To Gain or not to overGain

Discussion of using SharpCap for Deep Sky Imaging
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turfpit
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Re: To Gain or not to overGain

Post by turfpit » Tue Sep 11, 2018 11:58 pm

Mitch

In discussions with Nick @ Altair, he has been suggesting gain values of 2% - 4% which translates to gain=100 to gain=200 in SharpCap. These figures seemed to work well for my M31 and M52+NGC7635 in the Gallery. The black level=100 was to ensure no black clipping (decided by looking at the Min Image Statistic for a single frame stretched with ArcSinH(x) using FITS Liberator. Until I get the histogram 'right' then I dont start an imaging run.
M31-FITS-image-statistics.PNG
M31-FITS-image-statistics.PNG (196.73 KiB) Viewed 1011 times
I have no experience (yet) of how these settings will stand up for fainter objects but I imagine some adjustment will be needed.

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Re: To Gain or not to overGain

Post by AstroDude » Wed Sep 12, 2018 3:26 am

Sounds Good Dave will do...
going to run ther brain then image ... then use what iwant to use then see how the same target turns out in the wash as a compare.

the BRain says: Gain 400 offset 0 x 98 sec subs for 3 hours .
my take is Gain 600 offset 50 x 240 sec x 6 hours ...

will see.
Mitch

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Re: To Gain or not to overGain

Post by oopfan » Wed Sep 12, 2018 4:09 am

Hi Mitch,

Offset 0 is never a good idea especially with short exposures. Even with long exposures I would dial in some offset.

My experience with gain is this: the lower the gain the higher the dynamic range, the higher the gain the lower the dynamic range. For example with high gain the difference between black and white may only be a couple magnitudes, whereas with low gain the difference may be many magnitudes. Almost without exception I use the lowest gain setting. Using a 50-second exposure I capture 9th magnitude stars just below the saturation point, and the faintest stars come in at least 18th magnitude. If I were to choose a higher gain then that magnitude range would shrink.

Brian

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Re: To Gain or not to overGain

Post by admin » Wed Sep 12, 2018 6:09 am

oopfan wrote:
Wed Sep 12, 2018 4:09 am
Hi Mitch,

Offset 0 is never a good idea especially with short exposures. Even with long exposures I would dial in some offset.

My experience with gain is this: the lower the gain the higher the dynamic range, the higher the gain the lower the dynamic range. For example with high gain the difference between black and white may only be a couple magnitudes, whereas with low gain the difference may be many magnitudes. Almost without exception I use the lowest gain setting. Using a 50-second exposure I capture 9th magnitude stars just below the saturation point, and the faintest stars come in at least 18th magnitude. If I were to choose a higher gain then that magnitude range would shrink.

Brian
What you are not considering here is that if you increase the gain and reduce the exposure (2x gain, halve exposure), you can double the number of subs in any given time. Each sub has lower dynamic range than the longer subs did, but the final stack is likely to have very close to the same dynamic range as the stack of longer subs (for moderate gain increases).

An interesting thread - do remember that what Smart Histogram is saying to you is

"You will not see more faint detail by extending sub-exposure length past this point"

That doesn't mean that you can't run longer subs if you want to and your guiding is up to it. What we are trying to counter is the blind tendency to want to use 5min, 10min, 15min subs because the guys who've been doing this for ever with CCD cameras use long subs...

cheers,

Robin

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Re: To Gain or not to overGain

Post by oopfan » Wed Sep 12, 2018 12:17 pm

Robin wrote:
What we are trying to counter is the blind tendency to want to use 5min, 10min, 15min subs because the guys who've been doing this for ever with CCD cameras use long subs...
To be fair to the CCD guys, they take long subs because the full-well depth of their cameras are much deeper than CMOS cameras. I came across an old Starlight Xpress MX716-C on the internet yesterday for 155 pounds with a full-well depth of 70,000 electrons compared to my CMOS camera's FWD of 15,000 electrons. That tells me that they can capture very faint objects along with very bright objects without saturating the bright objects.

I agree with you that there is a sweet spot of gain vs exposure for a given sensor when you are interested in spending the least amount of time at the telescope. My primary goal however is to reduce bloating of 9th magnitude stars in the FOV that I can't crop out. If I need to spend more time at the telescope collecting subs to reduce noise then so be it.

To be fair I should experiment again with doubling my gain from 100 to 200 and halving my exposure from 50 seconds to 25 seconds. I tried it earlier this year and was dismayed by the bloat of nearby 9th magnitude stars. This is what started my journey into ever lower gains.

I know that the term depth-of-field is not correct in the way that I use it but I find that images taken with low gain have a greater depth-of-field, meaning that there is a 3-dimensional quality to it. By comparison, higher gain images seem flat and uninteresting to me.

Brian

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Re: To Gain or not to overGain

Post by turfpit » Wed Sep 12, 2018 1:24 pm

This thread gets more interesting by the hour.

When I took up Astro-imaging as a hobby a couple of years ago, I aspired (and still do) to create the images like those at http://www.orionoptics.co.uk/imagegallery.html.

I will leave it for anyone interested to look up the specs of the cameras used. ;)

Dave

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Re: To Gain or not to overGain

Post by admin » Wed Sep 12, 2018 6:47 pm

oopfan wrote:
Wed Sep 12, 2018 12:17 pm

To be fair to the CCD guys, they take long subs because the full-well depth of their cameras are much deeper than CMOS cameras. I came across an old Starlight Xpress MX716-C on the internet yesterday for 155 pounds with a full-well depth of 70,000 electrons compared to my CMOS camera's FWD of 15,000 electrons. That tells me that they can capture very faint objects along with very bright objects without saturating the bright objects.
Actually the thing forcing them to take long subs isn't just the opportunity of the high well depth but the curse of the high read noise. Typical CCD read noise is 7-8e and the optimal exposure time is proportional to the square of the read noise. With CMOS a sweet spot can often be found with a read noise of ~2e, which means optimal exposure times are 16 times shorter (from a noise point of view).

cheers,

Robin

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Re: To Gain or not to overGain

Post by mAnKiNd » Wed Sep 12, 2018 8:18 pm

Very useful points!

I'd also like to add that with high-gain, short exposures apart from the aforementioned, you also increase the probability of tighter stars in your final stack.That and also less trails and other anomalies as they will most probably affect only one out of your many short exposures, rather than ruining your single exposure.

Cheers
Minos

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Re: To Gain or not to overGain

Post by markmac99 » Wed Sep 12, 2018 10:56 pm

Just to update on this - I unticked the setting in Sharpcap and now DSS finds dozens to hundreds of stars.
Yay :)
Mark M
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Re: To Gain or not to overGain

Post by oopfan » Thu Sep 13, 2018 12:30 am

Great news!

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