ATM Re: Cassegrain

["Bill Cook" <atmj1@conceptsnet.com>]

Tony Wrote:

>my 8.5" primary is ground out and polished out to a perfect sphere. The
F/ratio is f/2.6. This would make it ultra-compact. What size of secondary
should I have for this piece and about how thick should it be. I have some
scrap 1/4" thick glass but have no idea if this is thick enough or if it is
stress free.  Also i have some 15micron pits at the edges of the mirror I
cant seem to polish out. If I put this on my grinding/polishing machine how
long would it take. What kind of design of a cassegrain should I have.

Tony<

Tony:

The f/2.6 FR will make your scope ultra-compact if compared to a Newtonian
but
not a Cassegrain. Today's 8-inch SCTs have FRs around f/2. This is, however,
good.
A long focal length can hide a multitude of aberrations (paraphrase courtesy
of the
Book of James). If you will note the Cassegrain instruments made around the
mid-1900's,
you will see that they were were almost always a bit long. Thus, a little
less convenient
focal ratio will leave you with a flatter field and better correction!

A secondary COULD be made from 1/4-inch plate. HOWEVER, why should you do
that?
The secondary is going to have a amplitude factor of around 5. So, it needs
to be made
VERY well. On secondaries, neatness counts.

The size can be determined by a first order optics ray trace, taking into
consideration
how much back focus you will need. This is just pencil and butcher paper
stuff.

If your pits won't polish out, it just means that you stopped short on
grinding. That, in turn,
means it would probably be faster for you to return to gringing until the
pits are of a UNIFORMLY
small size before proceeding.

As to the kind of Cassegrain should you have....wow. That is a question that
might have been
asked before the project was started; hopefully, a good one. While you are
at it, you might want
to take good notes and photos. I don't recall a Cassegrain article in ATM
Journal yet. I would
be eager to see one.

Just some thoughts,

William J. Cook,

William J. Cook, Opticalman Chief, USNR-Ret.
Manager, Precision Instruments & Optics, Captain's Nautical Supplies,
Seattle
Editor / Publisher, Amateur Telescope Making Journal
yata, yata, yata