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Question about over-correction with flats

24 Mar, 2020 22:01
Hi, I use an ASI 1600 MM cool Pro CMOS camera.  On a 16-bit scale (65550 total adu output) I try to keep my Flat adu to about 1/3 of total, or about 20,00 - 24,000 adu.  I understand that this will keep the flats in the linear range of the camera's response.  Sometimes however,  my flats will leave white-ish motes instead of the dark motes that I would have if no flats were used.  I usually use 30 single flats to integrate into a flat master in PI.
This issue has happened twice now with my Ha filter.

1. Has anyone seen this before ?, and
2. Is the fix to lower the adu read out for the individual flats to say 15,000 adu.

Thanks for your comments.
30 Mar, 2020 14:39
Hi Bob,

I recently got the same camera and have been having a difficult time with the calibration frames.  I am a beginner so take this with the proverbial grain of salt.

Initially I was having the same problem of the flats not working.  There was vignetting and "amp glow" and correcting with flats left one side too dark and the other (amp glow) too bright, best case.  Also these areas fell within the same adu as parts of the galaxy image so taking them out  in a stretch also wiped out parts of the image.

Scouting the forums: There are some discussions on CN and elsewhere that discuss nonlinearities in the sensor and electronics of this camera.  The old rules for calibration frames from CCD cameras need to be modified a bit.  The recommendations were:

Flats - keep exposures over .2 sec,  .5 optimal.  My filters are new and little if any dust so I used the Ha filter to get exposures this long (tshirt method, daytime, .5 sec).  I used the peak at 60% on the SharpCap histogram.   (about 39,000adu).  I took other flats as well but this worked.

Bias - short exposures don't work correctily.  Use 1 second.

In the one image I have taken and  processed since following this advice, I have gotten very a good result using flats, dark flats, and bias frames.  The vignetting and amp glow are gone, and the image is intact and full range.  This is monochrome, I've only processed the L image (last night).

Unfortunately, the dark frames do not work.  I'm running the camera at -20C so this isn't as bad as uncooled.  I'm not seeing hot pixels.  But the histogram for the darks is bizarre - the low end peak is at a higher adu than the light frames.  So using the flats (I'm using DSS) ruins the image by taking out much of the midrange.   It is as though something is turning up the gain or offset on the dark exposure, but this isn't shown in the camera setting logged by SharpCap.

I'm a newbie so there may be something else I'm doing wrong.  But such are my results to date.  I'm going to email ZWO and ask for guidance on calibration frames.  I'll also retake the darks.  I took them during daytime with the telescope (C11) capped and black plastic bags covering the scope and image train.  Maybe some light leaked in, but I don't think so.  That would be non-uniform and I don't see that on a stretched dark frame.

This camera is really popular so you would think there would be some definitive info out there.

30 Mar, 2020 17:49
I also have the ASI1600MM Pro and also had a problem with overcorrection of flat-frames. It could resolve the problem by using calibration frames for my flats (flat darks and/or bias frames). If you take dark frames matching the temperature, gain and exposure length of you flat frames for each filter and correct you flats with a master dark flat before making the master flat, the overcorrection should not appear anymore, at least that worked for me.

30 Mar, 2020 18:15
Correction to 3rd paragraph from the end of my post:
"So using the flats (I'm using DSS)"  should have been "So using the darks…"

Ben, in DSS there isn't specific control over use of the calibration files (or if there is, I don't know how to do it)  but using dark flats and bias frames with the longer exposures noted did work and the vignetting and amp glow are gone.

But…the darks still ruin the image if I use them.

30 Mar, 2020 19:57
Thanks so much for the replies, I'll go back and give things another try !
Stay well,

31 Mar, 2020 01:19
It worked !!  I got rid of my over-correction ( bright donuts and corners) using a flat dark master to calibrate the flats AND I unchecked Optimize in the master dark section when calibrating the lights.

Thanks again !

Bob, aka, Happy Camper  !
02 Apr, 2020 19:55
Well, on my next capture, LRGB, the same process worked on L and G, but there was still overcorrection on the R and B.  Unfortunately I don't have a flat lower  than the 60% I used so I'll have to make a flat library for experimentation.

I asked ZWO for guidance and suggested they produce a guidance doc.  We'll see.  If we could really understand what was going on we could deal with it on a more rational basis.

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