Re: ATM 90mm f-11 APO

[The Silent Observer <silent1@ix.netcom.com>]

"Brian S. Boothman" wrote:
> 
>   I have done some remedial polishing on commercial
> optics with good results. I feel that if I am going to
> spend the time grinding and polishing I would like to
> do it on a scope that is the best I can afford.
> Not the biggest.

I'd have to say that if image quality per dollar is your criterion, an
8" Newtonian on a Dob mount is the way to go; you might easily spend
that on the glass blanks for a 90 mm apo.  I built mine for under $500,
and wasn't particularly careful in scrounging because I was working
toward a deadline (made it, too -- mirror and scope completed a few days
before I took off for a star party 1800 miles from home).

The "best" -- a well made 8" Newt with 20% obstruction has no color
(barring that produced in the eyepiece -- generally almost undetectable
with good EPs), no spheric aberration, if longish focal ratio very
little coma -- and about four times the light grasp (accounting for the
obstruction) with over twice the resolution capability.  A five foot
truss tube on a low Dob mount will take up much less space than a
refractor tube and tripod, and transport more readily in a small car. 
Also, the longer focal ratio gives higher magnification with the longer,
higher quality eyepieces (possibly in combination with a Barlow -- a 10
mm and 2x Barlow will give better images for less money than a 5 mm, in
my experience), and can still be run down to 50x or so with a 25mm or 32
mm -- and all of this with 1.25" eyepieces, no expensive 2" size
needed.  A fine 8" mirror in good collimation will support 400x, and on
a very good night might well go to twice that if you have the eyepieces
and Barlows.

And you can grind an 8" mirror in the space of a 55 gallon barrel and
room to walk around it, or the space of a four foot plank laid across a
couple stools or some blocks, or even a corner of a table and room to
put a chair -- the exact same space you'll need to work lenses.

Assuming you have room to build the mount, you can get much more
telescope for much less money with a Newtonian than with any refractor. 
Oh, and I might add that my 8" is my first ATM scope -- I strongly doubt
I could have made a successful, much less excellent achromat, but my 8"
has a Strehl of around .95 and performs with commercial 10" SCTs.

-- 
It is well to approach all wonders and miracles by gradual stages or
degrees . . .
                                                           Fritz Leiber

Donald Qualls, aka The Silent Observer           NAR # 70141-SR Insured
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Opinions expressed are my own -- take them for what they're worth
and don't expect them to be perfect.