ATM The perfect telescope, sort of

[Bratislav Curcic <epabcc@epa.ericsson.se>]

Anthony,

what about all spherical cassegrain with subaperture corrector(s)?
There was a number of designs published in ATMJ by fellow Aussie
ATMer whose name escapes me now (how embarrassing!). Basically,
spherical primary and secondary are teamed with some kind of
Maksutov subaperture corrector,  which takes care of overall
correction. 

I don't think that anything but spherical is viable for that 
28" f/1.8 primary.

> Neither the Cass or the Loveday is sealed, which, as Tex said, makes a 
> difference.

It may make difference for 10-12" class instrument, but 20+ inch one
is going to be just trouble if it is closed IMHO.

I'd definetely try to come out with something that uses mostly spheres,
even if it means several lenses for fiddling with field curvature. 

> 25" in 
> diameter and 1.2 inches thick edge to center with a 4.25 inch hole in the 
> center and is formed to an f/0.58 curve back and front.  It is strain and 
> striation free and appears to be a modern pyrex.

Although 'modern pyrex' can be surprisingly uniform, I don't think it
can be THAT good to be used for a critical refractive element.
Especially considering size. You may save youself a monumental amount of
trouble if you give up the idea of using that for a full size Maksutov
corrector.

Good luck,
Bratislav