How do you figure out what filter size for a given scope / camera combo?

[Stonius]

How do you know if you need 2" or 1.25" or one of those other weird sizes in between (38mm unmounted? Why do people need that?)

I assume it must have something to do with aperature, focal length, sensor size and the point on the optical train where the filter will be inserted. Fair enough.

Is there a formula for this (preferable), or even an experimental way of determining *before purchase? There must be, surely!

Markus

[Merlin66]

Filter size= (Distance from filter to chip)/ Focal ratio + Diagonal length of the chip.

[Stonius]

Thank you sir,

You're an officer and a gentleman...and a wizard at spectroscopy too it seems. Something else I'd like to explore at a later date but I'm putting it off for the moment. I'll scale this particular learning-cliff first.

Thanks

Markus