Brightness and White Balance Adjustments after Smart Histogram Brain

[poolemarkw]

Here's another with the gain at 350 and brightness at 45, color space at 16 Raw. I've attached the SharpCap screen as well as the FITS Liberator screen. You can see that the peak is also bimodal.

[admin]

Hi,

Ok, I see now that you want to use the settings for darks and that the offset is too low for those frames. There's no harm in using a higher offset than the brain recommends as long as you use the same offset for the dark frames in the light frames. I can see that it might be a good idea to update the brain algorithm to use an offset that would properly separated dark frame from the left-hand side of the histogram.

I think the dual peaked histograms are just a reflection of the different sensitivities of the different pixel colours on the colour sensor and nothing to worry about.

Cheers, Robin

[poolemarkw]

Here is my final chart showing the Brightness setting required to get a Histogram minimum around 500, for several gain settings. My plan is to:

1) Use the Histogram Brain function to get recommended Gain/Exposure combinations and select one of the options along the curve. If I understand correctly, lower gains and higher exposure times are more desired to improve dynamic range.
2) Use that selected Gain value and my chart below to set my Brightness level.
3) Use those settings to capture my Darks
4) Use those same setting to perform my Live Stacking capture using the Darks for pre-processing.

Please let me know if I've missed anything!

[turfpit]

Mark

1) yes, unity gain seems to be a good starting point but not necessarily the end point. Gain=125 looks an interesting one from your graph.
2) yes
3) exposure same that planned for lights when capturing darks
4) match dark exposure when capturing lights but you might have meant that anyway

1 - 4 feels more like being in control to me (as opposed to the 'playing with sliders' from my dim and distant past)

The outcome should be interesting.

Dave

[poolemarkw]

Thanks Dave! I really appreciate your (and others) help. Unity Gain is at 117 according to the ZWO site. I'm curious to see just how long of an exposure will be required to get that number. I don't autoguide so it may not be possible.

[turfpit]

Mark

I don't autoguide so it may not be possible.

Don't be in a hurry to go down that route - look at the basics first.

I have a Celestron AVX mount, 2nd hand, cost me £500 (£800 new). I use it without guiding. Good reasonable polar align via polarscope, careful 4 + 2 mount alignment, then All Star Polar Align (ASPA) to finish. Focus with bahtinov mask viewtopic.php?f=18&t=1151.

Here are some unguided images.

600s with Ha filter viewtopic.php?f=16&t=1217

180s exposures viewtopic.php?f=16&t=1204

120s exposures viewtopic.php?f=16&t=1199

Good mount setup and work on the histogram will go a long way.

M31-capture.JPG (70.84 KiB) Viewed 704 times

Image drift for 20 x 180s

1-hour-capture.JPG (68.24 KiB) Viewed 704 times

Dave

[BlackWikkett]

@ poolemarkw

Looking at your graph for gain vs brightness it seems your curve increases brightness level as you increase the gain. If you follow this plot at some point will you not start clipping the white point?

I use ASI294MC Pro and ASI183MC Pro. The way I use the brightness (offset / black level) is to increase it as gain goes down. This has the effect of moving the histogram off the left edge aka not clip black point. I've had good results with this method.

-Wikkett

[oopfan]

In my experience saturation of the brightest stars in the field is something that is almost impossible to avoid. 9th magnitude stars are at the saturation point at my lowest gain setting and 50s exposure.

With regards to reducing offset while increasing gain, it all depends what your starting point is. I think that you may be running with a needlessly high offset at low gain. If that is the case then your perception is correct.

To avoid confusion I use the method that Dave explained: take bias frames at each gain setting. Adjust the offset until you achieve a symmetrical energy distribution without clipping the left hand side of the histogram. I don't get too upset if the minimum pixel value is zero. This can be explained by the existence of one or more cold pixels. I use the minimum offset required. Dave advocates for increasing it more but I am not convinced that there is any benefit.

Brian

[poolemarkw]

Dave and Brian,

Many Thanks for your help in explaining this clearly. I'll be out Friday and Saturday evenings to do some tests with this new information!

Mark

[turfpit]

Folks

If you follow this plot at some point will you not start clipping the white point?

Yes, therefore intelligent use of the histogram is required in combination with the settings. With an Altair 183C capturing at 5000x3000 I usually push it and end up with a small number of pixels at or around 100%. Acceptable (to me) on a 20 mega-pixel frame. In line with Brian's suggestion, I have been easing up on the offset but maintain a centrally balanced bias frame to establish an offset value for a given gain.

For me it has been a year long journey to relate histogram to finished image. One thing I have become aware of is that by capturing what I believe is better data, the amount of processing required has significantly reduced - sometimes down to just a single 2-point 'S' shaped stretch in curves.

Using the Atik 314E CCD camera I recently acquired has been a revelation - only one setting, exposure, so that really does focus in on the histogram.

I got a lot of value (and improvement) from reading Craig Stark (Nebulosity) articles on offset, bias and histograms. There is some really useful info in the Nebulosity manual (comes as part of the download).

Dave