What are these stains?

[jones]

Hi,
I took my second picture with my 50mm lens, being my guider cam broken. After processing with Iris and doing the asinh, I have tuned the visu to expose gradients, and look what I found!
http://i.imgur.com/228DS.jpg

What's going on there? It looks like there's a pattern. It's also visible with the image properly stretched:
http://i.imgur.com/gxdtS.jpg

Any idea what could've caused that?
Thanks!

[jjjnettie]

It could be marks left from cleaning the sensor.
Taking flats should clear it up.
But I wonder if you've captured some very faint Northern Lights?

[DavidU]

That's an interesting thought. The sun is flaring.

[multiweb]

I reckon as well. The marks are rectangular and follow the sensor shape.

[jones]

Hi everyone. I have taken flats, a number of them.
Also, I don't think those are northern lights because what you're looking at is the sigma-clip stacking of 51 pictures.

[jones]

PS: I exclude this has anything to do with the sensor because it didn't happen when shooting at prime focus of my ED80 scope. So it must be the lens, mustn't it?

[acropolite]

If there are city lights nearby, it could be skyglow off thin cloud. I have images with similar patches caused by skyglow from a nearby city.

[jones]

LOL, acropolite, I am in a city. The light pollution is very evident. But I have to rule out the thin cloud hypothesis because of 2 reasons:

1) The imaging happened over 2 hours, so it should've been a very stationary cloud.
2) More importantly, the same pattern appears on another image taken the next night.

I wonder if there's a problem with my flats (both images share the same). I'll check that tonight.

[jones]

I have checked my flats and they don't show that pattern. What on earth is it?