Sensor Analysis on QHY165C

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Re: Sensor Analysis on QHY165C

Post by admin » Fri Nov 02, 2018 12:17 pm

Hi,

I think I came to the conclusion that for this camera the offset is applied before the gain, so that the same offset adjustment has a bigger effect if the gain is bigger. The sensor analysis code is not really designed with that set up in mind, since in pretty much all the other models of camera the offset added is independent of the gain.

This means that SharpCap picks an offset that it thinks is sensible for doing some of the measurements, but that offset becomes too large as the gain is increased pushing the whole image into overexposure. I don't have a solution for this problem yet - it seems to be restricted to this model of camera and QHY is still trying to work out a better way to do the gain on this camera, so I am reluctant to invest a lot of time into a solution that might not apply if they change their approach.

sorry,

Robin

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Re: Sensor Analysis on QHY165C

Post by bwells » Fri Nov 02, 2018 2:49 pm

Robin:

Thank you for your help and for trying to make SharpCap Sensor Analysis work for me! I will continue to use SharpCap for all of my imaging needs and maybe eventually, we can revisit this as well.

Do you have any recommendations for me when it comes to selecting the best gain and offset for my imaging?

What does your insight into how offset and gain work tell you I need to do when I select offset and gain?

Thanks
Bryan

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Re: Sensor Analysis on QHY165C

Post by bwells » Fri Mar 15, 2019 1:34 am

Hi Robin

I am going to see if the new QHY Driver is any help, but I wrote QHY Support to ask about setting Gain and Offset for the QHY165C and SharpCap after you said you the offset is applied before the gain and you thought they might "fix" this problem in a future driver. They wrote back a very short message which I pasted below:

Hi, It is CMOS characteristics, the gain of this chip is digital gain, so offset is prior to offset. Analog gain has to be adjusted by R B G. Because the analog gain of the chip is coloured, this is different from most CMOS. Best Regards

So anyway, I doubt this is helpful as it seems to confirm your point, but do you see any way forward for me to use SharpCap to get a gain and offset I can rely on?

More generally, is there any way you can think of overall for me to figure out a good gain and offset for my particular sky conditions? I have tried using the iterative approach of setting different gains and offsets by Todd Benko & Brant Felix, titled "How to Scientifically Determine CCD Gain and Offset Settings" but I cannot do what they suggest for the same above reason. I am coming from a DSLR and I cannot believe something as basic as gain and offset is so nebulous <excuse the pun>!

Bryan

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Re: Sensor Analysis on QHY165C

Post by turfpit » Fri Mar 15, 2019 2:44 pm

Bryan

To establish offset for a given gain, I take a bias frame staring with the lowest offset - then look at the histogram in FITS Liberator. If the histogram is not central then Increase offset, take another bias frame and repeat until I get a centralised histogram. Unity gain is a good place to start. Once you have the offsets for the different gains you use then job done.
M42-bias-frame-used-to-set-black-level.JPG
M42-bias-frame-used-to-set-black-level.JPG (104.52 KiB) Viewed 315 times
Have a read at this https://www.qhyccd.com/uploadfile/2018/ ... 334403.pdf.
coming from a DSLR and I cannot believe something as basic as gain and offset is so nebulous
Agree with you there.

CMOS - user has to deal with exposure, gain, offset
DSLR - user has to deal with exposure & gain (ISO)
CCD - user has to deal with exposure

Interestingly the bias frames from my Canon DSLR and Atik CCD look just like the histogram above.

Dave

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Re: Sensor Analysis on QHY165C

Post by bwells » Sun Mar 17, 2019 1:32 am

Excellent. Thank you. I will give this a go and certainly end up better off than I am now. I tried using the BasicCCDParameters script in PixInsight, but I was never sure if it was giving me the data I needed.

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Re: Sensor Analysis on QHY165C

Post by turfpit » Sun Mar 17, 2019 7:24 am

Bryan

Please feed back your experiences, they may help others with this frequently asked topic.

Dave

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