Gradient in the center of dark frame

[mkalika]

What might be the reason for a circle form gradient in the middle of the dark frame?



As I can see the root cause are dark frames... But what might cause this gradient to appear in dark frame suddenly? All in all that was very short 3 sec exposure at 1600 ISO.

I made longer exposures in the past, but never had this issue.

I am shooting with BackyardEOS and Canon 550D.

Thank you!

[Alejandro Tombolini]

Strange. Only a guess. Was the viewfinder of your camera covered when you took the darks?

Saludos, Alejandro

[mkalika]

Thank you Alejandro!

Yes, I closed viewfinder using the standard cap that comes with camera. Like this:



I also imaged from my balcony in city, so there might be some light around me... but the telescope was covered and the camera viewfinder as well.

[NGC7789]

Just grasping at straws but does the LCD come on at any time during dark capture? Are you using a battery or a AC power supply?

And if you take a longer dark, say a minute or more, is the effect still evident?

[mkalika]

Thank you all!!!

I found the reason... My primary mirror leaks light... I covered the bottom part of the scope using black garbage bag and now I receive good darks... no gradients and glow anymore.

Looks like I have to tailor pants for my scope