1.25" Vs 2" Imaging Train

[Knight of Clear Skies]

A simple question I'm struggling to find the answer to. I've been asked to look into buying a filter for a scope and I'm not sure if there would be any benefit in using a 2" version. Am I right in thinking that 1.25" is sufficient for a APS-C sized sensor?

[michael.h.f.wilkinson]

For the Canon EF-S size it may (just) fit, as it has a 26.7mm diagonal, compared to about 27.3mm for the maximum field-stop size of a 1.25" EP. There might be some vignetting if the filters have thick mountings. The Sony and Nikon cameras have a larger format (28.4mm), so slight vignetting is more likely.

[craigg]

Personally I would go with the 2" version.  One reason is if you decide to also use a focal reducer / coma corrector, these are normally 2" and therefore you can fit the filter to the reducer and get the best of both worlds.  That's what I have for use with my 8" newtonian when using DSLR (cropped sensor).

[Knight of Clear Skies]

Thanks for the info.

[spaceboy]

2" all the way as you'll never run the risk of vignetting.