Artifacts in HX516 Images

I would like to illustrate a problem that I have found flat fielding images from my Starlight HX516 camera. It appears that the flat field did not compensate for unevenness in chip response. I took great care in the flats. I took 50 exposures of the flat field and 75 exposures of flatdarks. I set up an even gray cardboard on a wall next to the scope. I pointed the scope straight at the board. I believe I had the board very evenly illuminated. Of course it must have been totally out of focus. I used exactly the same scope setup even the same focusser position for the flats. Still the calibrated image showed artifacts.

The magnitude is about 1.6% in count between dark areas and bright spots. So its not a lot, but still ruins the image if you want to get the best out of it. The calibration would surely work on shorter exposures or images with less background. You have to remember that the signal of the galaxy is just a few counts above the background level, and faint detail will only become visible if the background level is exactly subtracted.

I suspect a temperature effect, as flats and raw images were not taken at exactly the same ambient temperature. The Starlight camera does not have a regulated cooling and so chip temperature is just a function of ambient temp. So would this imply that unless I take care only to image at the same ambient temperature at which I took the flats my images will always have artifacts? Should I look for another camera with active temperature control?

Does anybody have similar experiences with a Starlight HX516?

Any help or comments are appreciated.

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Below is a collection of the images if you want to take a look yourself.

Average of 35 dark frames at 4min each (taken at 20C)(FITS)
Average of 51 flat frames at 5sec each (taken at 18C)(FITS)
Average of 78 flat-dark frames at 5sec each (taken at 17C)(FITS)
Calibrated flat field (flat-darkflat) (FITS)
Gzip file of 22 image files at 4min each (taken at 22C)(GZIP)

To access the above files right click on link and select 'Save Link As'.

This is a jpeg of the master dark frame:

This is a jpeg of the averaged flat

This is a jpeg of the averaged flat-dark

This is a jpeg of the dark subtracted flat

This is a image that was created using the following sequence:
1. Calibrate individual raw images. (cal = (raw-dark)/(flat-darkflat)
2. align and average calibrated raw images