What happened to my dark frame

[jm_rim]

After a whole night of (hopefully) successful imaging, I started my early morning by finishing up with calibration frames.
But know, after getting all the files into DSS, I noticed that my dark frames look very strange. The first 4 looks fine (though with a circle in the middle), but the rest keeps getting lighter with a circle in the middle (no change in ISO or shutter speed) - which really annoying since the last 21 dark frames cannot be used. Have anyone else experienced this or somebody know what the reason for this might be? 
Since I'm using a DSLR and the dark frames shall have the same temperature - can I try do take new darks tonight when the temperature is roughly the same as last night?

 

Thanks, Jesper

 

[Ouroboros]

That’s probably what I’d do. Also worth spending time today looking for light leakage when taking darks. It looks odd though, even for light leakage. Do you close the view finder? That’s one way light can get in. Or could it be a thermal effect with one part of the sensor getting slightly warmer after a night of imaging?  

[tooth_dr]

I would be quite certain it's a light leak.   Have you considered not using darks, but doing a process called dithering during your light image capture?

[jm_rim]

41 minutes ago, Ouroboros said:

even for light leakage. Do you close the view finder?

Yes, I'm using a D700 which have can close the view finder. 

 

42 minutes ago, Ouroboros said:

Or could it be a thermal effect with one part of the sensor getting slightly warmer after a night of imaging?  

This is a good question - I started taking the dark frames about a half hour after my last light frame. However, the dark frames keep getting brighter, which, at least in my logic, does not indicate a light leak. If it is a light leak should all the dark not look the same and not gradually get brighter?   

[jm_rim]

22 minutes ago, tooth_dr said:

 Have you considered not using darks, but doing a process called dithering during your light image capture?

I don't have a guide camera - only a RA motor. However, between each frame there is a very small downward shift (not start trails), maybe this can be consider as a kind of dithering?  :-)   

[tooth_dr]

6 minutes ago, jm_rim said:

I don't have a guide camera - only a RA motor. However, between each frame there is a very small downward shift (not start trails), maybe this can be consider as a kind of dithering?  🙂

Unfortunately that doesnt work