Sensor Analysis on QHY165C

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Re: Sensor Analysis on QHY165C

#41

Post by admin »

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

#42

Post by bwells »

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

#43

Post by bwells »

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

#44

Post by turfpit »

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 11311 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

#45

Post by bwells »

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

#46

Post by turfpit »

Bryan

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

Dave
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Re: Sensor Analysis on QHY165C

#47

Post by bwells »

Hey Robin,

I tried the newest SharpCap last night with my Qhy165c. I saw the 3 RGB gain sliders. But I recall you said I should set these RGB sliders to a low value like 1 and adjust the other gain setting to the actual gain I want to use.

First, I did not see any gain adjustment other than the 3 RGB sliders. In the user interface.

Second, in one of the header fields, I saw a gain value in the header that was set to 20, in addition to the RGB gain settings.

Can you please tell me how I need to set the gain for my QHY165C? I did not try the sensor analysis yet, but I thought I would ask if you think the sensor analysis will work now?

Anything else you can think of I should be aware of to use Qhy165c with SharpCap?

Thanks!
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Re: Sensor Analysis on QHY165C

#48

Post by admin »

Hi Bryan,

I think you should see a 'digital gain' control appear alongside the three colour gains for that camera. It's the digital gain that you should set to the minimum value as far as I can work out. It's possible that that's also what is appearing in the fits header (although it's also possible that that could be coming from the green gain slider – I tried to set up the green to be the one that would be used in other parts of the program when it wants a single gain value).

Hope this helps, Robin
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Re: Sensor Analysis on QHY165C

#49

Post by bwells »

Hello Robin

This morning, I ran the latest SharpCap Sensor Analysis this morning on my QHY165 and it ran successfully. So I thought I would post this back and ask you if this looks valid and to ask if you have any new information on using Sensor Analysis using SharpCap with my QHY165C. What are your thoughts?

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Re: Sensor Analysis on QHY165C

#50

Post by admin »

Hi,

as far as I can see, the QHY165 uses the IMX071 sensor - the same as the ASI071MC. Now, I have the ZWO version of that camera, and it's a 14 bit sensor, whereas your data is coming back as a 12 bit sensor.

That's odd... It could be an issue with the QHY SDK, or it could be some other camera settings (white balance, etc) not being at default, which could lead to the bit depth being misread. The fact that you are seeing roughly the right full well depth makes me think it might be an SDK thing, but it's not 100%.

Now, the impact of the bit depth coming through as 12 not 14 means that the e/ADU values are all about 4 times bigger than the ZWO equivalent values, and that in turn means that the read noises come out higher (the read noise can never be much less than half the e/ADU for any gain).

The final result of that is that the camera is a much less attractive proposition in 12 bit mode with a minimum read noise of 5.8 (according to your data) compared to the ZWO version with a read noise of 3.3 or thereabouts.

Actually, having another look at the QHY page for the 165, it actually says a 12 bit ADC, so I suppose the actual performance of the camera matches the documentation. Not sure how ZWO get 14 bit (or how QHY don't) - it is certainly a big difference!

cheers,

Robin
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