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

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

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.
(Note that this advice is given, even though there is no hard rule on what
constitutes an *oversized* exit-pupil anyway!) I think that this notion is
falsely supported by talking about wasted light/aperture, because surely
waste is bad?! This is why I dislike the use of *waste* - because I think
it is misleading and confuses the issue, especially for the less experienced
observer.

Dennis

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