Re: [ATM] Or over-correction?

[spoon@pageamerica.net]

Here's something I posted to the TMB group a few years ago concerning some
(mostly) empirical observations concerning eyepiece designs with respect
to primary f/ratios.
--Mike
**************************************************************

This past year I've made a couple of faster mirrors and some
interesting observations came to light in the process concerning
eyepiece performance. One of these mirrors is a 14.5" f/3.3. The
other I am working on is a more conventional 12.5" f/4.2. I realize
TMB objectives are slower than this but from my experiences I found
some objectionable traits even at f/7. Plus many TMB owners may have
a faster reflector in their stable also.

Most of my concerns with the fast objectives was the spherical
correction and the findings from my field reviews (I did a LOT with
the f/3.3) is that Plossls, Orthos and barlows are hugely
undercorrected with these wide light cones as a general rule. The
most neutral correction I saw was with a 3mm Radian and a 4.7mm Meade
UWA. A Pentax 5.2mm XL showed slight undercorrection and an older
Type 1 nagler 7mm showed overcorrection (as did all my older
Naglers). Note that I tried a Zeiss 4mm ortho and a couple of other
brand orthos (also 4mm) and they all exhibit largely the same
undercorrection (a wave or two would be my guess.) My cheap 2x
Celestron barlow accentuated it even worse and made the good
eyepieces bad also. A 2.5x Powermate was definitely better but still
it was obviously stressed at f/3.3.

At f/4.2 I am still seeing the 7mm Nagler as showing a bit of
overcorrection and the Radian is nicely neutral. I haven't tried the
Plossls or orthos as I'm simply checking for spherical in the star
test compared to my bench results which show a good null. Based on
previous use, I'd think they would be usable but it's certainly
something to consider if trying to build a fast planetary scope (and
a good reason I consider a TMB 9" ~f/9 as one of my dream scopes). It
also needs to be considered when star testing fast optics.

At slower f/ratios I find field astigmatism in the Plossls becomes
objectionable to me at about f/7. While they remain sharp on axis, to
my eye they have some problems not far from axis. Of course they get
worse as the light cone becomes wider. This isn't a concern for
driven scopes but for undriven use it comes into play. The orthos
seem a bit better but this may be due to their slightly smaller FOVs
or perhaps they're better corrected for astigmatism. Since most of
the scopes I've been using lately have had faster mirrors, I haven't
used the older design eyepieces as extensively. For planetary use on
axis my guess (and I stress it is a guess only) is that orthos and
Plossls are likely excellent choices down to f/5 or so.

Since this has been an empirical review so far I'll mention a bit of
more objective study on the subject. A few months ago I was visiting
a friend at the U of Arizona/Steward observatory mirror lab and he
mentioned he had been raytracing some eyepieces designs using Zemax
and wanted to show me the results. He had standard Plossl, ortho and
a Nagler prescription from the Rutten and Venrooij book "Telescope
Optics". I mentioned the findings as above and he ran some traces
using a fast primary and seemed impressed as I correctly identified
in advance the spherical corrections we would find in each case.
Since I'd spent months tinkering with the f/3.3 mirror correction, I
knew it had a decent figure using autocollimation and it was nice to
see that the computer raytraces agreed so well with the field
testing. Unfortunately he didn't have prescriptions for the Radians
or other exotic eyepieces from Meade and Pentax but I expect we'd
have found good agreement to what was observed in the star test.

I apologize if this is a bit off topic but I simply couldn't find
much definitive info when I went looking for it in the past. This is
not surprising as using extreme f/ratio objectives is not very
commonplace but they certainly can expose eyepiece characteristics in
actual use. I might note that I eventually did use the 14.5" at
slightly over 800x during testing and it held up nicely though I
personally think more moderate scopes would be a preferred choice for
such high magnifications.

Best,
--Mike Spooner

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