How is flat division actually done in ASTAP?

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Expand view Topic review: How is flat division actually done in ASTAP?

Re: How is flat division actually done in ASTAP?

by steffens » 27 Feb 2020, 07:10

Sure, much clearer now.
Thanks!

Steffen

Re: How is flat division actually done in ASTAP?

by han.k » 12 Feb 2020, 16:13

Hello Stephan,

Your correct, the text is wrong. I have corrected it as follows:


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

1. 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.
2. The darks will be combined to an average master dark.
3. Optional on the master flat a 2x2, 3x3 or 4x4 mean filter is applied to reduce noise.
4. From each light frame the master dark will be subtracted to extract the pure deep sky signal.
5. Each light frame will be flattened by dividing it by the master-flat resulting in the corrected light frames.
6. 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.


There are no intermediate calibration files kept.
Does this clarify it?

Clear skies, Han

How is flat division actually done in ASTAP?

by steffens » 12 Feb 2020, 13:21

Hi,

I've got a question regarding how flat division is actually implemented in ASTAP.
The documentation on the web site describes this in the stacking process as:
...
2) The combined dark frames called master dark will be subtract from the light frames to extract the pure deep sky signal.
3) The combined light frames, dark compensated in 2) will be flattened by dividing it by the master-flat resulting in the final deep sky image. Prior to this a small 2x2 or 3x3 median filter is applied the master flat to reduce noise.
Does that actually mean, that flat division happens after alignment and stacking of the dark compensated light images? That would seem wrong to me, as after alignment (translation, rotation, scaling), the light image does no longer match the flat image. Dust bunnies are no longer in the same place as in the flat!

In all other stacking software I know of, flat division is part of the calibration / pre-processing step that is done for each source light image before alignment (registration) and stacking.

Is this probably just wrong wording in the documentation on the web site?

Best Regards,
Steffen

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