A Schiefspiegler as first telescope (was Re: ATM 3-Vane or 4-Vane Spiders)

["Ernest Tomlinson" <hyacinthus4@yahoo.com>]

From: Patrick St. Jean <stjeanp@pat-st-jean.com>

> On Fri, 5 Oct 2001, Nils Olof Carlin wrote:
>
> > Well what are the alternatives? a Schiefspiegler, an optical window, a
> > refractor, no telescope at all?
>
> What about the, for lack of a better word, non-Newtonian reflectors I've
> seen pictures of?  The ones where the primary is angled so that the main
> tube is clear of obstructions...  Is there anything special that needs
> to be done when making a mirror for them?

A Schiefspiegler is the simplest such telescope, requiring two mirrors, a
concave long-focus sphere, and a convex sphere matching the radius of
curvature of the concave.

I'm a sometime amateur astronomer, and I've owned and used a number
of telescopes, including a 2.4" Meade refractor and a 10" Cave-Astrola
Newtonian reflector.  I've always wanted to make my own telescope and
grind my own mirror(s) for it; and I've also been long fascinated with the
various tilted-component designs for unobstructed reflectors.  So I've found
myself wondering lately whether I couldn't construct a Schiefspiegler for
my first try.  But I've never successfully ground a mirror before--on two
separated occasions, I've started, but external circumstances and
interruptions
conspired both times to halt the projects.

Now everything I've read suggests that the easiest first mirror to attempt
making is a 6" or 8" medium-focus (f/6 to f/10) parabolic mirror for a
Newtonian.  The Schiefspiegler requires a sphere of f-ratio at least f/15.
(I played with Jose Sasian's TCT program to test to see what happened
if the f-ratios of the mirrors were reduced to f/12; the astigmatism and
coma
of the resulting scope seems to me too great.)  There's also the added
complication of figuring the convex mirror.

So, what do you all think--is it a valid first project, or should I embark
on
something simpler to start out with?

Ernest Tomlinson.