Color balancing the ASI224MC

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calan
Posts: 35
Joined: Mon Feb 13, 2017 7:37 am

Color balancing the ASI224MC

Post by calan » Sun Feb 26, 2017 11:02 am

After picking Robin's brain and a lot of experimenting, here is what I've found works best for getting good color balance out of my ASI224MC and SharpCap.

1). Select a gain and exposure, based on mount, sky, etc. I usually use G300 and from 2s to 15s due to the ever-present wind and my old mount.

2). Set the image controls to defaults. I use gamma = 50, brightness (offset) = 140, and both white balance values at 50 with Auto off. Open the histogram (green icon) while viewing your target, and make sure you have some signal and that nothing is really odd-looking. If the hump is off the left edge, change the gain/exposure or bump up the brightness a bit to move it to just right of the left edge. Don't worry about anything else at this point.

3). Capture some darks. I usually just accept the default of 10 frames for 10 seconds and up, and select 20 frames for exposures below 10 seconds. Let a few frames fly by between gain and exposure changes, before pressing the capture button.

4). Get on target, and assign the appropriate dark frame you just captured, based on the gain and exposure you are going to use.

5). Using the histogram (green icon again; not the live stack histogram)... verify that the hump still just clears the left edge. If not, bump up the brightness slightly until it does. Leave the gamma at 50.

6). Use the blue and red white balance sliders to get the white, green, red, and blue traces all aligned on top of each other. As you increase both blue and red, the green will move to the left and the white will move to the right. I've found that at G300 and 10s exposures, my camera almost always requires blue at about 58 to 64, and red 55 to 58.

7). Leave the auto buttons turned off. Every time I've tried using auto turned on, the results are unpredictable.

It is important to capture the darks with the white balance at 50, and then adjust the white balance sliders once the dark frame is being applied and you are on target. If you create your darks with the color balance at some value other than 50, you'll get an additive effect that is almost impossible to get rid of. By capturing the darks with the white balance values at 50, selecting the dark frame, and THEN balancing the sliders with the object in view, you'll be adjusting the true color balance of the object, rather than amplifying color imbalances in the dark frame. (At least that's the way it appears to me. Robin may have a better technical explanation. :) )

Leave gamma at 50, as there is really no reason to change it. If you increase it, it will widen the sky hump and increase color separation causing false color. To get more signal, use exposure time, gain, or the brightness slider if necessary.

You can also display and adjust the area of the image that is used for the histogram adjustment. It's the small icon next to the reticule icon at the top. If you want to adjust for sky background, use the drag handles to select a small dark area. If you want to color balance the object you are interested in, adjust it to surround that object, or the whole capture area.

This workflow seems to provide the best color balance for me, and is repeatable for any object that I want to capture and view. I usually just shoot a single set of darks for 5s, 10s, and 15s at my chosen gain once everything is cooled down, and maybe again later in the night if the temp drops a lot. When I change the exposure time, I load up the appropriate saved dark, adjust the color balance, and save the profile. I rarely have to readjust the white balance between objects during the night for any given gain/exposure length, unless it's something really colorful and I just feel like tweaking it a little.

Here are some images saved directly from SC, with no processing whatsoever. You can see that the color balance is actually pretty good using this process (M81 is just slightly green; I probably got in a hurry while adjusting the sliders).

NGC 2024_Stack_1264frames_6320s_sm.jpg
NGC 2024_Stack_1264frames_6320s_sm.jpg (15.27 KiB) Viewed 217 times
M81_Stack_386frames_3860s_sm.jpg
M81_Stack_386frames_3860s_sm.jpg (16.89 KiB) Viewed 217 times
M37_Stack_1068frames_2136s_sm.jpg
M37_Stack_1068frames_2136s_sm.jpg (29.9 KiB) Viewed 217 times

db48
Posts: 17
Joined: Mon Feb 27, 2017 1:24 pm

Re: Color balancing the ASI224MC

Post by db48 » Sat Mar 04, 2017 9:07 pm

I have a general question concerning the white balance procedure of a CMOS camera, and I would like to share my (limited) experience and doubts with other users.
According to photography textbooks, a "neutral" image is necessary to adjust the R and B balance parameters.
I place in front of the scope a piece of photographic 18% neutral grey paper, illuminated by bright sunlight. Both the gain and the acquisition time of the camera are set at the minimum level to avoid saturation effects. R and B balance parameters are then adjusted to get te best overlap of the R, G, B curves in the hystogram.
There is a lot of discussion in the web about the effective "neutrality" of 18% photographic surfaces, but - as far as I know - it is not easy to find an alternative. In principle, a Color check palette is much more accurate, but it should be placed far enough from the scope to reach the focus.

Any suggestions about this topic?

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