Re: Testing telescope eyepieces

On Jan 29, 8:55 pm, Bill Turini <Respon...@xxxxxxxxxxxxx> wrote:
The short answer is in any regard.

The only test method I find on searching the Internet is subjective
tests on astronomical objects using objectives of (possibly) varying
quality.

What I'm wondering is if there is some way to quantitatively test an
eyepiece alone without any supporting optics that need to be separately
tested, or accounted for. (tests for nodal points, exit pupil position,
etc. are not included in this discussion)

There have been suggestions of testing an eyepiece by using it in a
telescope to view resolution targets. This seems to me to rely on the
quality of the objective.

Certainly, if one had the design parameters, one could see what the
theoretical performance of an eyepiece is, but how do you tell if the
manufacturer built it according to the design.

All I've heard from people is "Try it and if you like the performance,
then it's a good eyepiece."


Hi Bill,
That's what Selby said in 1948, and he was familiar with eyepiece
design, objective design, the aberrations of the eye, optical bench
testing, and probably a lot of other stuff.
Think about it. The eyepiece is part of a system, and systems can
vary. For example, let's consider the action of an eyepiece if you
run the
light backwards through it (which is the way most eyepieces are
designed.)
Perfectly collimated light passes exactly through an external pupil
(without
vignetting), goes into the eyepiece, and is focused on the image
plane.
Is that image plane flat? Are the axial rays aimed so they continue
on
through the focus to pass through the center of the objective? What
if two
objectives have different focal lengths? Axial rays converging on the
center of one objective will miss the center of another.
Also, the image plane of an eyepiece is usually not flat. It is
usually curved. Let's take an eyepiece whose field is
concave toward the observer. If you are using the eyepiece on a
Gregorian, then the image plane of the telescope and the eyepiece
nearly coincide for excellent imaging, but the same eyepiece, used on
a
Cassegrain, which has a focal plane that is convex to the observer,
would not perform nearly as well. Most of the field would be out of
focus. Even the Pretoria's apparent field curvature changes for f/4
paraboloids of different focal lengths, because the objective's focal
plane curvature changes.
Asking if there is an independent quantitative method of evaluating
an eyepiece by itself is like asking if there's a way to evaluate
the flint half of an air-spaced achromat, without knowing anything
about
the crown lens, the eyepiece, or the wavelength range. You can
look at the coatings (some Chinese eyepiece manufacturers think
they are an unneccesary expense on the internal surfaces) or the
surface quality, but evaluating it in your system is the best test.
However, If you'd like recommendations for telescope eyepieces,
I would say that the Vernonscope Brandons are best for planetary
observing (few glass pieces and coatings to scatter light) and
Televue Naglers are best for wide fields. I haven't used the Nikon
eyepieces much. And, for what it's worth, Astro-Physics
refractors give the best view of stars, bar none, that I have ever
seen. Perfect diffraction patterns rising out of the blackness.
They are great.

Wade Kelman
.

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