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I note that there is a wide range of gain settings used in image aquisition on Astrobin. I am wondering what the rationale is behind choosing a gain setting to use. I would like to pick one setting and stick with it so that I can have a small reusable calibration library. Any advice would be very helpful. Cheers.  |
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I will preface this comment by noting that I am a novice imager, not an expert... But I use this camera and my experience has been that unity gain (111) works great. I have also used a gain of 53 in an attempt to get greater dynamic range but have not noticed much of a difference. I once tried -0- gain to get the maximum DR and ended up with banding that ruined the results of that imaging session. To expand a little on the use of this camera, I encourage you to look on Cloudy Nights for some great information about using it. There are some very smart and very accomplished imagers that have posted great information there that is invaluable. One specific example I will toss out there is that there are some tables of ideal exposure times for the 183 that take into account sky brightness, gain, and whether one is contemplating broadband or NB imaging. Really great stuff. |
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| Thanks for the input Mike. I have looked at and admired your images. I note that you use gain of 139 in a lot of them and I am currently experimenting with that myself purely on the basis of your results but I will give cloudy a nights a thorough look thru as well. I find that there are so many conflicting pieces of advice from various sources that I wonder if there is any "right" setting at all. |
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Hi I`m using the ZWO183 MC Pro. So far with the limited time ive actually used the camera i`ve stuck to unity gain 111 and offset 10. I`ve stuck to 180 secs using my TS65 Quad and Skytech L Pro filter. HTH Alan |
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Hi Allan, Thank you for the nice comments! I use the 139 gain with my ASI1600 which is unity gain for that camera. So much of this is related to the sky conditions but to get a place to start, Alan Hancox’ comment is right on... IMHO of course, lol... I have a library of darks that I’ve compiled with several different temperature settings and a number of different exposure lengths. If I was starting over, I would do that 5he same way. one other thing I would add is that I do not use bias frames with this camera and I have no issues with amp glow.. I hope that helps a little... CS Mike |
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Hi, I think gain is very similar to ISO, just different ways to express how much the signal gets boosted (pre-amplified) before being recorded. ISO is "equivalent fim sensitivity" while gain is basically multiplication in decibels. Ideally you want your gain to be high enough for fixed pattern noise and other problems of your sensor to become negligible compared to the signal, and low enough to not start burning your highlights or introduce other nonlinearities. I believe the rule of thumb given an exposure time is to use the highest possible gain which a) will not cause clipping to the right of your histogram and b) keep the bulk of the data to the left 1/3rd of the histogram Or something like that. Exposure is the dominant parameter (I am assuming for the simplicity that the subject and conditions are the same, that f/number and aperture is constant so you can only change exposure and gain). Because exposure affects how much signal you are collecting, not how it gets amplified. You generally want to have the longest exposure possible and lower the gain. But if you can do only short exposures (say 15 seconds) because your tracking is less than optimal, because there is light pollution, because whatever, you want to bring the gain high enough to satisfy (a) and (b), otherwise you subs will show very high fixed pattern noise. The situation becomes a little more complicated because for all sensors there is a value for gain above which there are no significant gains in terms of fixed pattern noise elimination while there are detrimental results in terms of colour detail. Otherwise everyone would be shooting at the maximum gain (or the maximum ISO) and we wouldn't need tracking or big telescopes at all  The reason you are seeing conflicting advise is because all factors (subject type, exposure, f/number, aperture, sky background levels, tracking, gain) affect each other and there is no single optimum value for everybody, even with the same sensor/camera. Planetary lucky imaging presents very different constraints from deep sky imaging, light polluted sites are different from dark sites, guided can expose longer than unguided and so on and so forth. There is a very good analysis here: https://clarkvision.com/articles/iso/ it speaks in terms of DSLR cameras and ISO, but it's practically the same thing. Cheers, D. |
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Hi Allan, If you want to get optimal, look up Robin Glover's vid on the topic, there is a lot of excellent information explained. "Deep Sky Astrophotography With CMOS Cameras by Dr Robin Glover" I have the Altair equivalent 183M, and I use Sharpcap for a couple key things, accurate polar alignment, and the sensor analysis, this is absolute magic, really. It measures the LP buildup in a dark patch of sky close to your target and works with your prior measured sensor noise data to give you a recommendation for gain / exposure. You can run it prior each session to understand the best exposure tonite with filter x. you can understand what exposure to run with any gain, and what you loss of dynamic range and noise component will be. The other useful point is it tells you where in the histogram your exposures need to be, if you are shooting @ 12bit. There is a point if passed, overcomes the benifits of shooting 12bit, you might as well shoot with 8 bit if you want to run it like that. really helped with these couple worries/questions I had from early on.... making sure I nail polar aligment easily each run, and then what gain and exp. settings to use. hope its useful Vik |
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| I can't fault what is said in the previous comments but when using Sharpcap brain I often have to shoot some more darks and I end up with some at 57.4 seconds or 45.3 seconds and so on. I personally would choose one setting for narrow band, another setting for RGB and another for luminance which makes having a library of calibration frames less of a chore. However, I am also not an expert and altering my exposure times/gain/offset to suit is the least of my problems in acquiring an image that is pleasing to the eye. |
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| Hi Stephen - there is no need to use the precise values it gives you, its a recommendation that you can work with. I would have 'snapped' to the closest in my dark lib for each - 45s and 60s, rather than get the .4 and .3 in there and then need another set of darks. If it falls between my dark lib times, Ill shoot the longer one or do a few test captures and see how I like it. |
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| USE unity gain forever and ever。 |
