Author Topic: Preprocessing and combining images with different gain settings  (Read 359 times)

Offline VoidPointer

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Hi,

During image capture over several nights, I accidentally captured part of my data with a different (higher) gain setting on my camera.
The data is still good but I'm unsure how to treat it appropriately during preprocessing.
Clearly, I need to create matching bias and dark frames for the higher gain setting.
Should I also use a separate flat for the higher gain, or can I use the same (calibrated with the lower gain bias/dark) flat I use for the other set of exposures?
Finally, how should I go about integration? Align+integrate all exposures at once or rather integrate separately and then combine those two results?

Cheers!
Lars

Offline pfile

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Re: Preprocessing and combining images with different gain settings
« Reply #1 on: 2019 August 26 07:03:13 »
i think the flat should be OK once calibrated with the right calibration frames. it's the equivalent of shooting flats with a DSLR set to a different ISO than lights, and that works fine as far as i understand.

as for integration i think integrating everything together in one go should also work.

rob

Offline VoidPointer

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Re: Preprocessing and combining images with different gain settings
« Reply #2 on: 2019 August 27 01:08:35 »
Thanks Rob, that's helpful...

BTW, can Batch-PreProcess figure out the correct master-bias/dark to use for calibrating a given frame also based on the gain? I know it works for exposure time using EXPTIME fits-keword. There is also the EGAIN keyword... will BPP also select bias/darks based on that keyword?

Cheers!

Offline pfile

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Re: Preprocessing and combining images with different gain settings
« Reply #3 on: 2019 August 27 07:44:24 »
i'd have to read the source code to give a proper answer, but BPP was conceived quite a while ago before the wide popularity of CMOS cameras so my guess would be no, that it doesn't look at the gain.

just grepping for "GAIN" in the batchpreprocessing folder i don't find any hits there, so it looks like it does not check that keyword.

rob

Offline vicent_peris

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Re: Preprocessing and combining images with different gain settings
« Reply #4 on: 2019 August 27 10:37:06 »
Hi Lars,

The workflow is as follows:

- Acquire the bias and dark frames at the right gain for those lights.

- Configure BatchPreprocessing as follows:
   - Load the individual bias an dark frames for these lights.
   - Load the master flat you created in the previous with different gain. Don't worry since this master flat is already calibrated and only contains the proportion of light response per pixel.
   - Check the "use master flat" option. Keep unchecked "use master bias" and "use master dark" options.
   - As registration reference, select the same reference as you selected in the previous run.

- Run BPP in a different output directory. Don't activate to apply the light integration.

- Manually integrate all the lights for each filter. You can mix images with different gains, ImageIntegration will weight each frame according to its noise level.



Good luck,
Vicent.

Offline VoidPointer

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Re: Preprocessing and combining images with different gain settings
« Reply #5 on: 2019 August 28 01:52:16 »
Thanks for the detailed response Vicent. I got everything integrated nicely based on that!