Question about over-correction with flats

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.

Steve

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.

Steve

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  !

So what seems to work for me generally, is taking darks, dark flats, and flats, processing using DeepSkyStacker.  No bias frames.  This seems to take care of both the vignetting and overcorrection of the vignetting.

Nothing from ZWO.

Steve

Hi Bogdan,
This was an issue with narrow band. I was using 8 second flats and going for 18 - 22k  adu values (out of 65,550) or about 1/3 of brightness range.  I tried using 8 sec darks to subtract out the camera noise. I'm not really sure that helped very much. I think that what worked was in Pixinsight I UNchecked "Optimize" under the master dark settings in the Calibration Process. This seems to work now every time as long as the flats are between say 15k and 35k. I don't know if this is something most people have already figured out or if I just got lucky playing around.
I also have used a brighter light to get 20k values faster at 3-4 sec with narrow band.

On a lighter note, I was surprised to find that I had some dust on my sensor widow that I could only barely see. I had thought that the dust was on my reducer and wanted to wait until I finished my current project before taking things down to clean. I tried a couple of blasts from an air bulb to blow off the dust. I could not believe how clean my flats were last night !! I felt really lame.

Live, learn, and be well !
Bob

🖖

Steve, that sounds good. Please let me know if it works for you. I got an EL panel from Amazon for $40 and it has worked really well.
i do rotate the scope to zenith to do flats. My wife got me some of those round cross stitch frames to hold my tshirt fabric in place so I don’t have to fumble around with rubber bands anymore. Works great! I also use a square piece of semitransparent plastic which also helps to diffuse the light. A thin sheet of neutral density film also helps if necessary.

CS
Bob

-<