Comparison of sequentially taken flat frames with HX516

Flat images were taken on a cloudy night so some developments could be due to changes in the cloud structure. However clouds were pretty uniform. Exposure 30sec.

Here I subtracted a dark frame from the images. The dark frame (30sec.) is the average of 50 exposures.

Then I divided the calibrated images. If the images were of uneven brightness but actual brightness values were a linear function (i.e. all pixel in img_a are 10% brighter that same pixel in img_b) then the result should be an even gray.

The divided image was histogram gauss equalized and is displayed on the left.
On the right is the image after gauss filtering with sigma=4 and the histogram gauss equalization.
 
 

raw = divided calibrated images histogram gauss equalized
filtered = divided calibrated images gauss blur filtered sigma=4 and then histogram gauss equalized

#50 - #40
raw                                                                                Gauss filtered sigma=4
     

#50 - #30
raw                                                                                Gauss filtered sigma=4
     

#50 - #20
raw                                                                                Gauss filtered sigma=4
     

#50 - #10
raw                                                                                Gauss filtered sigma=4
     

#50 - #01
raw                                                                                Gauss filtered sigma=4
     

Two immediately neighboring images
#50 - #49
raw                                                                                Gauss filtered sigma=4
     

With more time in between exposures more structures become visible.
 

Statistics:

 

From the std-dev of images 1-20 vs 50 it appears again that the camera was not fully in equilibrium. Perhaps still cooling down? These were the first images taken. Images 30 to 50 seem to be pretty homogenous.