Beware ROI

Discussion of features and issues relating to Altair Astro Cameras
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admin
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Re: Beware ROI

Post by admin » Sat Jul 14, 2018 9:14 pm

Thank you Brian,

the first thing I can see is that it's just an offset issue - see the screenshot below
Capture.JPG
Capture.JPG (270.52 KiB) Viewed 569 times
The left is one of your 1280x1024 images with the higher mean, the right is 1920x1280 with a lower mean, now look at the range (max-min) on each - on the left it's 288ADU, on the right 272ADU. If we had scaled by a factor of 16 then we'd expect these to be very different ranges. Similarly you can see that the standard deviation of both images is ~13.3 and that the histogram at the bottom has levels that are 16ADU apart in both (you need to hover the mouse for this).

So, what we have is a rogue offset of approx 4800 ADU (16 bit ADU) or 300 12 bit ADU. It turns out that for these cameras the black level control value (in MONO 12 at least) corresponds pretty well to the mean image brightness in 12 bit ADU, so with your selected black level of 20 you should get

black level 20 : Frame mean 20 (12 bit ADU), 320 (16 bit ADU)

To get the mean ADU you are seeing in the problem frames would imply a black level value of 320!

I do wonder if somehow the black level control is not being applied correctly under some circumstances, leading to the extra offset. However the thing that counts against that theory is that the csv histogram files you included with your 'problem' frame batch all had peaks at an ADU of about 320 or so (12 bit ADU), which would imply that SharpCap was reading the right values to be able to write that data to the histogram.

All I can say for now is to keep this under observation. If you do see the problem again try to spot if the histogram level in SharpCap itself is also offset more than expected, or is it only an issue in the saved fits files (I think it being an issue in only the fits files is very unlikely, but who knows). Also, if you see the issue try adjusting the black level control a little - if the brightness suddenly corrects itself then it is likely that the black level is somehow being set wrongly by SharpCap or mishandled by the camera.

hope this helps,

Robin

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Re: Beware ROI

Post by oopfan » Sat Jul 14, 2018 11:40 pm

Robin,

Re: "All I can say for now is to keep this under observation. If you do see the problem again try to spot if the histogram level in SharpCap itself is also offset more than expected, or is it only an issue in the saved fits files"

I had a clue that something was awry when I was taking darks and I saw a wide gap on the left-hand-side of the histogram. I erroneously attributed this to an increase in sensor temperature. It sure would have been nice if the manufacturer included temperature readout. I wouldn't have taken this topic down that path. Unfortunately I did not take a screenshot.

I wonder when this rogue bias is being introduced? So far it looks like it kicks in early in a session such that it affects all lights, darks, flats, and bias frames. I'm fearful that it could happen midway through a sequence.

I'll install the latest version of 3.1 and keep an eye on it. I think I read that there was update of the driver SDK in there somewhere.

Thanks,
Brian

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