Correct Exposure

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SteveJP
Posts: 38
Joined: Sat Apr 28, 2018 8:35 am
Location: Melbourne, Australia

Correct Exposure

#1

Post by SteveJP »

Hi Robin,
Your tutorial"Picking the correct exposure for Deep Sky" is very interesting and informative.
Essentially as you explain, the correct exposure is primarily dependent on read noise and sky background brightness. I wonder if you could answer a couple of questions?
1-Obviously when one sets the gain high to reduce read noise the dynamic range of the camera is reduced. You don't directly address this issue, but it is what I hear said when the suggestion is made to use higher gains. Many astrophotographers seem to regard unity gain as the ideal setting. At a gain of 300 for example, my ASI071MCPro is reduced from 14 to 10 bits approx. Does it just not matter? For a constant total exposure, does the increased number of subs make up for the reduced dynamic range and not utilizing the full well of the chip?
2-I'm wondering if there is a known relationship between SQM and the photons/pixel term in your equation - given a exposure time, gain and quantum efficiency?
Thanks
Steve
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Re: Correct Exposure

#2

Post by admin »

Hi Steve

Yes, you're right that the reduced individual sub exposure time allows a larger number of subs in the same total observing time and this tends to offset most of the loss of dynamic range caused by moving to higher gains (the exact amount depends on how quickly the read noise drops off as you increase the gain – under some circumstances the effects can be enough to increase the total dynamic range of the final stacked particularly if your camera is one of those that shows a cliff drop in read noise at a particular gain). This is all covered in parts 6,7 and 8 of this post –viewtopic.php?f=35&t=456

I also put together a calculator that can be used to convert between sky brightness and electrons per pixel per second – you can find it here https://tools.sharpcap.co.uk

Hope this helps, Robin
SteveJP
Posts: 38
Joined: Sat Apr 28, 2018 8:35 am
Location: Melbourne, Australia

Re: Correct Exposure

#3

Post by SteveJP »

Brilliant - Thanks Robin
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