From: Guy Brandenburg 
To: kenneth durkin ,  atm{at}shore.net
Reply-To: Guy Brandenburg 


My opinion - FWIW - is that an f/3.5 scope will be worthwhile for
wide-field views, but will be **very** hard to parabolize and will have
lots of coma at the edge of the field (meaning, that stars will look like
comets or seagulls), and will be useless for high magnification on
anything. It will be a very short scope, easy to carry around, but it will
also need to be collimated to perfection every time you use it.

A Ronchi test is a very easy-to-perform, qualitative (not quantitative)
test of the overall smoothness etc of a figure, and is a really good
adjunct or precursoro to doing the classical knife-edge test. Since a good
ronchi grating only costs a few dollars and can be sent very easily in the
mail, I would recommend ordering one. See Peter John Smith's excellent
website on ronchigrams; you can also download RonWin.

Two tests are better than one.

If you want to have an f/4.5 (which is already very short, IMO) instead of
an f/3.5, then you will have to go back quite a few steps. Sorry.

Good luck.

Guy Brandenburg

kenneth durkin wrote:
> I was shooting for an F4.5 but now realize that I'm at
> an F 3.5.  Since I'm well into polishing I would
> rather not go back sevearal steps.  Will an F3.5 make
> a worth while scope?  Will the Foucault test be
> sufficient for determining parabolization or will I
> have to do Ronchi testing as well?  Thanks, Ken
>
> =====
> Kenneth Durkin
> U.S. Consul
> Casablanca, Morocco
> Visit my website at www.geocities.com/kendurkin/index
>
>
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