Re: ATM 8" f3.8 -- Worthy project?

["Howard Banich" <hbanich@teleport.com>]

Hi Matt,

I made a 8" f3.4 a few years ago and it was a bear to parabolize - but that
was in the days before I discovered sub-diameter laps. I used the straight
up Foucault (three zone measurements) and Ronchi to get close and smooth,
then star tested to get very close to right on. The star test is a good idea
no matter what type of bench test you're going to do.

The difficulty I found was at this f-ratio a full size lap tended to take
out parabolization if the normal W stroke was used. A tangential stroke,
similar to what I used to hog out the blank in rough grinding ended up doing
the trick for me.

The scope this mirror ended up in works wonderfully with a Paracorr, giving
satisfyingly sharp images of both wide star fields and of the planets. My
guess is that you'll be able to get a fine figure on your f/3.8 - it's not
all that far from the f/4.25 you've already done.

Deep figures are the industry norm for making Schmidt-Cassegrains (f/2?) so
perhaps the reason very short focal length Newtonians aren't more popular is
because the use of coma correctors is required for sharp images across most
of the fov. Just a guess.

The other thing to keep in mind is that the tolerance for a well corrected
parabola down here is smaller than at a more popular f-ratio of f/5 or so,
making it a tougher target to hit.

Howard
Forest Grove, Oregon

-----Original Message-----
From: Matt Terry <mterry@tampabay.rr.com>
To: ATM list <atm@shore.net>
Date: Wednesday, October 25, 2000 2:35 PM
Subject: ATM 8" f3.8 -- Worthy project?


>     A couple of quick questions:  How  many folks have made sub f/4
>smallish mirrors?
<snip>
>Would such a mirror prove
>exponentially harder to figure than, say, the 6" f/4.25  I managed to bring
>to one-eleventh wave?  That doesn't seem likely, but there must be a reason
>such short focalities are not oft made, eh?
>
>  Thanks,
> Matt