White balance and master darks

Discussions of Electronically Assisted Astronomy using the Live Stacking feature.
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saguaro
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Joined: Tue Feb 14, 2017 9:04 pm

White balance and master darks

Post by saguaro » Sat Oct 26, 2019 7:10 pm

With my ZWO 385MC and my 1600MC-Cool cameras I have always used 50/50 for R and B (under Image Controls) when creating my master darks in SharpCap. However, my live stacked images always had a greenish cast to them that I could not balance out using the live stacking color sliders. When I create my master darks using the same color balance as my light frames, the greenish cast was not present and I get a better white balance during live stacking.

https://www.astrobin.com/aitf98/B/?nc=c ... n&nce=1157

https://www.astrobin.com/407388/?nc=collection&nce=1155 (scroll down to see the description)

This is not camera specific or filter specific since I’ve seen the green cast with both cameras and using different filters using different scopes. I can only conclude that it must be something going on in SharpCap but I’ve no idea what it is. I just know that I get better results when my master dark has the same R and B values as my lights.

Robin: can you shed some light on what is going on? Thanks!

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Re: White balance and master darks

Post by admin » Sat Oct 26, 2019 7:44 pm

Hi,

Yes, that would be what I would expect to happen. The colour balance controls for most cameras are just an extra gain that is applied to the individual colour channels making them brighter or dimmer. If the settings are different between your darks and your lights then the darks may not subtract properly – just as if you took two sets of darks with different colour balance settings you wouldn't expect the data to be the same.

Maybe the red channel was set higher in the lights than in the darks, which would mean that the subtraction in the red channel would be incomplete. This would be very difficult to fix in postprocessing as the error that is introduced isn't just a linear scaling of the colour channels – it's a small offset.

Overall the best approach is to change absolutely nothing between your darks and lights except for covering the business end of the telescope!

Cheers, Robin

saguaro
Posts: 68
Joined: Tue Feb 14, 2017 9:04 pm

Re: White balance and master darks

Post by saguaro » Sun Oct 27, 2019 2:57 pm

Thanks for the explanation!

I know you recommend just leaving the R and B sliders at 50/50 and doing all color balancing using the live stacking color sliders. And that means using a matching master dark with 50/50 for the R and B sliders as well. When I use this method, I either get a background with a red or green cast (depending on the camera and filter I'm using) that I can't balance out during live stacking.

I find I get the best results when I do an initial color balance using the Smart Histogram to get all the R, G, and B values to match before I start live stacking. Then when I start live stacking, generally either one of the auto color balance controls (using Histogram peaks or Star Colors) gets me very close to a good color balance with a fairly dark background (no red or green cast). But this means also using a matching master dark that has the same color balance. This technique may not yield the best results for everybody but I find it works best for me.

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