Could this be caused by internal reflections?

[pyrasanth]

I use a Celestron RASA with a camera adapter that puts the CCD at the correct back focus. I know that the image circle of the RASA is 43.6 mm at the correct back focus of 72.80 mm & my Atik 11000 camera sits very close to this. I measured the clear diameter of all the apertures and these exceed the 43.6 mm aperture required.

What I'm getting on most images is a faint ring of light as in the image below. I looked inside the adapter supplied & even though it is black painted it is black shiny. If I put the adapter against a bright back ground say a computer screen I can see internal reflections inside the adapter- could these be the source of trouble. I can deal with the vignetting through flat fields but a bright ring through an image is very destructive.

I've just ordered some flock paper from FLO to flock out the inside of the adapter but it would be useful to get the forums opinion as to the possible source of the ring as seen in the frame below. I would expect to see an even drop off towards the edge of the field & not a bright ring being meet by the sudden change to vignetting. This seems odd.

Please let me know your thoughts. I should have the flock paper tomorrow so I can retest.

 

 

 

[andrewluck]

It does seem very likely. This post has some very useful information on this subject: http://www.sbig.com/about-us/blog/flat-fields-the-ugly-truth/

Andrew

[Daniel-K]

hummm, it also looks like you have a streak going through the middle of the image aswell. internal reflections can be a pain in the ASS.  i think your doing right by flocking it to eliminate a possibility. the top right vignetting looks heavier then all the other corners. is this cause of camera tilt/ rotation? 

[Ewan]

Yep that looks like reflections, just last night I pointed the 80mm APO + 460EX at Vega as a test as I am just getting back into DSO imaging, a big FOV with Vega central displayed some very weird reflection, almost looked like a planetary neb, not only that but when I slew the scope down the reflections go up & vice versa.

[Ewan]

[Jessun]

My first guess would actually be dew on the sensor glass or some other glass close to the sensor.

/Jesper