eyepiece limits

David Knisely wrote in message ...

Potential aperture *is* wasted, which is why it may
be more cost effective to use a smaller scope for wider fields.
You have essentually "maxed-out" your brightness if you go beyond
the pupil size of the eye, so there is no further gain by going
ower in power (other than perhaps obtaining the larger field of view).

Gary Seronik addresses this quite nicely in the October issue of S&T.
I'll recast his argument in terms of my own situation, and explain
why I just bought a 40mm eyepiece which I intend to use in my 12.5"
F/5 Dob -- among other scopes -- despite the fact that its 8mm
exit pupil is "wasteful."

At the moment, the widest-field ocular that I use on my 12.5" F/5
Dob is the 27mm Panoptic, yielding a 5.4mm exit pupil that is an
almost perfect match for my eyes, and is not wasteful in that sense.
However, I find it very frustrating to view the North America Nebula
through that scope/EP combination because the FOV is so restrictive;
in fact, in some ways, I prefer the view through my 7" F/5.4 scope
with the same ocular.

Now when I use that 40mm ocular on my 12.5" scope, my own pupil is
going to vignette the scope down to be the equivalent of an 8.5"
scope. Is this wasteful? Well, yes and no. The fact is, I don't
own an 8.5" scope. And although a 40mm widefield EP isn't cheap,
it's a whole lot cheaper than an 8.5" scope! I could take my
7" scope with me whenever I want to view the N.A. Nebula, but
since I've already got my 12.5" scope for other subjects during
the same viewing session, carrying my 40mm ocular is a whole
lot easier than carrying an extra telescope just for that one
subject.

- Tony Flanders