Re: Testing telescope eyepieces

On Jan 28, 9:27 pm, Bill Turini <Respon...@xxxxxxxxxxxxx> wrote:
How are telescope eyepieces quantitatively tested? A search of the
Internet doesn't turn up any discussion of methods or results.

Thanks,

Bill

Hi Bill,
I don't know how they are tested. I do know how they could be
tested, but any method I come up with presupposes the use for which
the eyepiece was designed. Telescope eyepieces can be designed to
work with a "perfect" objective of some given focal ratio, or they can
be designed to be a component part of a system and compensate for its
aberrations.
For example, the eyepiece on a pair of binoculars is designed to be
part of a system that has converging beams going through a lot of
glass (the erecting prisms), and the old Pretoria eyepiece was
designed to compensate for the coma of a 6" f/4 Newtonian primary.
Use these on another system, and they don't look so good.
However, if you don't have the optical prescription and don't know
its intended use, you could just test the eyepiece for the purpose you
want it for.
If that purpose is a telescope, then place the eyepiece in the
telescope in question, make some resolution calculations, and then get
(or make) some appropriately-sized Air Force targets (Edmund sells
them for microscopes - you could adapt them for telescope use), and
look at the target (at an appropriate distance). That should give you
a rough idea of the eyepiece's MTF, at least around the contrast limit
that's important to you. Tilt the telescope to estimate off-axis
resolution. If you want to get fancy, you could illuminate the target
with monochromatic light. A pure blue and a pure red illumination,
simultaneously, can really open your eyes to the eyepiece's chromatic
aberration. (Some Nebula filters will do this for you.)
As you probably know, the focal ratio of the telescope matters. I
once saw a guy randomly pick two surplus achromats out of a bin, tape
them together in a custom tube, and get an eyepiece that was visually
perfect when used on an f/25 telescope. On an f/4 telescope, they
left something to be desired.
You can find more information about eyepieces in the book
"Telescope Optics" by Rutten and van Venrooij, if you don't already
own it.

Wade Kelman
.

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