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Old 22-11-2015, 11:20 PM
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John K
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Join Date: Sep 2005
Location: Melbourne
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Determining optimal exposure levels for a new cooled CMOS ASI 174 mm camera

Hi guys,

I have ordered a cooled ZWO ASI 174mm camera which will cool to 45 degrees below ambient in order to upgrade my 120mm for planetary, but also because I wish to do some mono deep sky imaging. So far, have spent several years doing mono planetary imaging and OSC deep sky, but looks like my two worlds are about to collide!

As I was not prepared to spend several thousand dollars on a mid-range mono CCD camera, I am hoping to see what can be achieved with this new style of CMOS Sony chips. My scope is a 12.5 f/5 scope and I will image from the ASV's dark sky site outside of Melbourne with a portable setup generally shooting one object per night over 4-5 hours with LRGB filters.

I will be using Firecapture to control the camera very much like I do when I do planetary imaging. The software will be able to set the camera temperature, and like with planetary, the user can control the Gain and Exposure length which then determines the Frames per Second / Minute.

The question now becomes, with this type of Camera / CMOS Chip, what is the optimal Gain / Exposure level for this camera for Deep Sky Objects vs optimal Noise / Signal and how do I determine that? For planetary it's the histogram normally and the noise level that I use to determine what my exposure / gain settings are in Firecapture.

Most people that I have seen using this camera in the "un-cooled" version are generally going up to about 60 seconds and stacking several hundred/thousand frames. My thinking is that this camera will be good for "Lucky" style imaging for Deep Sky objects, and based on how I capture and processes my Planetary don't have an issue with this, but need a formula of some sort to determine what may be best for this camera to optimise the Signal / Noise Ratio.

I know that most of you will now say well John it all depends!!.... So, I am attaching some info on things such as Peak Noise (e-); Dynamic Range (dB); Gain (e-/ADU) etc etc with the hope that some of the well informed people on IIS can assist. The data sheets don't however show much in terms of the effect on any of these levels vs temperature and although I have seen some calculators on-line I am struggling to make sense of the best approach.

Otherwise, my approach, will simply be to minimise the amount of noise through low gain settings and then maximise the exposure level to no more that 1-3 minutes per captured frame and be guided by the data I see in Firecapture in terms of the live histogram and noise levels as well as the visual appearance of the subs (which is quite an unscientific approach!)

It may well end up that I simply have to do some noise / gain tests myself on various objects and temperatures??

A long thread to probably a simple question for some! but would appreciate thoughts and comments.

Clear skies and thanks in advance.

John K.
Attached Thumbnails
Click for full-size image (Zwo Camera Noise vs Signal.jpg)
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Click for full-size image (ASI174mmSpec.jpg)
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Click for full-size image (asi-174-cooled-kamera-1000.jpg)
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Click for full-size image (ASI174mmSpecB.jpg)
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