1.25 inch T-adapter vs. 2 inch T-adapter astrophotography

[AutoBoof]

Can anyone compare astrophotography using a 1.25 inch T-adapter to using a 2 inch T-adapter?

What t adapter should I buy?

Thanks

[RickJ]

It depends on your telescope's f ratio, its usable field of view and your chip's size. The opening needs to be big enough to accommodate the light cone and chip size without vignetting, same as with filters. Too large is better than too small. I use 3.1" threaded adapters. Even a 2" T adapter would vignette my chip. Avoid nose pieces if possible. They can be the source of lots of problems ranging from tilted focal planes to slipping focus to dropped cameras. Use a safety line when using one.

Rick

[AutoBoof]

Thanks for the response, that makes sense. Saved me some money and frustration. My logic would lead me to think that the larger the opening, the more light is collected so a 2inch would be better than a 1.25inch. I never thought about the camera chip size. I have a t2i, I don't think a 2 inch t adapter would do me much good.

Thanks again!
Chris

[RickJ]

From what I just looked up the chip in that camera is 5184x3456 pixels at 4.3 microns. That is a 26.79mm diagonal which is larger than the 25mm typical internal opening in a 1.25" adapter. You need the 2" size.

Rick