Filter sizes for mono cameras

[Jammy]

I've just been looking at mono cameras and filters and was wondering why the filters and filter wheels come in different sizes?

For example, ZWO do a filter wheel and filters in 1.25" 31mm and 36mm.  Does it make any difference which you buy?

The bigger the better to stop vignetting?  Are the 1.25" and 31mm not the same, or are they threaded differently?

Hope someone can enlighten me.

Merry Xmas

[Demonperformer]

Obstruction is the thing. 1.25" filters in a filter wheel can cause significant vignetting. However, the ZWO cameras you can thread the filter inside the nose right up against the window, which is much better. The 31 & 36 mm filters are unmounted.

[Jammy]

Thanks for the reply.

I've just been reading up on mounted vs unmounted.  Unmounted is for fast scopes, but not necessary for F5 and above.  Is this correct?

I'm thinking about getting a new scope, I'll have to give this some thought.