Starlight Xpress increasing data loss through capture session?

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bilbo
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Joined: Tue Aug 13, 2019 12:50 pm

Starlight Xpress increasing data loss through capture session?

Post by bilbo » Tue Aug 13, 2019 4:57 pm

Dear forum members,

I stumbled upon a strange phenomenon and I cannot solve it.
I own a used Starlight Xpress SXV-H9C CCD camera, use it with SharpCap 3.2 (the recent version). I am learning its capabilities, shooting some example targets to learn how to use it.
Not long ago I targeted M27 Dumbbell Nebula, plugged in the camera about half an hour before shooting to let it cool. Then I made 60*1 minute light frames without binning and digital gain set to 10 (the scale was 1 to 100). I wasn't aware of the fact that the mentioned CCD doesn't have adjustable gain. I just was happy that at increasing gain setting I get brighter frames :)
I don't understand how, but the first frame contains a lot of data but then the nebula miracuously 'vanishes' and on the 60th frame there is only a faint trace of it. Nothing changed during the shooting session, those are continuous frames with no pause between them.

Here is the first and last raw, undebayered FITS converted to JPG:
2019-08-11-2256_1-dumbell 10gain60s_00001.jpg
2019-08-11-2256_1-dumbell 10gain60s_00001.jpg (125.99 KiB) Viewed 95 times
2019-08-12-0005_3-dumbell 10gain60s_00060.jpg
2019-08-12-0005_3-dumbell 10gain60s_00060.jpg (26.17 KiB) Viewed 95 times
Unfortunately I cannot read out the sensor temperature but I suppose it hasn't changed a lot since it cooled a lot before and after all, not only the noise changed but everything, the signal too.

I made a video about me scrolling the undebayered raw frames to give an impression what I am trying to talk about, here it is: https://drive.google.com/open?id=1oU-BL ... yd1TQs8ZJI

I copied all the frames and SharpCap settings plus logfile to this public Drive directory:
https://drive.google.com/open?id=1pHKGc ... lE8Fgaqoai

Yes, the frames are of very low quality and the stars are too bright but I was shooting only to experiment (and to check the longest usable exposure time with my new Celestron AVX mount without guiding).

I don't know what happened. There were no clouds, no obstacles, no dew, no UFOs (I suppose :D ).
I find it very strange that the background level is exactly zero, there is not even noise, as if something subtracted a growing number from all the pixel values. I am completely stuck. Maybe a camera, readout or software error? Maybe it's connected to the fact I tampered with the gain setting, not knowing I shouldn't do that?

(This is *not* a stretching issue, I am certain. That 'missing pixels' have exactly 0 value and the frames above are displayed with just the same stretch.)

Side question: if a certain camera doesn't have adjustable gain (like mine), what is the effect of that gain slider? Can it cause such 'clipping error'?

Any little thought or advice will be highly appreciated!

Have a good day and clear skies!

Krisztian from Hungary
Last edited by bilbo on Wed Aug 14, 2019 4:39 pm, edited 1 time in total.

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Re: Increasing data loss / clipping through capture session?

Post by admin » Tue Aug 13, 2019 9:11 pm

Hi,

The only SX camera that I have to experiment with is the lodestar, and I haven't seen any problems like this with it although I'm not sure that I've ever run it for an hour at a time in testing.

It may be worth doing some additional testing during daylight to see if you can reproduce the issue over any long run of imaging without wasting clear sky time on it. You can imagine perhaps putting a piece of dark card with a single small pinhole in it over the camera to effectively create a pinhole camera which would show an image but also need long exposures to allow you to reduce the same sort of set up that you Astro imaging in terms of exposure time etc.

You should be able to avoid using the digital gain and instead use the display histogram scratch control to brighten the faint nebulousity without adjusting the data coming from the camera which would help eliminate one possible cause.

Hopefully someone with experience using this brand of cameras will have some other suggestions.

Cheers, Robin

bilbo
Posts: 2
Joined: Tue Aug 13, 2019 12:50 pm

Re: Increasing data loss / clipping through capture session?

Post by bilbo » Wed Aug 14, 2019 4:38 pm

Dear Robin,

thank for your help! That's a great idea! I will set gain to 0 and trying to do similar sessions daytime. Hope it's not the fault of the camera, it's rather old but I love it...

Clear skies! :)

Krisztian

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