[ATM] Re: Aplantic Gregorian

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

" Final or total aberrations
will be increased due to the imaging element, obviously,  but I'd like to
be
able to find out what wavefront aberrations the Mersenne itself
contributes
, ideally to be able to quantify them and then decide what imaging
elements
to follow in order to insure a reasonable tradeoff for my project."

It would be good to clarify the terminology. Mersenne is a complete
system,
in which primary serves as imaging element for collimated beams. It does
have
aberrations created by the primary focusing collimated beams. What I
think 
you are talking about is only the two mirrors producing a bundle of
aberration-free
collimated beams. That is not "Mersenne"; it can be called "afocal
reducer", or in 
this particular case "Gregorian afocal reducer". The afocal reducer has
no inherent 
3rd order aberrations (I don't know about 5th order aberrations, but even
if they exist, 
they are negligible for relatively slow primaries).

Any aberrations are produced by imaging element focusing these collimated
beams.
There is no other aberrations to add or deduct. How the displaced
aperture stop
(better to say, stops, because off-axis points have it displaced both
axially and laterally)
will effect aberrations of the imaging system is specific to the imaging
system. The only
way to find it out is to specify the system and stop displacement. If the
imaging element
is primary mirror, the aberrations will be those of a paraboloid in
collimated light, with
astigmatism reduced due to displaced stop, and no effect on coma. If the
imaging element
is refracting objective,or any kind of small telescope, there will be no
added aberrations, 
since there wouldn't be optical surface enlargement factor. Off-axis
aberrations would
likely be reduced due to vignetting at such aperture, either due to part
of off-axis collimated 
beam being cut off, or due to diameter of collimated beams being smaller
than the 
aperture of imaging element (in the latter case, inherent axial
aberrations of the imaging
element would be reduced as well).

It would make it easier to pinpoint the answer if at least basic details
of your project
are known.

Vlad
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