Strange Gradient - Need Help Please!

Can anyone help me identify why this circular gradient is appearing in the centre of the image below? Funny how it's perfectly circular

Equipment in the imaging train;

Celestron 80EDF -> William Optic flattener -> Astronomik CLS filter - > ATIK 16HR




Its doing my head in. I checked for dew, fogging of filters CCD window. All ruled out. It looks like incident light coming from somewhere but I can't see any. I've never seen this before and i'm stumped.

Any ideas?

Vignetting? I get it with the Proline...

Dave

Dave G,

I got this effect sometime ago and still do. I originally thought it was light scattering from the Moon with dew. But tested the telescope at different locations and ruled that out. I also thought it was a higher relative humidity, 94 per cent RH causing some scattering in a central area. This usually shows up as mottling effect.

Finally, the culprit was vignetting. The same characteristics then showed up many times no matter what the circumstances with or without the Moon, RH, etc.

Eamonn A

Trial by elimination...

Does the anomaly appear when you image with a different camera - ATIK 16HRC?

Is the anomaly present on all filter wheel positions? Could be on just one filter OR something (a spider's nest!) in the blank space.

Remove the field flattener - still there or gone?

If you have a spare scope you can use the rest of the optical train on then do a test shot with that arrangement and see if it disappears.

The anomaly appears to be perfectly centred in the field and perfectly circular so my guess is that there are some strange internal reflections happening somewhere in the optical train. Is iths the first time you have seen this or does it appear in other shots?

Phil.

Can anyone help me identify why this circular gradient is appearing in the centre of the image below? Funny how it's perfectly circular

Dave,

I get this kind of result if I cool down my chip too quickly and where frost builds up.

I am hoping you have a series of subs and where inspection of each successive sub will show the circle to get bigger and bigger which is an indication that the ice is melting due to the heating of the chip via its use.

Last time I had that it was a result of dew on the refractor lens inside the OTA.