Brightness and White Balance Adjustments after Smart Histogram Brain

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poolemarkw
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Brightness and White Balance Adjustments after Smart Histogram Brain

Post by poolemarkw » Mon Nov 05, 2018 10:35 pm

I want to make sure that I'm clear on the process steps for setting all of the various components for good exposure and if I'm doing it the best way:

1) After completing the sensor analysis, I will use the Smart HIstogram Brain function to see the set of recommended gains and exposures. Let suppose I select an exposure time of 50 seconds and the associated gain.
2) Following this, I assume that I need to set the Brightness (ZWO ASI294MC). How do I determine the best brightness level to select? Is it just a visual adjustment to what appears best? Should the background be pretty dark? Should I be trying to achieve a certain histogram shape? Also, I have to wait for 50 seconds to the result of each change on the histogram, correct? Is there a better way?
3) Following this, I assume that I need to set the Red and Blue white balance sliders. Again, I have to wait 50 seconds to see the result of each change. Is there a better way?

Thanks!!

Mark

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by HC1492 » Tue Nov 06, 2018 7:50 pm

Hi Mark,

Have you checked out the Histogram documentation from the Sharpcap manual yet? This should tell you what you need to know.
https://docs.sharpcap.co.uk/3.2/#Using% ... %20Quality

Heather

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by turfpit » Tue Nov 06, 2018 9:38 pm

Mark
How do I determine the best brightness level to select
Brightness is ZWOspeak for offset. Have a read of this thread viewtopic.php?f=9&t=1209&start=20 around Oct 16. Create a bias frame which has a balanced histogram in FITS Liberator. For a given gain, set the exposure to the minimum possible and capture a single frame with the camera cap on (a zero exposure dark frame). Keep adjusting brightness until the FITS Liberator histogram is centralised. That is the brightness to use for that particular gain setting. Change the gain and you need to re-establish the offset (brightness) value.

It should make sense if you look at the images.

Dave

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by poolemarkw » Wed Nov 07, 2018 2:28 pm

Thanks Dave! I've read through that thread a couple of times and understand that I should use the Offset (Brightness) that provided a centralized histogram of the bias frame at a selected gain. I'm uncertain as to Brian's suggestion to use the "minimum offset". How is that minimum determined?

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by turfpit » Wed Nov 07, 2018 3:31 pm

Mark

I am sure Brian will be along soon to enlighten you.

Dave

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by admin » Wed Nov 07, 2018 4:30 pm

Hi,
The brain algorithm wants to separate the histogram from the left-hand side. Often it doesn't need to add any offset to do that because the sky brightness is sufficient. When that happens it will suggest minimum offset but you can always go a little higher.

Cheers, Robin

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by oopfan » Wed Nov 07, 2018 7:58 pm

I agree wholeheartedly that sky brightness contributes mightily to shifting the histogram rightward but the problem is with the darks not the lights. In my experience the Brain recommends relatively short exposures even though my skies are darker than average but so be it. With an uncooled camera like mine in the dead of winter a short exposure results in a clipped histogram (see attachment). During calibration this "missing energy" wreaks havoc especially when stretching the image to eke out faint nebulosity. My apologies if the Brain's algorithm has been improved since then. I haven't used it since that time. Now I advocate the approach I learned in the AAVSO CCD Photometry class which uses Bias frames to determine the proper Offset.

Brian
Attachments
M42 Dark.jpg
M42 Dark.jpg (211.69 KiB) Viewed 373 times

poolemarkw
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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by poolemarkw » Wed Nov 07, 2018 8:28 pm

I performed the bias test on my ZWO ASI294 MC and yielded the following data. SharpCap offers a gain setting from 0-570 and brightness settings from 0-80. Does this look reasonable? I also noted that a couple of the gain settings produced bimodal histograms. Why would this be?

Gain Brightness
100 10
150 15
200 18
250 18
275 20
300 25
325 35
350 45
375 50
400 60
425 75
450 80

Thanks,

Mark

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by turfpit » Wed Nov 07, 2018 8:44 pm

Mark

Can you post a couple of example FITS Liberator images as above. Also an example of a bimodal one - I might have seen that before, I will have a hunt round in my old data. What colour space is being used?

Dave

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Re: Brightness and White Balance Adjustments after Smart Histogram Brain

Post by poolemarkw » Wed Nov 07, 2018 11:11 pm

Here's the bimodal image, with gain at 150 (out of 570 max) and brightness at 15 (out of 80 max). The color space was Raw 16.

Image

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