My new setup just came in, and im kinda new with the zwo cameras. I have a ASI585MC Air and i was wondering how to setup the gain. I currently have an Askar 71f (without the reducer) as telescope and i use an Optolong L-enhance filter for nebulae shooting. So for my understanding the gain of the camera for faint objects should be at 252 to kick in the HCG (like the Jellifish Nebulae if its correct), is that correct? For bright objects should i keep it at 100-150 range (like orion or general galaxies )? And how much would the exposure be for both cases ? Thanks to anyone that can clarify 😄
ASI 585MC Air gain and exposure clarification
Engaging
You should use HCG mode, for the MC AIR it’s a gain of 200 not 252, check the performance charts. I would stick with that unless you have a problem with stars clipping. That can be easily controlled by not making your subs too long, 15 to 60 sec. usually works well.
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Michele Boseggia · Jan 22, 2026, 10:35 PM
i was wondering how to setup the gain
Hello… a few words to complement Tony’ post which provides excellent info.
With recent Sony (at least) sensors, the HCG mode (a king of magic) which boost the ratio of signal-to-(read) noise.
There is thus a sweet spot for the ideal gain which is the smaller value above HCG threshold.
Caveat : even if, at sensor level, HCG occur physically at a given H/W gain, camera manufacturers do not necessarily render this value with the same scale factor: the “magical” number may vary between one manufacturer and another, always refer to the camera manual.
For the Zwo dialect, the value is 150 (TBC) . Bye superstition (to be sure to be above the threshold) I add one (151) but it might be ridiculous… For that value you keep 12 stops range in light dynamics which is very good.
IMHO, for deep sky photo this is the one and only value of gain to use . Some wizards are using very low gain for spectroscopy (maybe to keep the maximum dynamics)
To tackle with the brightness of the target and sky conditions, you will play with exposure duration / number of exposures . The rule of thumb (you will have to dig into that) is to adjust single sub time to the local brightness of the sky background. The rule is to have a given ratio of background signal to sensor read noise (https://astroimagery.com/techniques/imaging/astrophotography-calculator-for-long-exposures/). This is a bit tough for a beginner, you have to dig into it. Some capture software like sharpcap offers a built-in tool for that.
Another simpler way around is to experiment. Try by non linear steps ( 1 5 10 30 60 120 240 s) . Examine the histogram to avoid excessive growth of background noise and star saturation.
Last idea : on a complex object like a nebula with surrounding bright stars (the academic case is Alnitak in Orion Horse Nebula) , if you have a single value for the exposure time, you will have weak nebula or burned stars. The trick is to capture the same image in two or more sequences, varying the exposure time : for instance 15s for stars and 12O or even 240s for nebulae… The two sets of stacked image may be recombined in a single frame through HDR techniques either :
with luminosity masks to keep in a given image what zone you want to keep or reject (needs some training)
with expert-grade software like PI in which these functions are now offered as standard. It costs a kidney, it’s not very simple in the beginning, but the results are breathtaking. Remember that adequate processing is 40% of the final results.
Clear skies
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