Matt Dawson

Just to mention (though it only applies to thinned CCD chips), I always use an IR blocking filter to get rid of those pesky diffraction patterns on my images caused by atmospheric moisture.

Hello I'm new to this forum, greetings from rainy Luxembourg. I see this subject has been well covered already but I have a specific situation I need help with. I have a f/3.67 20" (508mm) Newtonian,coupled with a TS-Optics Coma Corrector 0.73x Reducer, and an Apogee U4000 CCD (chip size 15.2mm x 15.2mm). I understand I need an over sized secondary to take full advantage of this f/2.7 super fast setup, but exactly what size secondary should I order? This setup has disadvantages that I cannot change like unforgiving collimation and focus (and some coma), but this is the one thing I can change for optimum performance. Can anyone tell me how to calculate this or, better still, calculate it for me? Cheers, Matt Dawson, Luxembourg