[ATM] Re: Maksutov corrector minimum thickness

[vladimir sacek <vladis.2@juno.com>]

Bob,

If the blank thickness is T and combined sagitta for the two radii is
"x",
final center thickness of the negative element is T-x vs. T of the
positive
element (in a  numerical consideration). Given radii, it gives only one 
possible outcome vs. center thickness for both elements. If design
requirement
vs. that final center thickness differ from that one outcome, it is
obvious 
that you can have proper thickness for both elements only if you use
blanks 
of different thicknesses or do extra grinding. Or, for a given center
thicknesses,
there is only one value of the combined sagitta that will result in equal
blank 
thicknesses needed.

That is what it comes to with required final center thickness of 1.8" for
the positive
element and 0.5" negative, with the radii of ~120" and 300", for a 20"
blank, as I thought 
they were. The combined sagitta is ~0.57", which would require ~1.1"
thick blank for 
the negative element vs. ~1.8" for the positive.

But my recollection of the radii was way incorrect. I run the design
again 
(20" aperture, f/4/16 Houghton Cassegrain with 1.46 index for both
elements), 
and the corrector looks like this:
R1:  2048.1mm
t1:  41.98mm
R2: -2213.7mm
spacing: 1.3mm
R3:  -2048.1mm
t2:  12.5mm
R4:  -2213.7mm

With this radii, both elements require about identical blank thickness.
So, see,
my thinking was correct, only my database wasn't. 

Thanks for making me correct myself :)

Vlad
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