[ATM] Re: Aplanatic Gregorian

["lou krajci" <loukrajci@comcast.net>]

>From: Michael Lindner <mikell@optonline.net>

>Reading Schroeder's "Astronomical Optics" (which is indeed a $100 book!) I
got
>interested in the aplanatic gregorian. After running through the equations,
>however, I discovered it was difficult to come up with a practical design.
>Secondary sizes tended to be 50% or larger and/or the mirrors became quite
>aspheric. My conclusion was that this is not an easy scope to make nor a
very
>useful design to build - better performance for less work out of almost any
>other 2 mirror design. I suspect that's why it's not in R&V or many other
>books.

Perhaps aplanatic Gregorian is too difficult.  What about a 'compensating'
non-aplanat Gregorian?  (I know, that's my term, and it may not be correct,
but so far other folks have only mentioned the standard Gregorian and the
aplanatic Gregorian).  I'll try to explain what I'm getting at.  First some
definitions.

A.  Standard Gregorian:  Paraboloid primary, concave elliptical secondary.
But not corrected for coma...so it's not an aplanat.  (And this is not, by
my use of the words...a 'compensating' Gregorian.)

B.  Aplanatic Gregorian:  (Type of primary?  type of concave secondary?  I
don't know yet.)  Corrected for coma (and spherical aberration).  (If I
understand it correctly this IS, by my use of the words...a 'compensating'
Gregorian.)

In my earlier posts I asked if there were any designs/guidelines out there
for a 'compensated Gregorian.'  My intent was to have a Gregorian that uses
two concave mirrors, that are both easy to fabricate and test (hopefully by
null testing at conjugate foci), that is corrected for spherical aberration,
but not necessarily for coma.  In other words it's not an aplanat (B above),
but it's not A either.  That's because by themselves either mirror can't
form a perfect on-axis image...like type A can with either of its mirrors
used individually.  (And I presume type B can't form perfect on-axis images
with a single mirror...but I'm not sure)

So...a compensating Gregorian is not like type A...but it could be type
B...although I consider type B an 'over engineered' compensating Gregorian
because it not only is corrected for spherical aberration, but coma as well.
(I suppose you could say that the aplanatic Gregorian is a subset of
compensating Gregorians.)

Do such compensating Gregorian (non-aplanat) designs exist?  Are they easy
to fabricate/test...or do you end up with very aspheric mirrors that make it
too much trouble, etc.?

Thanks in advance.

Tom Krajci
Albuquerque, New Mexico

PS.  If I'm using bad terms/definitions...please straighten me out.
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