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Dedicated astrocamera colors during totality

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#1 gbartha

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Posted 20 April 2024 - 09:30 PM

One of the things I did during the eclipse was use a Player One Ares-C color astrocamera to image the eclipse with an AT102ED telescope.  As the camera does not have an IR cut filter, it was recommended to use one and I used one from ZWO.  I used Firecapture for the imaging with the correct Bayer pattern and PIPP to break the .SER files into individual frames. This combination produced normal color images during partial eclipse phases but not during totality.  I have attached an image of the prominences and the color is very orange.  This indicates a lot of green color.  It is extremely difficult to color balance this.  It also has me concerned that when I use this camera for deep sky that the images will also be too green and be very difficult to color balance.  My expectation was, if anything, that my red sensitivity would be a bit high and need to be toned down.  So I am very confused.  Can anyone explain what is going on?

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#2 gbartha

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Posted 21 April 2024 - 11:26 AM

So after some additional investigation I think this extra green may be normal for astrocameras and somehow this gets fixed in post processing.  The explanation seems to be that the Bayer pattern has twice as much green as the other colors.  Although I thought that the green values get averaged rather than summed so that it is balanced but perhaps for astro it is desirable to have the extra luminance data.  The reason the partial phases appear to be relatively normal is that the tendency towards orange color matches the Sun.  So my questions are 1) how and where does this color get fixed in post processing software like DSS or Siril?  2) More importantly, how can I color balance this to be more like a regular camera?  When I use GIMP or other software it is like searching for a needle in a haystack to find parameters to color balance.  The software tools for doing this clearly were not designed to handle this well despite seeming like a very common problem.  Having an existing photo from a DSLR should make this much easier and yet there is no way to use this.  Ideally, one should be able to constrain the search space by setting regions of near white, near black and the specific color of the prominence.  If anyone knows of a guide for doing this please share.  My best results came from using channel mixer in GIMP but GIMP doesn't save the detailed history of parameters used and resets the values of this tool each time so I have not been able to reproduce the one time I got it close. 



#3 topcode

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Posted 21 April 2024 - 08:14 PM

So after some additional investigation I think this extra green may be normal for astrocameras and somehow this gets fixed in post processing.  The explanation seems to be that the Bayer pattern has twice as much green as the other colors.  Although I thought that the green values get averaged rather than summed so that it is balanced but perhaps for astro it is desirable to have the extra luminance data.  The reason the partial phases appear to be relatively normal is that the tendency towards orange color matches the Sun.  So my questions are 1) how and where does this color get fixed in post processing software like DSS or Siril?  2) More importantly, how can I color balance this to be more like a regular camera?  When I use GIMP or other software it is like searching for a needle in a haystack to find parameters to color balance.  The software tools for doing this clearly were not designed to handle this well despite seeming like a very common problem.  Having an existing photo from a DSLR should make this much easier and yet there is no way to use this.  Ideally, one should be able to constrain the search space by setting regions of near white, near black and the specific color of the prominence.  If anyone knows of a guide for doing this please share.  My best results came from using channel mixer in GIMP but GIMP doesn't save the detailed history of parameters used and resets the values of this tool each time so I have not been able to reproduce the one time I got it close. 

while it is intuitive that "2x green in bayer means green excess" that's not actually the case, green excess is rather caused by relative response/qe being optimized for green.

In deep sky images this is normally fixed with a color calibration based on star(s) in frame(for example, in siril), and fortunately, we have a star in frame with solar eclipse images. So, we can actually just use the sun from just before totality, to color balance the corona and such right when totality starts. Registax, should actually work really well for this, you can select a region of the sun as white, and use RGB balance to well, balance it. Then you can save the parameters, and apply the same to during totality.

Unfortunately, this does require that your solar filter is neutral density(creates no color cast) which not all of them are. But for example, the Baader solar filters are


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#4 gbartha

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Posted 22 April 2024 - 06:48 PM

Thanks for the suggestion topcode.  My solar filter doesn't qualify but I think you can get a similar effect by using a diamond ring shot that shows some pure Sun without a filter.  I tried the Registax RGB color balance with this and it sort of works but still not better than what I can do with GIMP after all my prior efforts.  I am just going to accept what I have and stick with regular digital cameras for solar eclipse imaging in the future. 



#5 topcode

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Posted 23 April 2024 - 03:19 PM

Thanks for the suggestion topcode.  My solar filter doesn't qualify but I think you can get a similar effect by using a diamond ring shot that shows some pure Sun without a filter.  I tried the Registax RGB color balance with this and it sort of works but still not better than what I can do with GIMP after all my prior efforts.  I am just going to accept what I have and stick with regular digital cameras for solar eclipse imaging in the future. 

Its possible that diamond ring would be clipped, and therefore not very helpful. The corona of the sun should basically appear a dim white, maybe try just balancing colors based on somewhere near the sun but not on any prominences?
 





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