[masraum]

Originally Posted by Rusty Heap

So how do I calculate all that? what to measure? I'm a newby

If the scope is an unknown, ie it's not a XYZ model from Brand 123, then I'm not sure. I'm sure there's a way. Mike/Pazazu would probably know. I don't think it's going to be an easy linear measurement, but maybe I'm wrong. I don't know if it's going to be the distance from the primary to the secondary or primary, to secondary to focal point of the EP. I'm not sure.

Without that info, then I would probably get the 26mm and use that a bit, then maybe shoot for something in the 21-17.5 mm range.

Another question is how much are you looking to spend on EP. You can get an EP for $20 or you can get one that's $200 or you can spend $600.

So maybe a budget would help with recommendations.