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Old 21-08-2017, 04:40 PM
tim.anderson (Tim Anderson)
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ASI1600MM-C Hot Pixels

I have been doing some imaging with an ASI1600MM-C camera and have noticed that there are hot pixels appearing in the light frames that do not appear in my master dark, even when I construct a master dark less than an hour before starting an imaging run (the master dark is constructed from >30 dark frames run at the same exposure and temperature as I use in the imaging run).

That is to say, the master dark removes a large proportion of hot pixels from each light frame, but not all of them. As a result, the additional hot pixels then show up as coloured smears throughout the stacked image (see attached pic).

Can anyone tell me if this is normal behaviour behaviour for this camera or do I have a dud? If it is normal behaviour, how do I get rid of the artefacts?

Any help appreciated.
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Old 21-08-2017, 05:13 PM
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Shiraz (Ray)
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Hi Tim. I think that maybe it is a design issue that carries over from the sensor's days in a terrestrial camera.

From what I can tell, looks like the camera erases hot pixels when they get above about 8000ADU. There are occasional pixels that are near the threshold and can disappear on either the dark or the light - but not necessarily both. Then you get a few pixels that are not corrected by the dark cal.

I find that dither plus minmax rejection when stacking cleans them up OK. Bad pixel rejection also works, but only with darks that are taken at a shorter exposure than the lights - just to make sure that no hot pixels have been "disappeared" in the darks, but not the lights.

Sam from ZWO confirmed that the camera/software implements a bad pixel rejection process and offered to check with his software person to see if there was a solution to the odd hot pixel that goes missing in action - didn't hear any more, so guess not. In any event, it is not a deal breaker. cheers Ray

Last edited by Shiraz; 21-08-2017 at 09:06 PM.
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Old 21-08-2017, 05:30 PM
glend (Glen)
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How are you processing? I have not noticed these in my 1600mm-c images. Are you using DSS? There is a cosmetic cleanup to remove any remaing hot/cold pixels in the final registered image. I must admit i don't examine individual subs too closely, other than grading them in both SGP and DSS, and discarding on that basis. I agree dithering could help, but found i have not had to use it on the 1600, my old Canon yes, jt had to be dithered.
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Old 21-08-2017, 06:21 PM
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Shiraz (Ray)
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just for interest, dug up a composite image that shows what goes on.

the leftmost panel shows a group of warm pixels at gain 100, time 120s.

the next panel shows the same group at 300s - same group, just 2.5x brighter.

the next panel shows the same pixels at 600 seconds - 2x brighter again, but the hottest 5 pixels have simply disappeared.

the final panel at gain 200, shows that all but dimmest 4 of the original group of 16 warm pixels have been "disappeared".

If a hot pixel happens to be near the threshold value, it can flick in and out of existence from frame to frame, depending on read noise. cheers ray
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