What size filter should I buy?

[billdan (Bill)]

Hi,

I would like to add Ha to my QHY-12 OSC images, could I get away with a 1¼ filter? The CCD chip diagonal dimension is 29mm and hopefully I should not get any vignetting from the 32mm filter.

Or is the criteria for choosing based on distance from the CCD chip to the filter? Would 50mm distance be OK?

Regards

Bill

P.S. I have a funny feeling I may have placed this in the wrong section. If so could the moderator move it to astrophotography.

[billdan (Bill)]

I found the answer, looks like I have to use the big ones.