Dark circles.

[mickhyde]

So what causes these dark circles in the left image? Flats removed them OK as you can see, but I'm curious.
I checked the glass window on the camera straight after imaging, and it looked pretty clean.
Dust, condensation??
Atik 460EX, cooled to -10C.

Many thanks.
Mick.

[joelshort]

Those are caused by dust or other things somewhere in the image train.  It could be on a reducer/flatener, filters, or the CCD glass.  The smaller the dark circles the closer the dust is to the CCD.

[topboxman]

Nice job calibrating flats with the lights. It not only removes dust motes but flattens the whole FOV very nicely.

Peter

[STW HH]

Hi Mick, all statements have been done.
 Condensation Looks like cristals and concentrate  not in the middle of the Picture.

By the way, how Long did you expose your flats?

Joe

[mickhyde]

Many thanks all for the comments.

I'm glad its dust in a way, don't want any nasty ice crystals.
Strange thing is I've cleaned the glass with a q-tip couple of times, but the circles never seem to move 
Also as you say, it has flattened the view.

The exposure for the flats was 0.100s, using a laptop screen with white background pressed against the Takahashi scope, with the infamous white t-shirt in the way.
I took a number of exposures and this histogram looked the best. Worked really well 
See attached histogram.

Cheers,
Mick.
http://www.flickr.com/photos/mickhyde/
http://astromick.blogspot.co.uk/
Twitter @mickhyde

[joelshort]

Try rotating the camera in the focuser and take another flat.  If the dust circles rotate with the camera, then you know the dust is somewhere on the CCD glass or in the CCD chamber itself.  If the dust circles don't rotate with the camera, then you know the dust is somewhere in the telescope or filters etc.

BTW, you are NEVER going to get rid of all dust circles.  That's what flats are for.

[mickhyde]

Thanks, good shout, I'll try that and let you know. 

Cheers,
Mick.