From: mcollins@wdc.sps.mot.com (Michael Collins)
Subject: Re: Telescope question
Organization: Motorola Western MCU Design Center, Chandler Arizona
Date: Fri, 4 Sep 1992 02:03:52 GMT

Introduction
------------

  In the following, I don't want to come across as some kind of
high-brow, esoteric-optical-system snob.  The best telescope is
any one that is in use regularly, and I know Doug McDonald is one
of the more active observers who contribute postings to this forum.
A few years ago, I was in the very fortunate position of being able
to have two Maksutovs built for me, at unbelievably low cost.  As
this optical system is somewhat rare in the amateur world, I decided
to jump into a recent thread where I can add some information
gained first-hand.

Doug McDonald wrote:

> The physics of it ... backed up by serious experimentation on my part...
> says that a perfect obstructed system (refractor or reflector, it of
> course does not matter) will be unequivocally better than an unobstructed
> system whose diameter equals (outer diameter of obstructed system - 
> diameter of obstruction). An obstructed system will appear, looking at
> planets, roughly equal to an unobstructed system with diameter 
> (diameter of obstructed system - diameter of obstruction/2).

  I'm not quite sure I follow the logic in the preceding paragraph.
A "perfect" obstructed optical system cannot equal the resolution
of a "perfect" unobstructed optical system of the same aperture, no
matter what.  The effects of the obstruction, and its support if
there is one, degrade the image.  A smaller secondary, typical of
a long focal ratio Newtonian, will not have a significant effect,
but a comparable unobstructed telescope of the same aperture will
still perform better if its elements are properly figured.
  In the case of short f.r. Newtonians, the situation is much worse,
for three reasons.  First, coma increases roughly with the inverse
square of focal ratio.  At f/6, it's not too bad across a moderately
wide field, but by f/4 it's really becoming a factor.  Conversely,
it's negligible for most observing at f/8, and essentially gone past
there.  With fast primaries you get a rapidly converging light cone.
This means you must use a larger diagonal to intercept enough light
to fully illuminate a wide-field eyepiece across the field stop.
As with coma, it's not too bad outside of f/6, but look at the size
of a reasonable diagonal for photography in a 13" f/4 telescope.
The Dobsonian crowd has an advantage here, in that they don't have
to accommodate either the fully illuminated field or back focus
requirements of photography.  The other serious difficulty with a
rapidly converging light cone are the severe demands it puts on the
eyepiece, and it makes focusing much more difficult no matter what.
I don't have one eyepiece I can get focused across the field on my
f/4 Newtonian, but they all work just fine in an f/10 catadioptric.
  Of course, all the above assumes "perfect" optical systems.  Ask
an optician which curve they'd rather figure for a given diameter --
f/4 or f/8.  I'll bet I can just about guess the answer.

> An 8 inch Questar would be inferior to an 8 inch Meade
> or Celestron because of the dramatically worse chromatic aberration
> in the Maksutov.

  Uh, Doug, have you ever looked through a Maksutov?  Your 1/10 wave
Meade, whatever else its performance, still incorporates the basic
Celestron compromise: the corrector is mounted directly behind the
secondary, not twice as far from the primary where it "should" be.
[Incidentally, there's been some discussion of moving the corrector
back on a C-8 elsewhere in this group recently.  Anyone considering
this should take a careful look at the effects such a change will
have on vignetting.]  Moving the corrector makes for a much more
compact scope, and permits the use of a smaller primary, but it
also _lowers resolution_.  Your best focus just isn't as good as
it would be if the optical system weren't compromised for the sake
of compactness.  Is this a good idea?  Well, considering that you
have an 8" telescope less than three feet long, including equatorial
mounting, I think so.  But you won't get the resolution of a good
Maksutov, and certainly not that of a comparable refractor.
  There are other differences.  The 2080 has a secondary about 75mm
in diameter, if I recall correctly (I had one for several years).  My
8" Maksutov, because the primary is slower, hence the secondary is of
lower magnification, has a 50mm secondary obstruction, yet the fully
illuminated fields are about the same diameter.  In your favor, my
corrector is about three times the average thickness of yours, so you
undoubtedly get more light at the focal plane from the same object.
I also see halos around brighter stars because my corrector isn't
coated, due to reluctance on the part of the optician to risk the
operation.  (He sent one from a 5" to be coated once, and it
returned warped.) My Meade was purchased before they were offering
coated correctors, but the effect is much worse on the Maksutov
because of the greater correction.  There is, however, no -- repeat
zero -- noticeable color.  The design accounts for refractive
effects.  Can't say if Questar does, though I would certainly hope
any 7" optical tube assembly costing $8K provides essentially
perfect achromatic performance.  Oh, my Maksutovs also have a little
bit of uncorrected spherical aberration.  It's more obvious on the
8" than the 5", seen as different diffraction patterns inside and
outside of focus.  The 8" still produces better images (w.r.t.
resolution) than my Meade or a friend's C-8.

Mike Edelman had earlier written:

->However....a properly figured newtonian of long focus (f/8+) with a small
->secondary can rival a refractor for contrast and resolution. The major
->mass-market manufacturers don't make such a telescope. Pity.

> The reason for that is that no one would buy them. Long focal length 
> implies mounting problems .... and if you can mount a 6 inch f/8,
> you can equally mount an 8" f/6 or 10 inch f/5, which will,
> with good figure, annihilate the performance of the 6 inch. There is
> no substitute for aperture.

  Some would also argue there's no substitute for resolution.  I
wouldn't trade my 8" f/6 Newtonian for anything, but I can sure see
a lot of aberrations I didn't before now that I've had two Maksutovs
for a few years.  You might be surprised just how much you could see
with a good 6" f/8 Newtonian, even compared to a 10" f/5.  I'd also
take a careful look at structural considerations before swapping a 10"
telescope for a 6" on the same mounting.  Give me a steady view and
a good image over a brighter, wobbly one any night.
--
mcollins@wdc.sps.mot.com
Newsgroups: sci.astro