Advice on Hyperstar Brain Output

Discussion of using SharpCap for Deep Sky Imaging
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caseyjones
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Joined: Mon Sep 20, 2021 4:52 pm

Advice on Hyperstar Brain Output

#1

Post by caseyjones »

Firstly, thank you Robin for Sharpcap and your excellent support of it.

I have a hyperstar under light polluted skys (no filters) and am trying to understand the output from the brain. I have watched the CMOS presentation several times.

Specifically, I have an ASI294MC camera on a 9.25SCT with a Hyperstar under Bortle 6 skys. My 'Sky Brightness is measured at 92 e/pixels/s. Running sharpcap 4.0.8173.0 - 9/14/2021. When I run the brain, requesting 'Max Dynamic Range' it typically tells me that the optimum exposure is a second (or less) at gain=121. This number of frames is tough to work with. If I set a 2 second minimum exposure it lengths the optimal exposure to 4.7s still at gain=120. If I increase the minimum exposure to 5s it says the optimal exposure is now 20.2s at gain=0. I have the screen prints if you want to see them.

We know the ASI204MC drops the read noise at gain=120

Two questions.

1.
The brain recommendation switches to gain=0 at longer exposures because the read noise is such a small contributor relative to the sky brightness at this point?

2.
Is the only thing I lose by taking longer exposures with gain=120 potentially dynamic range? In other words if I confirm I am not over exposing pixels I am ok to go longer? For dim DSOs 30 or 60 second exposures seem a sensible length what I understand these are longer than the minimum in terms of stacking noise given my sky conditions but as long as I am not over exposing pixels what is the downside?

I don't want to ignore the brain output but sub-second exposures are tricky to deal with, just want to check I am not doing something silly by going longer (as long as I am not blowing out pixels).

Thanks!
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Re: Advice on Hyperstar Brain Output

#2

Post by admin »

Hi,

so the simple answer is that you can certainly go for longer exposures than the brain recommends (subject of course to not saturating either the background or the stars too much, and the tracking limits of your mount). One way to do this is to set the minimum exposure in the brain window to say 10 or 15s. One of the plans I have for the future is a 'super' brain that will consider more factors such as background saturation, how many stars are detectable, etc, to give better results than the current brain.

You are quite right that in the context of your sky brightness, the camera read noise is pretty small, so the brain can switch to gain=0 once the exposure gets pushed past a certain level (seemingly about 5s or so) without needing to worry about the higher read noise.

As to question 2, yes, all you miss out on is potential final dynamic range. As you say, if you are not having problems with star saturation then you can basically ignore this. Using a longer exposure than suggested will not adversely affect the detection of the faint stuff :)

cheers,

Robin
caseyjones
Posts: 2
Joined: Mon Sep 20, 2021 4:52 pm

Re: Advice on Hyperstar Brain Output

#3

Post by caseyjones »

Thanks Robin. I'll switch to gain=0 with the longer exposures and see how that goes. My mount isn't the best but I am guiding which helps.
In practice the wind is usually my limiting factor. To track the overexposed areas I use the 'FX: Highlight Over Exposed' on an initial
frame and then the 'Pixel value readout' cursor as the stack develops.

A 'super brain' sounds a nice enhancement as you know this is an area that beginners (myself included) struggle with. It is somewhat
non intuitive so having answers based on the formulas for your specific setup and sky conditions is invaluable.
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