ASTAP, the Astrometric STAcking Program and FITS viewer



Questions, feedback to the ASTAP Forum

ASTAP introduction

ASTAP is a free stacking program for deep sky images. This program can view and stack astronomical FITS images using an astrometric solution. It uses a local version of or PlateSolve2 to find the astrometric solution. With the solution the astronomical position of each pixel can be calculated. 

Main features:

  1. Stacking images including Dark Frame and Flat Field correction
  2. Filtering of deep sky images based on HFD value and average value.
  3. Automatic call to local or Platesolve2 for the astrometric solution.
  4. FITS viewer with swipe functionality.
  5. Results can be saved to 16 bit or float (-32) FITS file
  6. Export to 16 or 32 bit integer TIFF files, grayscale or color or simple PNG or JPEG.
  7. Mosaic building covering large areas using the astrometric linear solution WCS or WCS+SIP polynomial. 
  8. Some pixel math functions and digital development process
Double click on one of the selected files to see it in the viewer:

Program requires FITS images as input It can also process PNG, TIFF or BMP files but only in 8 bit resolution. No import facility for DSLR raw files.

ASTAP is not:
A post processing software with advanced options like noise reduction and unsharp mask. It is only intended for convenient stacking of astronomical deep sky images.

Program installation:

Windows installation:
The single executable astap.exe can be used anywhere. Standard location could be at c:\program files\astap\astap.exe

Install a local copy of (via ANSVR  or  Astrotortilla) or   Platesolve2 including star database as the astrometric solver. Or laternatively if you have Win10, 64 bit Creation edition you use the new Linux sub-system

ANSVR: The ANSVR link contains a newer compilation of 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:


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 solver program
ANSVR installation:
Astrotortilla installation:
Win10 subsystem:

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

You need either a local copy of of Wine & PlateSolve2 

Installation of is described at installation. To get the source: 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

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


For PlateSolve2, install Wine first. Then extract PlateSolve2 at $HOME/.wine/drive_c/PlateSolve2.28   The separate star database UCAC3 could be extracted at $HOME/.wine/drive_c/PlateSolve2.28/UCAC3PS.  
PlateSolve2 requires the msvbvm60.dll to run. Try to find a copy and place it in $HOME/.wine/drive_c/PlateSolve2.28 or alternatively install winetricks vb6run
Run PlateSolve2 in Wine and set in menu FileConfigure Catalog Directories the path to the UCAC3 star catalog.  

Program operation:

Start the ASTAP program. (In MS-Windows astap.exe, in Linux astap)

Enter the approximate location of the images taken and the image dimensions unless the location and dimension are already contained in the FITS file header. You can test this by just opening one FITS file.

Call up the stack window. Select the images, dark, flats, flat darks (bias)

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.

Press the stack button.

Save result as FITS  ( IEEE float (-32) or 16 bit format), or export as 16 bit stretched, 16 bit or 32 bit TIFF.  The stretched export folllows the gamma and stretch setting of the display.

All the program settings and file selections will be save by leaving the program or click on the stack button.

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. If the oversize is set from the default 100 pixels to a  large value lets say 2000 pixels, the array is on all sides 2000 pixels larger then  the first image.  If the first image is 2000x1500 pixels, the stacking array will be 6000 x 5500 pixels large. So any following image will be placed in this 6000x5500 pixel array.  If the images are taken from different areas of the sky, the stacking will result in a mosaic as long the proceeding images are within this 6000x5500 pixels.

Here a suggested work method:
  1. Stacked method sigma-mean using the local for solving. (option --downsampling 2)
  2. Mosaic option ON, oversize 2500 pixels. (The orginal tile images are 2328x1760 pixels)
  3. Crop the stacked result to about 5000x5000 pixels. 
  4. If required, applie the median-equalising filter under tab Pixel Math to equalise darker areas.
  5. Applied DDP (digital development process) under Pixel Math to strech the mosaic.
  6. Adjusted 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.

De stack menu:

The astrometry menu:

    Some guidelines:

Typical cold CMOS master dark:

Application of Gaussian blur under sigma 5:

Gaussian blur under sigma 5 and hot pixel removed:

Additional functionality:

The  FITS viewer can call PlateSolve2 and add the plate solution to the FITS header. Also PNG and JPEG files can be plate solved. The program will convert them to the FITS format.  With the batch routine several FITS image can be "plate solved".   The viewer can convert raw images of sensors with a Bayer matrix to color.

ASTAP viewer screen shot:

 PlateSolve2 is a good plate solver but can’t write the plate solution to the FITS header using standard keywords. HNS_FV can call the PlateSolve2 program and writes the found plate solution to the FITS file header using standard keywords.

Working of the program:

In principle it should be possible to rename the program as Platesolve2 and call it instead of PlateSolve2. The solution will then written automatically to the FITS file. The plate solution is read from the APM file produced by PlateSolve2.

There is a big variation in FITS file keywords. If this version doesn't work, please send me the FITS file for testing.

CCD inspector

ASTAP has a CCD inspector under tools for median HFD value and Tilt measurement.  This will quickly show any focus problem in the corners:. The squares are 5 times the HFD value. 

Background equalization tool:

Send a message if you like this free program. Feel free to distribute !

Succes,  Han  K

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