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Over correcting flats with fix

Equipment Calibration
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Dan Brown avatar
Dan Brown avatar
Howdy all. I've been rather bummed with the quality of my images for the last 9 months. I have data collected for two projects that I never finished due to the poor quality. Recently I discovered that my flats were over correcting my LRGB images and generally doing a terrible job, here is a Lum example, (one hour, bortle 4, luminance),

My procedure for capturing flats hadn't changed but I did swap my filters late last year. Considering that, my first thought was that I had a light leak. I did some tests and determined that was not the issue. I did find it curious that the narrow band filters corrected fine.
I had another light panel so I gave it a go. Strangely enough the RGB and NB filters corrected fine with this panel. After that test I remembered that I had also purchased a new light panel around the time I installed the new filters which finally made me understand what was going on. The new panel was much brighter than my previous ones and my flat exposure times where too short. After adding some sheets of paper to the new panel I was able to calibrate the above image to the following, 

My narrowband images were calibrating fine with the panel since even though it was brighter, the exposures were long enough. 
My camera is a QHY600. The required exposure time is greater than 1 second for proper calibration. Does anyone know why this is? My only thought is that the camera is applying a gamma curve to the shorter exposures, essentially boosting contrast.
Thanks,
Dan
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Dave B avatar
Dave B avatar

I don’t know if this link will help or not, but others seem to be having issues with short flags on some cameras. But I would think that the QHY600 wouldn’t have that issue.

I have the QHY268M, which is the smaller cousin of your QHY600 (assuming yours is monochrome). My Lum flats are 0.49 seconds, and I don’t have an issue with them. But I had to put a number of layers of dimming material in front of my panel in order to get the flats to get to the midway point on the histogram. Assuming your flats were in the midpoint of the histogram, I don’t know why you were seeing your issue.

Sorry I can’t be of more help.

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Mikołaj Wadowski avatar
Mikołaj Wadowski avatar

The most common reason for overcorrecting flats is not subtracting the offset. Did you calibrate flats and lights with a master bias, or the corresponding master darks? Are you changing the offset value for different filters, either for flats or for lights?

I’ve heard that in some cameras, the readout mode can change on its own at very short exposure lengths, which could throw calibration off. So even if you are seemingly calibrating your flats properly, you’d essentially need a separate set of bias frames for the LRGB flats.

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Dan Brown avatar
Dan Brown avatar
The most common reason for overcorrecting flats is not subtracting the offset. Did you calibrate flats and lights with a master bias, or the corresponding master darks? Are you changing the offset value for different filters, either for flats or for lights?

I’ve heard that in some cameras, the readout mode can change on its own at very short exposure lengths, which could throw calibration off. So even if you are seemingly calibrating your flats properly, you’d essentially need a separate set of bias frames for the LRGB flats.

Flats were calibrated with dark flats. I use the same offset for all filters. Your thought about the camera changing the readout mode should be easy enough to test, I'll give it a try this weekend.
Thanks,
Dan
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Vandewattyne avatar
Vandewattyne avatar

Try to use your flatdark as bias and see if its better.