ATM Re: atm-digest V1 #1241

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

Ari Friedman wrote :

> (Lurie-Houghton) ... an f/4 scope, but in the closing, he states
> the corrector "eliminates the spherical abberation and coma". If all
> coma is eliminated, is a f/3 scope possible?

No. There is another field aberration that is NOT corrected - astigmatism.
Although lower than in an equivalent Wright for example, it still limits
the system to about f/4.

> Would the mirror be too hard to grind?

Not harder than any other spherical f/4 mirror :-)

> Also, could a clear non-optical glass (read: Pyrex) be used for the
> correctors? What about plate glass (and dealing with the green images)?

Pyrex is all but out of question, as I've NEVER seen Pyrex that is homogenous
enough to be a lens. Plate glass is fine, again if it is stress and striae free.

> green images)?

Most older SCTs have plate glass corrector. The big Palomar Schmidt used to
have plate glass corrector too. Noone complained about 'little green stars'
(dunno about little green men :-). My Wright has plate for corrector, and
reds from emission nebulae come out rather nice.

> set of detailed instructions anywhere that tell how to grind the correctors?

Not that I'm aware of. Check Newport home page for general recommendations
about lens grinding. You'll need a spherometer and something for checking
edge thickness. For steep lenses you'll need a proper jig with dial indicator, 
not just a micrometer.

Bratislav