Author Topic: OSC camera light/dark problem  (Read 354 times)

Offline MineralMike

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OSC camera light/dark problem
« on: 2018 December 23 11:11:28 »
My ZWO ASI1600MC Pro camera is causing problems with cooled long exposures, and I wonder if I can mitigate it in PI. Please see the attached dark and light frames stretched with STF. A 30-minute exposure with the camera cooled to -20c produces a lighter area around the edges of the frame, extending inward and decreasing in intensity. This is most obvious on the right side where there are two large "blobs," but close inspection shows all the pixels near the edges are brighter.

The "blobs" on the right are up to 17% brighter than the center area, and the bottom, left, and top are up to 6.5% brighter.

I have two questions:

1. Since these bright areas are present in the light frames and the dark frames, why aren't they removed from the light frame when I subtract a dark with the same exposure time?

2. Can I consider these bright regions to be sensor "defects" and create a defect map to remove them? Any suggestions on how to do this with such a broad span of pixels (not a simple column defect), and such small deviation from the dark region?

Thanks for any ideas. I'm still learning PI, so need all the help I can get.

--- Mike

Offline pfile

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Re: OSC camera light/dark problem
« Reply #1 on: 2018 December 23 11:24:22 »
i guess this is 'amp glow' which is extra dark signal caused by the heat given off by the sense amplifiers and other circuitry around the photoarray itself. it's a common problem with CMOS imagers.

it should be removed by dark subtraction, provided you don't try to optimize the dark... so make sure that is unticked.

rob

Offline MineralMike

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Re: OSC camera light/dark problem
« Reply #2 on: 2018 December 24 08:15:53 »
it should be removed by dark subtraction, provided you don't try to optimize the dark... so make sure that is unticked.

Thanks, Rob. Yes, I think I had Optimize checked. When I reprocessed, I was sure to leave it unchecked, and the perimeter glow was not present in the calibrated subs. This was true even if I used a 20- or 3o-minute dark frame applied to 15-minute subs. Ultimately I exposed some 15-minute darks and made a master from them, then used that on my 15-minute lights.

All is well now; I'll be sure to use exact-match darks for this OSC camera, and not scale them.

BTW, ZWO responded to my question to them, saying that long exposures are not needed with this camera, and three 10-minute exposures are better than a single 30-minute exposure. I'll be trying 15-minute NB subs tonight with the (barely) waning Moon.

--- Mike

Offline pfile

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Re: OSC camera light/dark problem
« Reply #3 on: 2018 December 24 12:49:04 »
yes i have heard that people take some pretty short exposures with these CMOS cameras - like 30s-2m - and adjust the gain to facilitate this.

anyway glad to hear it is working for you.

rob