Re: [ATM] Cassegrain vs. Gregorian (secondary testing)

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Richard F.L.R. Snashall wrote:

> I guess the greatest difficulty would be, given a pair of lenses,
> accurately modelling the test setup.  Oh and the price... Rolyn
> lists each lens at over a grand!!!!

Now that I think about this... of course it works.  The lenses
should have a focal length about equal to the RoC of the mirror.
If light were emanating from the CoC of the mirror, the lens nearest
the mirror would collimate the light and the second would refocus
it with an effective RoC about equal to the RoC of the mirror,
but this is pretty much the nature of the test setup.

The light is collimated between the lenses, so, assuming the achromats
are pretty much corrected for infinite object distance, both would
effectively work at their best correction for spherical aberration.
It should resemble a Null test for a convex spherical mirror.  In
this case, it is a bit off, with the Null occurring for a mirror
conic of about +1 (oblate) instead of 0 (spherical).  It's not a
great Null, but we're dealing with f/5 achromats here.

-- 

Rick S.

http://users.rcn.com/rflrs

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