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CMOS Camera Gain Setting - ZWO ASI 1600


woodblock

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Another question about this camera. The software gives me a number of options for setting the gain and I wondered what other people set the gain to.  I understand that low gain is for greater dynamic range and high gain gives lower noise but how do you decide which value to use?

 

There is a manual for the camera and some graphs which plot various parameters against gain in dB but the software allows you to set the gain manually between 0 and 100. Is it possible to relate the dB value with the gain (0-100)?

 

Cheers

Steve

 

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Steve I think on that particular camera you are best sticking with unity gain which I believe is 139 but I sold mine a few years back so can't remember the exact setting.

On all my current cameras I stick to the unity settings which on the ASI183MM and ASI183MC is (the rather odd) 111 and on the ASI294MM is 120. I set the offset to 8 on all three cameras. I have never had any issues at all with these settings.

 

The ASI294MM is unusual in that when set at 120 the magical HCG high gain mode is turned on, the readout noise will be largely decreased (as low as 1.2e), and the dynamic range is basically unchanged (binning 2x2 only for that though).This is a completely different sensor to that used in the ASI294MC OSC - not sure why ZWO called the mono version of the ASI294 what it is as the sensor is the IMX492 so it probably should have been named the ASI492MM - but I digress.

 

So to make it simple stick with gain 139, offset 8 and temperature -20C. I'm not sure what your mount is and whether you are guiding so uncertain on what exposure length you are comfortable with. However for arguments sake if you can do 3 minute exposures without any problem then you can do a library of darks of that same exposure. Try to get as many as you can with a minimum of 20 preferably more. You can keep them for 6 - 12 months and refresh them in the future. The reason for this is as the sensor ages it can develop hot/cold pixels etc.

Now you have a cooled camera there is no need to shoot darks immediately after a session.

 

Flats don't need to be cooled nor do bias frames. Just make sure you match the gain and offset that you shot the lights with and you should be good to go.

 

I won't get into the debate about no bias/bias, dark flats/darks etc as that has been spoken about on many forums.

 

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There are many recommended combinations on the web. Some of it backed by good science. It also depends on your imaging conditions/preferences/style too. 

 

Its usually recommended to use Gain 139 for LRGB and Gain 200 for SHO. That gets you *good * results relatively quickly.

 

TBH after playing around with several combinations I have reverted to those values. Most of my recent images in this forum are with those settings - they are probably more limited by my editing skills than the data. (If I'm imaging M45 I would probably use Gain 0 for LRGB since the signal is so strong for the stars.)

 

I like to standardise on 30 second exposures for LRGB as it rarely clips stars. I'm currently using 120 seconds for SHO. Now my guiding seems to be under control I may use 300 seconds if the maths makes sense (the read-out noise of CMOS is so  low that you don't need to do long exposures to compensate for readout noise like you do for CCDs). The major benefit of longer CMOS exposures is reduced processing time after capture.

 

 

 

 

Edited by paul
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@TerryMcK The difference in noise is quite small and I have standardised on -15 degrees as the 1600 can get to that even on warm nights. But  on reflection those are rare in the UK when its dark. -20 would be better. (Time to make a new set of Master darks!).

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Thanks,

Why do they call it unity gain if you have to set it to 139? ie Not 1.

 

I had assumed that the gain was the equivalent to the ISO setting on my DSLR. Is that correct?

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1 hour ago, woodblock said:

Thanks,

Why do they call it unity gain if you have to set it to 139? ie Not 1.

 

I had assumed that the gain was the equivalent to the ISO setting on my DSLR. Is that correct?

Unity gain is where one unit measured == 1 photon. The values used (in this case 139) depends on the driver and circuits used by the manufacturer. (Cameras from different manufactures but using the same sensor may have a different scheme). 

 

So in most cases unity gain gives you the best compromise. Higher gains give more ADU units per photon, gains less than unity means you have to collect more photons to raise the signal by 1 ADU. Depending on your source of noise either approach can be useful.

Edited by paul
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I should make a small pedantic correction: 1 electron == 1 ADU at unity gain. As photons hit the sensor they "get converted" to electrons. With a Quantum efficiency of 60% then 60% of the photons get converted to electrons.

 

To save your eyes and my sanity I just deleted my a long BLAH BLAH noise on noise.😄 I'm not a  CMOS sensor designer, there are very few engineers who are - its a bit niche. If you are interested there is way too much opinion as fact on other forums. 

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