Well, considering that I played a lot with my scope (with / without focal reducer, with / without filter wheel...) it's not surprising that lots of stuff found its way into the light train. But where?
Somebody on the ccd-newastro mailing list forward me a link to the CCD Dust Shadows calculator from Wilmslow Astro (they have several other interesting calculators on that page: eyepiece projection, filter reflections, auto guider rates ...)
For these flats, the values are:
pixel size: 4.54 microns
focal ratio: 6.3
size of donuts: 200 pixels
Which leads to a distance of 5.7mm from the CCD chip. Which means on the camera window. http://www.arksky.org/asoclean.htm has some excellent instructions on how to clean optical surfaces (not mirrors). I love their first tip: TRY TO AVOID CLEANING AND ONLY DUST !!! So, I'm just doing the basic:
- Use a very soft brush (I still have such a brush from cleaning my camera sensor).
- First whisk the metal around the surface
- Then carefully brush top to bottom (not round or forth-and-back) with the brush