To: "lance clarke" 
Cc: atm{at}shore.net, owner-atm{at}shore.net
From: Hugues.Laroche{at}ses-astra.com
Reply-To: Hugues.Laroche{at}ses-astra.com



Hi Lance,

>Is there actually any reason for anyone to
>bother building a gregorian?

IIRC, the gregorian, contrarily to the cassegrainian, has an outward curved
focal surface, which should give a better match with many eyepieces
(especially not the
$$$ ones) Now the gregorian, as the
cassegrainian, has a coma-free
variant (Ritchey-Chretien for the cass). As a con, other things being
equal, the greg will have a longer f/ratio and a smaller field of view.

But, there are 2 variants which are interesting IMO.

1) the gregorian-mersenne, which is analogous to the scope of Clyde Bone.
Interesting in a big scope to be able to observe feet on the ground and
with a moderate focal ratio.

2) the other is the... Johnsonian(?) (see the archives in 1995). I have toyed quite
a bit with the idea to make one. You have a small secondary flat, VERY NEAR
the focus of the primary, and the gregorian tertiary is placed where you
would normally place the eyepiece. The focal plane is on the other side of
the secondary cage. However, it is tricky to tweak parameters while keep a
reasonable obstruction.

Well I think that's it so far :-)

Clear skies,

Hugues




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