of view of the camera should be available. This is the height of the image in degrees. See ∑ window, tab alignment, group-box astrometric settings. For FITS images this is normally automatic calculated from info in the file header.
Image height in pixels after ASTAP subsampling should be somewhere between 1000 and 3000. If it is higher set subsample at 2. If you specify 0 for subsampling, the program will select a subsampling factor automatically.
Search radius should be set large enough. See ∑ window, tab alignment, group-box astrometric settings. You could set this at 30° or larger up to 180°.
Stars in the image should be pretty round and the camera in focus. You could verify the star detection by the CCD inspector or by the Test button to show quads in the ∑ window, tab alignment). Most stars should be detected.
As a minimum about 30 stars should be visible in the image. They can be very faint stars, barely visible in the noise. For large field-of-view (>1°) expose 5 to 10 seconds but for smal field-of-view (<1°) far away from the Milky-Way plane you have to exposure longer 30 to 300 seconds and use the H18 star database.
For images filled with stars, only a small part of the stars may be saturated. The total exposure time could be hours as long it is possible to separate the brightest stars from the faint by intensity.
For bayered images (OSC cameras) or large images use the factor 2 downsample option. See ∑ window, tab alignment, group-box astrometric settings. Save settings if modified (Viewer pull down menu, file, exit (and save settings). You could also try auto auto selection of binning equals 0.
If your image is full of hot pixels you could adjust the "Ignore stars less then["] in tab Alignment. This is default set at 1.5" but could be set higher for long focal length.
An other option for hot pixels is option "calibrate prior to solving". Select in the darks tab, a dark or darks with similar exposure duration as the lights you trying to solve. If your select several darks with different exposure length, check-mark the option classify, "Dark exposure time".
The maximum number of stars to use should be defined. Typical set at 500. See ∑ window, tab alignment.
Hash code tolerance should be defined. Typical set at 0.007 especially for long focal length telescopes which are more sensitive to the seeing.. (In previous versions this was set at 0.005). See tab alignment.
For some failures you could force in ASTAP the option "slow" (-speed) for more reliable solving. The reliability will be very high but solving will be slower.
In case of excellent seeing and small band filter usage the imaged star could be smaller then 1.5". Reduce the setting "Ignore stars less then 1.5" in tab "Alignment". If the imaged star size is about one pixel after downsampling reduce the downsampling to 1 (auto is often 2). Improvements will be reported by a increased number of quad matches.
For long focal length telescopes with a field of view less then 60 arc minutes it is recommend to install the H18 star database and remove the G17 (or force use of the H18 in tab alignment).
For long focal length telescopes with a small field of view where objects like M13 fill the whole image ASTAP could struggle to solve. This is caused by the fact that the database has a better resolution then the image. Forcing option "slow" could help.

Warnings:

In case you get regular a warning "Star database limit reached" replace the G17 database by the H18. This could happen if your field of view is small (<30 arcmin). Remove the G17 files to force the use of the H18.

In the plane of the Milky-way plane it is easy to image enough stars for solving as shown in this ESA image showing the sky star density. If you look outside the Milky-Way plane into deep space there are less stars. The G17 will contain enough stars if your field-of-view is 0.5 degrees or more. For a smaller field-of-view you have to exposure longer to image fainter stars. The H18 could work well up to a field-of-view of 0.25 degrees is exposed enough. For large field-of-view (>1°) expose 5 to 10 seconds but for smal field-of-view (<0.3°) you have to exposure longer 30 to 300 seconds. Expose long enough till the solver detects 30-50 stars minimum or till the solver gives a warning "Star database limit reached" indicating you have reached magnitude 18.

For very long exposures and a FOV below one degree it could be required to reduce the maximum number of stars detected. This is default set at 500.If not stars with a magnitude fainter then the database will be detected..

Database usability:

The first letter indicates the type of database. G indicates 10x10 degrees tiles and a single magnitude Gaia blue. V indicates 10x10 degrees tiles but Johnson-V magnitude and additional the magnitude difference of Gaia Blue and Gaia red. The H indicates 5x5 degrees tiles and a single magnitude Gaia blue. The numbers 17, 18 indicate the maximum magnitude.

Support is available via the ASTAP Forum

Command line, ASTAP style

The program can be executed using command line options to solve images astrometric . E.g. ASTAP -f home/test/2.fits -r 30

The program will accept FITS files, JPG, PNG, BMP, XISF or TIF files.

	ASTAP astrometric solver command line		The FOV, RA,DEC options are intended for none FITS files. Not required for FITS files having the values in the header.
command	parameter	unit	remarks
-h			help info
-help			help info
-f	file_name		File to solve astrometric.
-f	stdin		Will accept raw images via stdin. Raw format according INDI. See 1)
-r	radius_search_field	degrees	The program will search in a square spiral around the start position up to this radius *
-fov	field_height_of_image	degrees	Optional. Normally calculated from FITS header. Use value 0 for auto. If 0 is specified the fov found by solving it will be saved for next time.(learn mode) *
-ra	center_right_ascension	hours	Optional start value. Normally calculated from FITS header.
-spd	center_south_pole_distance (dec+90)	degrees	Normally calculated from FITS header * The declination is given in south pole distance, so always positive.
-z	down_sample_factor	0,1,2,3,4	Down sample prior to solving. Also called binning. A value "0" will result in auto selection downsampling. *
-s	max_number_of_stars		Limits the number of star used for the solution. Typical value 500. *
-t	tolerance		Tolerance used to compare quads. Typical value 0.007. *
-m	minimum star size	arcsec	This could be used to filter out hot pixels.
-speed	mode	slow / auto	"slow" is forcing of reading a larger area from the star database (more overlap) to improve detection. *
-o	file		Name the output files with this base path & file name
-d	path		Specify a path to the star database
-analyse	snr_minimum		Analyse only and report HFD. Windows: errorlevel is the median HFD * 100M + number of stars used. So the HFD is trunc(errorlevel/1M)/100. Linux: see stdout
-analyse2	snr_minimum		Both analyse and solve the image
-extract	snr_minimum		As analyse option but additionally write a .csv file with the detected stars info.
-sqm	pedestal		measure the sky background value in magn/arcsec $2$ relative to the stars. The pedestal is the mean value of a dark.
-focus1	file1.fits -focus2 file2.fits .......		Find best focus point for four or more images using curve fitting. Windows: errorlevel is focuspos1E4 + rem.error1E3. Linux: see stdout
-annotate			Produce a deep sky annotated jpeg file with same name as input file extended with _annotated *
-debug			Show GUI and stop prior to solving
-log			Write solver log to file with extension .log
-tofits	binning	1,2,3,4	Produce binned FITS file from input png/jpg *
-update			Update the fits header with the found solution *
-wcs			Write a .wcs file in similar format as Astrometry.net. Else text style.
			* Defaults can be set in the program. Shortcut CTRL-A, tab alignment

Commandline parameters have priority above fits header values. Front-end programs should provide access to -z and -r options. Default value for -z should be 0 (auto).

Typical command lines:

astap.exe -f image.fits -r 50

astap.exe -f c:\images\image.png -ra 23.000 -spd 179.000 -fov 1.3 -r 50

For most FITS files the command line can be short since telescope position and field of view can be retrieved from the FITS header. If a FITS file is not available, preference is a non lossless image format like .PNG or .TIFF or RAW like .CR2. If possible in 16 bit or original 12 bit format. Not stretched or saturated, as raw as possible. For formats other then FITS the RA,DEC position and -fov (image HEIGHT in degrees !!) should be added.

If the FOV (image height in degrees) is unspecified in the command-line for RAW, PNG, TIFF files, ASTAP will use the FOV as set in the program, stack menu, tab alignment. This setting can be learned and updated automatically with the parameters -fov 0. ASTAP will try all FOV between 10 degrees and and 0.5 degrees. E.g.

astap.exe -f c:\images\image.png -ra 23.000 -spd 179.000 -r 30 -fov 0

After a successful solve, the correct FOV will be stored in the ASTAP settings. For the next solve using images from the same source the -fov 0 parameters can be omitted and solving will be fast.

The debug option allows to set some solving parameters in the GUI (graphical user interface) and to test the commandline. In debug mode all commandline parameters are set and the specified image is shown in the viewer. Only the solve command has to be given manually:

astap.exe -f c:\images\image.png -ra 23.000 -spd 179.000 -r 30 -debug

astap.exe -debug

1) The astap -f stdin command line option

Raw format send via stdin should start with three longwords (of 4 bytes). The first longword contains a format identification. The second longword contains the image width. The third longword contains the image height followed by the image raw data.

Raw format identification:

$31574152 or RAW1, 8bit mono image
$32574152 or RAW2, 16it mono image
$33574152 or RAW3, 24bit RGB image (R,G,B,R,G,B,R.....)
$43574152 or RAW4, 32 bit mono image
$66574152 or RAWf, 32 bit float mono image

The result is written to stdout. Only for the standard ASTAP Windows version the stdout is blocked (unless you use the command line version) and the result is written to a stdin.ini file at the ASTAP program directory. Use the -o command to specify any other file and directory. The .wcs file is always written but also here use the -o option to specify path and file name.

The ra,dec and fov should be specified in the command line. Example using a raw file as input:

e.g. /opt/astap/astap -f stdin -ra 23 -spd 179 -fov 1.2 -r 30 <./m42.raw

raw sample file which will solve using /opt/astap/astap -f stdin -ra 6 -spd 95 -fov 2.8 -r 30 <./16bit_rosette.raw

Loading of the image can be tested by adding -debug to the command line

Command-line, output files

In command line mode the program produces two output files at the same location as the input image. In case a solution is found it will write a .wcs file 1) containing the solved FITS header only. In any case it will write an INI file using the standard FITS keywords.

Example of the INI output file after an successful solve:

PLTSOLVD=T                                   // T=true, F=false
CRPIX1= 1.1645000000000000E+003                // X of reference & centre pixel
CRPIX2= 8.8050000000000000E+002 // Y of reference & centre pixel
CRVAL1= 1.5463033992314939E+002                // RA (J2000) of the reference pixel [deg]
CRVAL2= 2.2039358425145043E+001                // DEC (J2000)of the reference pixel [deg]
CDELT1=-7.4798001762187193E-004                // X pixel size [deg]
CDELT2= 7.4845252983311850E-004                // Y pixel size [deg]
CROTA1=-1.1668387329628058E+000                // Image twist of X axis [deg]
CROTA2=-1.1900321176194073E+000                // Image twist of Y axis [deg]
CD1_1=-7.4781868711882519E-004                // CD matrix to convert (x,y) to (Ra, Dec)
CD1_2= 1.5241315209850368E-005               // CD matrix to convert (x,y) to (Ra, Dec)
CD2_1= 1.5534412042060001E-005                // CD matrix to convert (x,y) to (Ra, Dec)
CD2_2= 7.4829732842251226E-004 // CD matrix to convert (x,y) to (Ra, Dec)
CMDLINE=......   // Text message containing command line used
WARNING=...... // Text message containing warning(s)

The reference pixel is always specified for the centre of the image. The decimal separator is always a dot as for FITS headers.

Example of the INI output file in case of solve failure:

PLTSOLVD=F // T=true, F=false
CMDLINE=...... // Text message containing command line used
ERROR= ..... // Text message containing any error(s). Same as exit code errors
WARNING= ..... // Text message containing any warnings(s)

The .wcs file contains the original FITS header with the solution added. No data, just the header. Any warning is added to the .wcs file using the keyword WARNING. This warning could be presented to the user for information.

1) Note the wcs file is default written as text file using carriage return and line feed for each line and is not conform the FITS standard. To have .wcs file conform the FITS standard add the command-line option -wcs.

Command-line, error codes

In the command-line mode errors are reported by an error code / errorlevel {%errorlevel%}. This is the same error as reported in the .ini file in case of failure.

Error code	Describtion
0	No errors
1	No solution
2	Not enough stars detected

16	Error reading image file

32	No star database found
33	Error reading star database

To analyse a FITS file you could do the following in a Windows batch file:

c:\astap.fpc\astap.exe -f c:\astap.fpc\test_files\command_line_test\m16.fit -analyse 30
echo Exit Code is %errorlevel%
pause

You will get
Exit Code is 326000666

where the HFD is 3.26 using 666 stars

Finding best focus based on four or more input images:

c:\astap.fpc\astap -focus1 D:\temp\FocusSample\FOCUS04689.fit -focus2 D:\temp\FocusSample\FOCUS05039.fit -focus3 D:\temp\FocusSample\FOCUS05389.fit -focus4 D:\temp\FocusSample\FOCUS05739.fit -focus5 D:\temp\FocusSample\FOCUS06089.fit -focus6 D:\temp\FocusSample\FOCUS06439.fit -focus7 D:\temp\FocusSample\FOCUS06789.fit -focus8 D:\temp\FocusSample\FOCUS07139.fit
echo Exit Code is %errorlevel%
pause

or with the -debug option

astap.exe -debug -focus1 D:\temp\FocusSample\FOCUS04689.fit -focus2 D:\temp\FocusSample\FOCUS05039.fit -focus3 D:\temp\FocusSample\FOCUS05389.fit -focus4 D:\temp\FocusSample\FOCUS05739.fit -focus5 D:\temp\FocusSample\FOCUS06089.fit -focus6 D:\temp\FocusSample\FOCUS06439.fit -focus7 D:\temp\FocusSample\FOCUS06789.fit -focus8 D:\temp\FocusSample\FOCUS07139.fit

Or with the special command line version:

Command-line pop-up notifier

If the ASTAP is command-line executed in MS-Windows, it will be shown by a small ASTAP tray icon on the right side of the status bar. If you move the mouse to the ASTAP tray icon, the hint will show the search radius reached. To refresh the value move the mouse away and back.

If the search spiral has reached a distance more then 2 degrees from the start position then a popup notifier will show the actual search distance and solver settings:

The first line indicates the search spiral distance (8º) from the start position and the maximum search radius (90º)
The image height in degrees.
Downsample setting and the input dimensions of the image to solve.
The α and δ of the start position.
Speed normal (▶▶) or small steps (▶)

See conditions required for solving to fix solve failures.

Tray icons are default off in the latest Win10 version. To set the ASTAP tray icon on, start a solve via the imaging program, go to Windows "Settings", "Taskbar", "Turn system icons on or off" and set the ASTAP tray icon permanent "on" as shown below:

Blind solving performance

Blind solving performance for a 90 degrees offset:

ASTAP blind solver performance. Solving a 50 seconds exposed monochrome image of M24, 2328x1760 pixels covering a field of 1.75 x 1.32° starting 90 degrees more north at position 18:17, +72d 00

Maximum stars set	Tolerance set	Astrometric solving time
500	0.005	147 sec.
250	0.005	103 sec
150	0.005	92 sec
100	0.005	70 sec

Reducing the "maximum number of stars to use" will result in a faster solving but also an increased risk of solve failure.

Usage as a solver with other programs or as PlateSolve2 substitute

Back to index

CCDCIEL, using ASTAP as solver.

ASTAP is a menu option in the CCDCIEL program. Install both ASTAP and the G17 star database. Select in CCDCiel menu ASTAP as solver and follow the guidelines in the help file.

Progress is shown in tray icon and popup notifier.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

Back to index

APT: using ASTAP as solver

Since ATP version 3.85.1, APT fully supports ASTAP. Just select ASTAP in the APT pointcraft menu after installing both ASTAP and the G17 star database. For solving it is important that the correct filed of view is calculated from the Tools, Object calculator. So set the focal length and "CCD width x height" correctly.

Once installed you could now test it with a saved image. The solver needs an initial position guess.

To modify the ASTAP default settings, see astrometric_solving

Progress is shown in tray icon and popup notifier. In case of solve failure please check the FOV indicated.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

Screenshot APT using ASTAP:

For older APT, Astro Photography Tool use it a PlateSolve2 substitute by renaming ASTAP.EXE program as Platesolve2.exe. and select it as PlateSolve2

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ModelCreator:

Installation as solver for ModelCreator:

Install ASTAP and additional install the G17 star database.
In ModelCreator select ASTAP as plate solver.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

Back to index

NINA, Nighttime Imaging ‘N’ Astronomy

For NINA version 1.9

Install ASTAP and additional the G17 star database.
Other settings as below:

To modify the ASTAP default settings, see astrometric_solving

Solving progress is shown in tray icon and popup notifier.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

Back to index

SharpCap:

Installation as solver for SharpCap:

Install ASTAP and additional install the G17 star database.
In SharpCap select ASTAP as plate solver.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

Back to index

SGP, Sequence Generator Pro:

The latest SGP versions support ASTAP as solver. Just install ASTAP and G17 database. Select ASTAP as solver. Set binning at 1x1 or 2x2. See conditions required for solving.

Solving progress is shown in tray icon and popup notifier.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

For older SGP versions: you could use ASTAP as PlateSolve2 substitute:

The orginal PlateSolve2.exe is located at C:\Users\you\AppData\Local\SequenceGenerator\ Where "you" is your user name. You can access this directory also directly by %LOCALAPPDATA%\SequenceGenerator

Install ASTAP and additional install the G17 star database in the same directory. Typical c:\program files\astap

Copy or move the astap.exe and all 290 files with extension .290 to C:\Users\you\AppData\Local\SequenceGenerator\

Rename the original Platesolve2.exe to something like PlateSolve2ORG.exe

Rename ASTAP.exe to PlateSolve2.exe

Test it with SGP. The confidence will be always 999. No PlateSolve2 window will be shown.

Progress is shown in tray icon and popup notifier.

If you select in SGP the PlateSolve2 setting "Max Regions" (=3000) this will force a search up to 90 degrees diameter. However the solver will be most likely stopped halfway by a SGP timeout.

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Voyager:

The latest Voyager versions support ASTAP as solver. Just install ASTAP and G17 database. Select ASTAP as solver.

Solving progress is shown in tray icon and popup notifier.

Solving should be reliable. In case of failure, have a look to conditions required for solving.

For older Voyager versions: you could use ASTAP as PlateSolve2 substitute. The original PlateSolve2.exe is located at C:\Program Files (x86)\Voyager

Install ASTAP and additional install the G17 star database in the same directory. Typical c:\program files\astap

Copy or move the astap.exe and all 290 files with extension .290 to C:\Program Files (x86)\Voyager

Rename the original Platesolve2.exe to something like PlateSolve2ORG.exe

Rename ASTAP.exe to PlateSolve2.exe

Test it with Voyager. E.g. in the OnTheFly section. Confidence will be reported as 999. E.g. Solved (J2000) => RA 19 59 36,167 DEC 22 42 36,31 PA 277,4 Res. 1,17 [as/px] FL 1131,77 [mm] Star/s 999

Back to index

Installation of the external Astrometry.net solver:

MS-Windows:

Install a local copy of Astrometry.net (via ANSVR or Astrotortilla) as the astrometric solver. Or alternatively if you have Win10, 64 bit Creation edition you use the new Linux sub-system

ANSVR: The ANSVR link contains a newer compilation of astrometry.net made for SGP. It runs as a Linux program under Cygwin in MSWindows. Follow up to installation step 9. The link you have to put in ASTAP is as follows:

C:\Users\user_name\AppData\Local\cygwin_ansvr\bin\bash.exe

Adapt "user_name" to the login name used in Windows.

The server program ANSVR is not required. Remove the ANSVR shortcut in the startup menu. Location:

C:\Users\user_name\AppData\Roaming\Microsoft\Windows\Start Menu\Programs\Startup

Alternative Linux sub-system in Win10 64bit Creators edition

Path for the astrometry.net solver program
ANSVR installation:
    C:\Users\user_name\AppData\Local\cygwin_ansvr\bin\bash.exe
Astrotortilla installation:
    C:\cygwin\bin\bash.exe
Win10 subsystem:
    C:\Windows\System32\bash.exe

Linux installation:

The single executable astap could be used anywhere. Standard directory could be c:/opt/astap but also at your home folder.

If you want to use astrometry.net this is described at installation. To get the Astrometry.net solver type: sudo apt-get install libcairo2-dev libnetpbm10-dev netpbm libpng-dev libjpeg-dev python-numpy python-pyfits python-dev zlib1g-dev libbz2-dev swig libcfitsio-dev You also have to download index files.

Path to the astrometry.net solver program "solve-field" could be:

/usr/bin/
or
/usr/local/astrometry/bin

Appendix 1, the stack process:

The stacking process for one shot color color camera's will be mathematically executed as follows:

The master flat is calculated as:

master flat: = (1/n ∑ [flat] - 1/n ∑ [flat darks] )

Where a bias image could be used as flat-dark image. The master flat should be averaged by a 2x2mean to remove Bayer matrix artifacts.

Each image is calculated as:

(image- {∑ [darks]/n} ) / master flat

Then the Bayer matrix is applied and finally the images are stacked in mode average or sigma clipped.

So for average stack:

final image:= 1/n ∑ Bayer(image)

Back to index

Appendix 2, the 1476 star database format:

The .1476 format divides the sky in 1476 area's and 1476 corresponding files with the extension .1476. The 1476 areas have a minimum width or diameter of 5 degrees. The boundaries are defined by constant RA and DEC values. For the similar 290 file format see the HNSKY help file

Each star is stored in a record of 5 bytes. The files start 'with a 110 byte header containing a textual description and the record size binary stored in byte 110. The RA values are stored as a 3 bytes word. The DEC positions are stored as a two's complement (=standard), three bytes integer. The resolution of this three byte storage will be for RA: 360*60*60/((256*256*256)-1) = 0.077 arc seconds. For the DEC value it will be: 90*60*60/((128*256*256)-1) = 0.039 arc seconds. The starts are sorted on magnitude and the magnitude is stored in a byte of the special preceding header with an offset to make all values positive.

  Example of star Sirius RA and DEC position:
  The RA position is stored as C3 06 48 equals: (195 +6*256 +72*256*256)*24/((256*256*256)-1)=6.75247662 hours equals: 6:45:8.9
  The DEC position is stored as D7 39 E8, equals: 215 57 -24. The DEC is then (215 +57*256 -24*256*256)*90/((128*256*256)-1)=-16.7161401 degrees equals -16d 42 58

Record format:

The 290-5 or 1476-5, short record size of 5 bytes for one star without designation:

    type
      hnskyhdr290 = packed record
      ra7 : byte;
   ra8 : byte;
      ra9 : byte;
      dec7 : byte;
   dec8 : byte;{magnitude and dec9 are written once in preceding header-record}
      end;

Stars are sorted from bright to faint in the "0.1" steps. Within the magnitude range, the stars are additional sorted in DEC. For a series of stars with the same DEC9 value, a header-record is preceding containing the DEC9 value stored at location DEC7. Since the stars are already sorted in 36 declination bands, the number of DEC9 values is already limited by a factor 36.

1476-5 header-record example: FF FF FF 20 06 This indicates the following records have a DEC9 value of 20 -128 offset and a magnitude of (06 - 16)/10 equals -1.0 (new method, +16 offset).

The shorter records methods become only space efficient for very large star collection of a few million stars. In these large collections many stars can be found with the same magnitude and DEC9 shortint. The Gaia database is only issued in the 1476-5 format of 5 bytes per star. or in an older format 290-6 (V17) or 290-5. The 290-6 has one more byte for the colour information. This is documented in the HNSKY planetarium program help file.

So the record sequence will be as follows:

header-record {new section will start with a different magnitude and dec9}
record
record
record
record
header-record {new section will start with a different magnitude and dec9}
record
record

The 1476 areas: Each ring is divided by lines of constant RA such that the minimum width in RA is about 5 degrees (deltaRA*cos(DEC)
.

Declination minimum	Declination maximum	Ring	RA cells	RA step north[degr]	RA step distance south[degr]	DEC step		Files
-90.00000000	-87.42857143	0-1	1					0101.1476
-87.42857143	-82.28571429	1-2	3	5.38377964	16.10799190	-2.57142857	{=90/(2*17.5)}	0201.1476, 0202.1476, 0203.1476
-82.28571429	-77.14285714	2-3	9	5.36933063	8.90083736	-5.14285714	{=90/17.5}	0301.1476, . . . . . , 0309.1476
-77.14285714	-72.00000000	3-4	15	5.34050241	7.41640786	-5.14285714	{=90/17.5}	0401.1476, . . . . . , 0415.1476
-72.00000000	-66.85714286	4-5	21	5.29743419	6.73757197	-5.14285714		0501.1476, . . . . . , 0521.1476
-66.85714286	-61.71428571	5-6	27	5.24033376	6.31824883	-5.14285714		06 . . .
-61.71428571	-56.57142857	6-7	33	5.16947632	6.00978525	-5.14285714		07 . . .
-56.57142857	-51.42857143	7-8	38	5.21902403	5.90674549	-5.14285714		08 . . .
-51.42857143	-46.28571429	8-9	43	5.21991462	5.78564078	-5.14285714		09 . . .
-46.28571429	-41.14285714	9-10	48	5.18296987	5.64803600	-5.14285714		10 . . .
-41.14285714	-36.00000000	10-11	52	5.21357169	5.60088688	-5.14285714		11 . . .
-36.00000000	-30.85714286	11-12	56	5.20082354	5.51859939	-5.14285714		12 . . .
-30.85714286	-25.71428571	12-13	60	5.15069276	5.40581321	-5.14285714		13 . . .
-25.71428571	-20.57142857	13-14	63	5.14839353	5.34991355	-5.14285714		14 . . .
-20.57142857	-15.42857143	14-15	65	5.18530082	5.33887123	-5.14285714		15 . . .
-15.42857143	-10.28571429	15-16	67	5.17950194	5.28678585	-5.14285714		16 . . .
-10.28571429	-5.14285714	16-17	68	5.20903900	5.27280509	-5.14285714		17 . . .
-5.14285714	0.00000000	17-18	69	5.19638762	5.21739130	-5.14285714		18 . . .

0.00000000	5.14285714	18-19	69	5.21739130	5.19638762	-5.14285714		19 . . .
5.14285714	10.28571429	19-20	68	5.27280509	5.20903900	-5.14285714		20 . . .
10.28571429	15.42857143	20-21	67	5.28678585	5.17950194	-5.14285714		21 . . .
15.42857143	20.57142857	21-22	65	5.33887123	5.18530082	-5.14285714		22 . . .
20.57142857	25.71428571	22-23	63	5.34991355	5.14839353	-5.14285714		23 . . .
25.71428571	30.85714286	23-24	60	5.40581321	5.15069276	-5.14285714		24 . . .
30.85714286	36.00000000	24-25	56	5.51859939	5.20082354	-5.14285714		25 . . .
36.00000000	41.14285714	25-26	52	5.60088688	5.21357169	-5.14285714		26 . . .
41.14285714	46.28571429	26-27	48	5.64803600	5.18296987	-5.14285714		27 . . .
46.28571429	51.42857143	27-28	43	5.78564078	5.21991462	-5.14285714		28 . . .
51.42857143	56.57142857	28-29	38	5.90674549	5.21902403	-5.14285714		29 . . .
56.57142857	61.71428571	29-30	33	6.00978525	5.16947632	-5.14285714		30 . . .
61.71428571	66.85714286	30-31	27	6.31824883	5.24033376	-5.14285714		31 . . .
66.85714286	72.00000000	31-32	21	6.73757197	5.29743419	-5.14285714		32 . . .
72.00000000	77.14285714	32-33	15	7.41640786	5.34050241	-5.14285714		33 . . .
77.14285714	82.28571429	33-34	9	8.90083736	5.36933063	-5.14285714		34 . . .
82.28571429	87.42857143	34-35	3	16.10799190	5.38377964	-5.14285714		3501.1476, 3502.1476, 3503.1476
87.42857143	90.00000000	36-37	1					3601.1476

The 1476 areas:

South pole view of the 1476 areas:

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Succes, Han K

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