From: dsc@gemini.tmc.edu (Doug S. Caprette Bldg. 28 W191 x3892)
Subject: Re: Fluorite scope for sale
Organization: NASA GSFC CDP VLBI
Date: Fri, 17 Apr 1992 20:20:21 GMT

In article <56280004@hpscit.sc.hp.com> jackz@hpscit.sc.hp.com (Jack Zeiders) writes:
(A rather one sided criticism of the Gregory Maksutov design.  I challenge some
of the remarks below, and elaborate on some of the deficiencies in the refractor
design.  For brevity, I have deleted paragraphs fromt the original posting, to
which Mr. Zeiders was responding.  Hopefully, this does not throw his
statements out of context, as they were quite general.

However, I do not express a preference for either design.  The simple fact
of the matter is, that if I were to have available the funds required to buy
one of these optical equivalents to Ferraris, I would spend it building a 
Newtonian of vastly greater aperture, which *WOULD* blow any of these exotics
out of the water (sky?) through sheer brute aperture.  While it is true that
aperture is not the whole story, there is none the less no substitue for it.

Incidently, I have a 10 inch f/6 Newtonian on a dobsonian mount, which I have
taken on vacation with a tent, sleeping bag, clothes, camera, binoculars,
charts eyepieces, and all necessary tools, (barely) within the 70 lb three bag
limit imposed on domestic air travel. So Ptffff as to portability arguments
as well.  (:-) )

>	Can you say unobstructed?
>	    Central obstructions are not a desirable optical attribute,
>	    reduced contrast results.
>

Can you quantify this for us?  No one denies that a central obstruction dcreases
the optical performance of a system.  But no one denies that increased aperture
improves performance, either.

>
>	Can you say scattered light from mirror coatings?
>	    Evaporated metel mirror coatings look like sandpaper at high 
>	    magnification, they scatter light throught the field further 
>	    reducing contrast.
>

Lense surfaces, coated or uncoated, will also 'look' rough, given enough
magnification.  It is generally recognized that scattering from surface 
defects in a lense, is about half as serious as scattering from similar
sized defects in a reflecting surface.  It is also recognized that it is 
easier to figure a reflecting surface to a given tolerance, than it is to
figure a (single surface of a) lense, to the same tolerance.

>
>	Ever hear of chromatic aberation? 
>	    Do you think a thick singlet meniscus corrector adds color?
>	    You bet it does.
>

A meniscus corrector with concentric surfaces, is achromatic, that is, the
dispersion introduced by refraction through the front surface is corrected 
by dispersion at the second surface.  The surfaces are parallel, same as for 
a window.   As they are spherical, and of different radii, the difference 
being equal to the thickness of the corrector, spherical aberration is 
introduced that is not fully 'cancelled.'  This is not a defect,
(not that any one said it was) rather, it is the purpose of the corrector.

In practice, the two surfaces in an actual Maksutov corrector may not be
precisely concentric. But they are always very close.  In theory, the 
corrector is fully achromatic, in practice, it will always be close.

The chromatic aberration, in a compound lense, is only reduced to zero,
in the case of infinite focal length, same as for any other lense, simple
or compound (unless all of the elements have parallel surfaces).

A florite doublet is not wholly free of chromatic aberration, in theory or
in practice.

>	The Gregory-Maksutov is a compromise design at best.
>	    The Questar secondary is an aluminized spot on the spherical 
>	    miniscus corrector. Good for production, not opticaly optimum.
>		Ask your "Master Opticians", f/15 helps here though.
>
Right, the various radii are chosen so as to minimise overall aberration,
over some minimal useful field of view, within the contraint of three radii,
one used twice.  Incidently, a Grgory Maksutov with all sperical surfaces,
is free of off axis coma.  The system is optimal, in the true sense of the 
word optimal, for a given aperture and focal ratio.  A variation on the
Gregory Maksutov, with a different radius of curvature for the secondary, 
could also be optimised, by selecting appropriate four radii.  What's the
point?  A similar argument exists for the refractor.  E.g. refractors are 
only optimal if the objective has the same index of refraction for all 
frequencies of interest.  No such material exists.  The use of any specific
finite number of elements is a compromise design.  By the standard implied in 
your paragragh above, a refractor can only be optimal with an infinite number
of elements, as some incremental improvement can be shown to occur in any 
design by the addition of another element, provided one ignores practical
considerations.

Any optical system with multiple curved surfaces, attempts to optimise over-
all correction, by introducing  compensating errors.  A refractor, florite
or otherwise, is no different in this regard.  The spherical and chromatic
aberration in one element, is corrected by the other(s).  
>
>	Think of a Questar as a folded 3.5" f/15 refractor with one thick non-
                                                                          ^^^^
>	color corrected element, two light scattering mirrors, and a spherical
        ^^^^^^^^^^^^^^^^^^^^^^^^  
        This is incorrect, as stated above.                          
	>
> 	magnifying secondary. Add another mirror or two more air-glass inter-
>	faces for a star diagonal or prism. You get both chromatic and reduced
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	This same addition is, of course, required for a refractor, if one
	wants a convenient eyepiece position.

>	contrast problems in one package.
>
I'm not about to think of a Questar as a refractor, because it is not.

>	Aperture is not the whole story.
>
>	Some of us like a wider field, f/15 does not do it. The little Apo'
>	will do both low and high magnification nicely. 
>
Ah,, yes.  This is where they truly come into their own.
>
>	How long are the exposure times for astrophotography at f/15? The 
>	Fluorite Apo has a field flattener-focal reducer that takes it to f/5.
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
			   Gee, how does one go about making one of these that
			   doesn't scatter light, introduce chromatic and 
			   spherical aberrations, and vignetting?

Aside from which, if you're doing astrophotography, you're not making full
use of the available resolution and contrast anyhow.

>
>	Many times, I have used several Questars, both 3.5" and 7". I've also 
>used a few OTI Quantums, two 4" and one  6" model, and other Maksutov systems
>up to 11 inches aperture. I own a 5" Maksutov-Cassegrain.
>...
>The Questar owner didn't like it either that night.  We could see a darker ring
>around the polar cap and the fact that the cap was split in several places 
>was not seen in the Questar.
>
The 3.5 inch Questars, and the 4 and 6 inch Quantums (Quanta?) have always
impressed me in actual use.  Oddly, I have used a 7 inch Questar twice, and
both times nearby 8 inch Newtonians showed much more planetary detail.  Talk
about Questar owners being unhappy!  In these cases I would have to say that
the Questar was, indeed, blown out of the water.

I would not be surprised to see a 70mm apo edge out a 3.5 inch Questar, in
actual use, or, vice versa.  But if either one were to blow the other out,
I'd hazard a guess that the inferior instument had a serious defect.

Here's a question for the expert opticians, that needs to be clarified for
the telescope purchasing FAQ:

Many years ago, when refractors with triplet objectives first began to be 
advertised, they were refered to as apochromatic refractors, which 
differentiated them from the refractors with doublet lenses, achromatic
refractors.  I assumed from the context, that an apochomat, was a triplet
lense.

Now, it has become common to refer to refractors as apochromatic, if the
have a doublet lense, with one florite element.  Is this a corruption of
the terminology?  Or, is there some other precise definition of apochromat,
that is accepted in the optical field?

--
 |  Regards,         |   Hughes STX                |    Code 926.9 GSFC        |
 |  Doug Caprette    |   Lanham, Maryland          |    Greenbelt, MD  20771   |
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