eyepiece limits

A nice real-world example - thanks. You know, the word *waste* is not the
main issue, but rather the frequent recommendations in this newsgroup that
folks not buy/use eyepieces that will result in over-sized exit pupils.


When the "wasted aperture" issue has been discussed in the past, I have come
down on both sides.

The way I look at it, it is just another trade off/compromise. A scope has a
maximum possible field of view that is determined by the focuser diameter and
the scopes focal length. It has an aperture. So, one can think in terms of
"wasted aperture" vs. "wasted field of view."

In some situations, "wasting" FOV to achieve maximum effective aperture is a
wise choice, in some, "wasting" effective aperture to achieve maximum FOV is
the wise choice.

It is my belief that it is important for someone buying an eyepiece to
understand these various issues and certainly exit pupil vs. eye pupil dilation
is an important one, especially for someone such as the original poster who is
using a 1.25 inch focuser.

If a longer focal length eyepiece buys one a wider field of view, then it may
well be a wise choice even though some of the aperture is not being used. On
the other hand, buying a 1.25 inch 40mm eyepiece is not likely to be a wise
choice is one is using an F5 scope. The field of view will be about that of a
32mm Plossl with similar brightness at less magnification.

(Note that this advice is given, even though there is no hard rule on what
constitutes an *oversized* exit-pupil anyway!)

This is because each of us has a different maximum eye pupil dilation so it is
impossible to set a hard fast rule. However the 7mm number that is often used
as a guide seems to be an upper limit, for many the number is closer to 5mm so
that effective aperture maybe even less than is stated in various examples.

If one does not want to use the term "wasted aperture" I think the term
"effective aperture" is a reasonable choice. If someone is aware that their
personal maximum pupil dilation is say 6mm, then they can be aware when they
use that 40mm eyepiece in an 8 inch F5 scope that the effective aperture is
only 6 inches.

Jon Isaacs