Judson Mitchell said:
Can someone point the advantage of a 40 mm plossl? Since the 32 has the same actual FOV and higher magnification, what are the reasons for using a 40mm?
Judson:
Just to be clear, I believe we are discussing 40mm 1.25 inch eyepieces.
I have two 40mm 1.25 inch eyepieces, they don't get used much but they do get some use. One is an old Japanese Meade MA, the other a Celestron Plossl.
The general thinking does seem to be that they are of little worth because they provide the same true field of view as a 32mm Plossl (at least a good quality 32mm, some do not have a 27mm+ field stop) but lower magnification, they are essentially worthless. There is some truth in that attitude but it stems from a fast scope paradigm.
Not all is field of view and magnification... Sometimes a larger exit pupil is desired. If you are operating at F/6 or even F/7 a 32mm gives you a generous exit pupil but when with slower scopes, even F/8 but more commonly F/11 or slower, a 40mm eyepiece can be useful, even if it is only for aesthetics.
Consider an F/13.3 refractor. a 32mm Plossl provides a 2.4 mm exit pupil, a 40mm provides a 3.0 mm exit pupil, neither is overly large but certainly there are situations with any scope where the brightness of a 3mm exit pupil is desirable. In an F/13.3, a larger nebula like the dumbbell can be quite interesting with a 40mm... It would have the about the same brightness as a 18mm in an F/6.
This is the way I look at it: If I had to choose between a 32mm Plossl and a 40mm Plossl, the choice would be simple, the 32mm. But I don't have to make that choice so I can have both and I do have both. I do believe that the 40mm Plossl ought to have an asterisk with a note at the bottom pointing out the limited usefulness of the 40mm. Unfortunately I think that often people purchase the 40mm thinking they will be getting an even wider true field of view.
Bottom line: In a slow scope, I find that sometimes sometimes a larger exit pupil is desirable, even if it comes at the expense of apparent field of view.
Jon
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