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Old 12-11-2020, 09:14 PM
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multiweb (Marc)
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Gain/offset with CMOS cameras

I'm wondering if the procedure to set the gain and offset for CMOS is the same as CCDs. With both my cooled CCDs I would typically set the gain to 1 and find an offset value that would give me a bias frame with an ADU ~1000. Do you do the same for CMOS?
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Old 12-11-2020, 10:32 PM
glend (Glen)
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ZWO publish graphs that are very useful for picking Gain and offset settings. Here us an example,
Just scroll down to the graphs.

https://astronomy-imaging-camera.com...asi1600mm-cool

You can see that Gain is marked at about 139 (ADU), and is usually referred to as Unity Gain in those cameras. We used to play around with different Offset figures (like 20), when these cameras first appeared, and I believe ASI cameras now come with a fixed Offset (others may comment further on this). I used to shoot alot at Gain 139, and Gain 76 (which seemed to be a good compromise setting where well depth is better, Dynamic Range slightly better, and noise was not significant enough to be an issue. Now I believe ZWO fix Offset in the driver, but again I could be wrong, as I sold my ASI1600 a couple of years ago.
A IIS member (Shiraz, aka Ray) actually published a series of charts for various ZWO cameras and he worked out an ideal ADU target, which i recall was around 600-800. I suggest you contact him as he was the expert on the figures for a range of ASI camera models.
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Old 12-11-2020, 11:12 PM
sunslayr (David)
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Dr Glover (sharpcap) gives a good breakdown on the theory in his talk on youtube Part 1 Part 2. It basically comes down to lowering the noise to below the background of the sky you're shooting while not losing to much dynamic range. It also covers things like sub length and cooling temperature.
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Old 13-11-2020, 05:39 AM
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I haven't tried it myself but have seen it in threads. Sharpcap has a feature that analyses your camera and gives you a table of various settings and the resulting dynamic range etc. That would be a good place to start.

Greg.
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Old 13-11-2020, 07:42 AM
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Merlin66 (Ken)
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I use FireCapture (for solar imaging). FC has a default offset of 10.
I find using the Unity gain is a fair compromise.
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Old 13-11-2020, 07:57 AM
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Thanks for the links and info guys, will do some reading.
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Old 13-11-2020, 08:15 AM
Startrek (Martin)
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Quote:
Originally Posted by sunslayr View Post
Dr Glover (sharpcap) gives a good breakdown on the theory in his talk on youtube Part 1 Part 2. It basically comes down to lowering the noise to below the background of the sky you're shooting while not losing to much dynamic range. It also covers things like sub length and cooling temperature.
Iíve watched Robin Glovers YouTube tutorials on Gain and exposure etc.. and enjoyed watching them very much , whilst very informative, interesting and theoretically correct , they donít quite work with the latest batch of QHY and ZWO cameras that entered the market in the last 12 months as camera technology with these new Sony sensors has advanced so far since he presented those tutorials and is advancing every 6 months as well. In fact if you followed his proposed settings rigidly with these new cameras ( eg ZWOASI2600MC )you would not be taking advantage of the excellent performance of this camera with respect dynamic range , read noise etc.. as these new cameras have a QE of 80% +
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Old 13-11-2020, 11:42 AM
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Quote:
Originally Posted by glend View Post
ZWO publish graphs that are very useful for picking Gain and offset settings. Here us an example,
Just scroll down to the graphs.

https://astronomy-imaging-camera.com...asi1600mm-cool

You can see that Gain is marked at about 139 (ADU), and is usually referred to as Unity Gain in those cameras. We used to play around with different Offset figures (like 20), when these cameras first appeared, and I believe ASI cameras now come with a fixed Offset (others may comment further on this). I used to shoot alot at Gain 139, and Gain 76 (which seemed to be a good compromise setting where well depth is better, Dynamic Range slightly better, and noise was not significant enough to be an issue. Now I believe ZWO fix Offset in the driver, but again I could be wrong, as I sold my ASI1600 a couple of years ago.
A IIS member (Shiraz, aka Ray) actually published a series of charts for various ZWO cameras and he worked out an ideal ADU target, which i recall was around 600-800. I suggest you contact him as he was the expert on the figures for a range of ASI camera models.
It is possible to change the offset on ZWO cameras with the ASCOM driver (at least with my 533 it is) and I am fairly sure the same is possible with the native ZWO driver, but I don't use that. The default 'unity gain' settings of 100 gain 70 offset on the 533 deliver a minimum ADU of around 2500(ish) whilst lowering offset to 20 brings the minimum ADU down to around 620. I have not yet established whether this makes any real world difference to imaging.
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Old 13-11-2020, 03:56 PM
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Originally Posted by Craig_ View Post
It is possible to change the offset on ZWO cameras with the ASCOM driver (at least with my 533 it is) and I am fairly sure the same is possible with the native ZWO driver, but I don't use that. The default 'unity gain' settings of 100 gain 70 offset on the 533 deliver a minimum ADU of around 2500(ish) whilst lowering offset to 20 brings the minimum ADU down to around 620. I have not yet established whether this makes any real world difference to imaging.
Correct. There is an offset setting available in the ASCOM driver as well in Neb4 for the 462MC but not the older 120MM so I guess it depends on the camera. Using the 462MC as an ASCOM camera is not a show stopper because I don't need a fast frame rate for deepsky. It was only an issue for planetary. I've also noticed that ASCOM doesn't seem to return a value to Neb4 when the exposure is finished so if you set an interval of X seconds delay in between takes it doesn't read it. I have to experiment a little more but for now I would assume pausing guiding or dithering with PHD doesn't work either.
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Old 13-11-2020, 05:07 PM
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So what I did, seemingly back in the distant depths of time, was to take a bunch of bias frames with different offsets. This showed me that at the default values in the presets weren't to my liking...the histogram was all bunched up at the left hand edge (zero end).

What I was looking to see was the Gaussian distribution of the bias frame to pull its extremity away from the zero end, such that the bias/dark frames would subtract from my light frames correctly.

I did this for several different gain values...notably gain 75, 139 and 200 on my ASI1600. In the end, as much out of laziness/convenience, I settled on the offset value of the highest gain I would practically use, where I felt I could calibrate accurately. This worked out to be (drum roll) an offset of 50. This, coincidentally, is the value ZWO set by default in the ASCOM driver of the ASI1600 Pro and later models.

Using a little magic maths in Shiraz/Ray's thread ( http://www.iceinspace.com.au/forum/s...hlight=asi1600 ) for exposing , I "calculated" target ADUs for different gain values I might use. Due to the relationship of gain/ADU and read noise, the target ADU increases with gain. I'm sure there was a formula in there somewhere but I don't see it right now. It was something like:

Target ADU = bias + (read_noise^2 * 10) / gain

[Note: ensure all your values used are of the same word length, i.e. 12-bit or 16-bit]

I haven't tried it with my ASI462, but I've used that "technique" with every other camera I've used subsequently, and it's my guiding light for achieving sky-limited exposures. Keeping exposure times to a minimum helps maximise the dynamic range, but also compensates for mainstream mount performance

So I think, to answer Marc's original question, the maths is the same as with a CCD, only the numbers are different
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Old 13-11-2020, 05:51 PM
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Originally Posted by multiweb View Post
I've also noticed that ASCOM doesn't seem to return a value to Neb4 when the exposure is finished so if you set an interval of X seconds delay in between takes it doesn't read it. I have to experiment a little more but for now I would assume pausing guiding or dithering with PHD doesn't work either.
I don't use Neb4, but the ASCOM driver when used in APT does tell the software when the exposure is finished - any delay set between subs has worked fine for me. I would have thought the driver would have the same functionality regardless of what software you use, but maybe not.
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Old 19-11-2020, 09:21 AM
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multiweb (Marc)
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Quote:
Originally Posted by Craig_ View Post
I don't use Neb4, but the ASCOM driver when used in APT does tell the software when the exposure is finished - any delay set between subs has worked fine for me. I would have thought the driver would have the same functionality regardless of what software you use, but maybe not.
Thanks for that Craig, I'll experiment some more. I've also posted on the ZWO forums see if I get a bite there.
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