(ATM) Cassegrain types?

[Steve Lee <SL@AAOCBN1.AAO.GOV.AU>]

>  What about the Gregorian-type Cassegrain? I don't know what the proper
>  name for this type is but it would have concave secondary in the
>  Gregorian configuration. How does this type rank against the others for
>  coma, aberration, etc?

The Gregorian was the first reflecting telescope to be designed (but not
made).  The Scottish mathematician Gregory started with the "pure" paraboloid
to image a star, then an concave elliptical secondary to re-image the star
back through the hole.  Like the straight Newtonian and classical
Cassegrain, it has coma proportional to it's focal ratio.  The problem
(and feature) of the Gregorian is that it typically has a very high
focal ratio.  The secondary amplification for a Cassegrain is typically
4, while for a Gregorian it is 6.  Team that with an f/4 primary and you
get a good planetary telescope and little else.

Also, because the secondary is outside the primary focus, the design is
far less compact than a Cassegrain.  It is, however, far easier to make
and test.  The primary is a standard paraboloid, while the secondary is
a concave ellipse which can be null tested at conjugate foci (as can the
primary of the Dall-Kirkham).  It's longer tube length and very high
focal ratio are probably the main reasons for its non-popularity.

A recent ATMJ had an article about improving it's wide-field capabilities
by introducing a pair of small lens near the conjugate focus.

Steven Lee