Nasty splodge on flat frames

[Tim star]

Still learning the ropes with my new Atik 460 coupled to SW ED 80 with Televue FR and Celestron filter wheel. The wheel lets in a lot of light but stopped this with foil and a shroud made from the leg of some discarded joggers. Decided to start building up a library of darks since it was cloudy, all went well. Thought I would do some flats too though I am thinking flats probably need to be done at each imaging session since any imperfections may vary from session to session (and filter to filter I suppose).

I know there is some tolerance of dirt on the optics which is why we do the flats but this splodge seems a bit much and I cant work out where it is coming from. Have cleaned the FR and (very carefully) the front of the objective. It doesnt rotate when I rotate the camera so does that mean it is on the sensor?

[Zakalwe]

If it doesn't rotate when you rotate the camera then it's probably on the sensor chamber window. if it was on the sensor then it would be in sharper focus.

[ollypenrice]

Agreed. Chip window. That isn't partularly bad, by the way, but the danger is that it might move. Best to clean it, I think.

By the way, don't take darks on the scope. Even the tiniest amount of light will wreck them. Atik supply a thick metal screw on cap so that you can get perfect darks.

If your camera stays on the scope then flats will be pretty durable. I need to re-do them every couple of months, no more than that. Anything on the objective will have no effect whatever on flats. Think of the huge central obstruction on a Ritchey Chrétien. That doesn't show!

Olly

[Tim star]

Many thanks both. I will clean the sensor window very carefully. Major help on th darks Olly thanks. I had got that quite wrong. Much easier with the cap on! Tim