Re: (ATM) RE: How much quality?

[Chuck Grant <grant@aretha.llnl.gov>]

> From: clark@witch.mitra.com (Clark MacDonald)

> > Classical Cassegrains have far too much off-axis abberations to make
> > very good wide angle scopes. The small field of view and long focal
              ^^^^^^^^^^
> > ratio hide the flaws of the classical Cassegrain design.  Any design
> > would work great at f/20 with a small field of view.]
> > Chuck
> > 
> 
> OK. What is it that I do not understand? What off-axis abberations
> are we talking about? Other than curvature of field introduced by
> the secondary magnification factor, typically 4X - 5X, what is the

curvature of field, flaw 1

> flaw in this design.

All designs have flaws.  It doesn't mean they are not usable.
It means they are not perfect.

> 
>       The secondary obstruction is usually < 20%

Having an obstruction at all is a design flaw.  Given an
obstruction, it will be much larger if you try to make the scope
wide angle, flaw 2

>       Coma is that of a newtonian of same f#

Having any coma is a flaw. this is particulary bad for wide angle,  
i.e. low f#, flaw 3

>       Long FL is in a small package

This is fine IF you want a long focal length.
long FL is bad for wide angle, flaw 4

astigmatism flaw 5

All of these flaws are reduced to managable levels by using
long f#'s

> 
> Perhaps the thing to mention here, is that the Classical Cassegrain
> was never designed to be a low power, wide field instrument. This

Right. It is not very good as a low power wide angle scope.
Exactly my point.

> means that adequate baffleing should not be a problem, because
> FOV is not a problem.

The Cassegrain's baffling requirements.  Flaw 6

> 
> A Classical Cassegrain has a parabolic primary. My scope has
> an F4 primary making it a Very Fast, Very wide angle 
> newtonian scope.

If you are changing the optical configuration to use it as a 
Newtonian, then it is no longer a classical Cassegrain.
Anyway I do not consider an f/4 Newtonian to be a great optical
performer for wide fields due mainly to coma.  The Newtonian
has its own set of flaws.

>
> I do not know of a more flexible instrument.

Flexible sometimes means equally bad at everything.

This is not to say that you can't make a fine classical Cassegrain
telescope.  But it will have a high focal ratio.  And just about
any design can perform well at high focal ratios.

Chuck