From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


I found the same issue with a small Mak-Cass.  It helped to make sure the
scope was well cooled.  Star test improved greatly.

Incidentally, the star test pattern you described is Undercorrection, no?
The secondary shadow appears earlier inside focus?

A small Mak-cass can  be a good casual scope, but you will need a
long-focus newtonian to satisfy your optical craving. May I suggest a 150mm
f/8 or 200mm f/7.

You need to really be clear about what degree of correcting is needed
before trying to compensate.  a strongly curved meniscus lens in the system
will cure spherical, but it is trial and error and will introduce other
abberations.  I'd say use it as is, and contemplate another scope.

Do any ATMers have only one scope?

Colin
-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Poul
Linnet
Sent: Monday, April 14, 2003 4:41 AM To: atm{at}shore.net
Subject: ATM Maksutov Cassegrain questions



I have just bought a new Maksutov Cassegrain tubus, 90/1200 mm, NoName,
with an internal flipmirror, design similar to the Meade ETX-90. The
mechanical finish is beautiful but the optical performance is ??.

Startesting on Polaris shows a very sharp outer ring with the ocular outside focus
and a very fusy star picture inside focus position. Comparing to Suiter
this indicates
approx. 1/2 wavelength spherical overcorrection.

* Is this normal in this simple 90 mm F/13.3 Mak/Cas design with approx. 15
cm distance between
   Mirror and Meniscus and the secondary mirror being just an aluminized
spot on the Corrector?

* Are there any possibilities of correcting this by external means ?

* Has anybody a OSLO LT dataset available for this design ?


Poul Linnet
Copenhagen, Denmark

--- BBBS/NT v4.00 MP