2" eyepiece vs 1 1/4

Tim · Guest

Hi
I have a new stellarvue 80mm nighthawk NG 550mm focal lenght scope with a 2" diagonal. I note that 2" eyepieces are about twice the money for comparable focal length, and tend to be longer focal lenght on average than 1 1/4 What, if any, is the advantage of a 2" over a 1 1/4" on a scope of this type? Also a 20mm is about the shortest 2" I have found, on a 550mm scope this will yield only about 27x magnification, would a barlow be nessessary with these big 2" eyepieces?
Thank you

Jne_K · Site Admin Joined: 24 Sep 2003 Posts: 5022

Hi

It's a matter of optical design constraints. When you hit 40mm focal length in a 1.25" eyepiece, the barrel size reduces the field of view possible. For instance, most 32mm Plossls will have a 50 or 52 degree field. The same eyepiece model in a 40mm drops down to a 40 degree apparent field of view. Bottom line: if you want a focal length than 40mm with a useable field of view, you must go to a 2" barrel - the optics just won't squeeze into a 1.25" barrel.

2" eyepieces are somewhat a special niche of eyepiece. They are long focal length (hence, low magnification) and wide field of view tools and are the darlings of folks who use very large scopes and search for the faint stuff and also for folks using refractors as CCD imagers or piggyback scopes. You don't need the 2" barrel design for shorter focal lengths. Some eyepieces in short focal lengths offer a dual sized barrel allowing you to use it in both 1.25" and 2" focusers. The idea is to accomodate users who have either size focuser. There is no optical advantage to a 2" eyepiece in a short focal lengths.
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