From: Jan Bentz 
To: Poul Linnet 
CC:  atm{at}shore.net
Reply-To: Jan Bentz 


Poul -- The back focal distance has a direct influence on the spherical
correction.  If the system was designed for a certain bfl, using it at a
different bfl will upset the correction.  On your system, if it is
possible, change the distance between the corrector and the primary
slightly and recheck the spherical correction.  The  bfl will change with
the square of the change in separation so go easy.  The other thing to
check is wether or not there is supposed to be a filter or even a plano
glass block just  in front of the focus.  If this is so it will add
overcorrection.

Jan Bentz

Poul Linnet wrote:

>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