From: "Dennis Woos" 
To: "Atm" 
Reply-To: "Dennis Woos" 


My 13 year old son Doug and I had our 10" f/5.6 flex newt at
Stellafane last year.  We made the mirror from Dan Cassaro 7/8" plate
glass.  It is a fine scope, but we do have a little astig which seems to
get worse as the temperature falls.  We have tried to correct it by
changing the pusher plate from 1/2" lexan to 3/4" mdf, de-wedging
the pusher ring, and being very careful about aligning the pusher ring and
rubber.  Because of the thermal element, it is hard to tell if the astig
has gotten a lot better or not. Also, because it is so slight I guess it
has not been worth it to try a lot harder to get rid of it.  Someday when
we build a bigger one we will rebuild this one.

We have another flex mirror scope, a 6" f/8, 1" Pyrex.  This is a
great scope, and I wish the 10" was as sharp.  The only scope I have
looked through that I though was better on Jupiter/Saturn was a 4"
Televue refractor, and that only slightly.  Of course, the problem with
this scope is the modest aperture.  Jupiter and Saturn are great, as are
open clusters (I don't know a better object for a 6" than M11). 
Splitting doubles is very satisfying, and is pretty much limited by the
seeing and limiting magnitude. Just this evening we happend to split both
pairs of Epsilon Lyrae at 118x, as nice as I have ever seen them split in
any scope (TV NEAF 10.5mm plossl
$25 - thank you Al Nagler).  Beautiful.  However, the 6" can't compare to
the 10" when it comes to globular clusters, nebulae, and galaxies. 
The astig is not detectable on DSOs, and globular clusters and nebulae are
tremendous in the 10" (we really want 2" OIII and UHC filters in
a filter slide!).  However, the 10" is to galaxies as the 6" is
to globulars, i.e. most are not all that satisfying.  So who doesn't want
more aperture?!

A final word of advice.  I don't know when or what we will flex next, but I
know that we will fanatically de-wedge everything, and be fanatically
careful about centering the puller.  In fact, I think you need to be
fanatical about the entire flexing process to achieve outstanding results,
at least in the larger apertures.  The 6" was our first flex mirror
and it was easy and/or we were lucky.  I can't express how satisfied we
are, not only with the mirror but with the scope.  A small dob of
reasonably long f-ratio with outstanding optics is a wonderful scope.  It
is amazingly inexpensive, easy to transport and setup, and delivers
outstanding viewing. Thank you Isaac Newton, John Dobson, and Bill
Kelly/Alan Adler!

Dennis Woos
----- Original Message -----
From: "CSC" 
To: "Atm" 
Sent: Sunday, May 25, 2003 7:29 AM
Subject: ATM RE: Flex mirror


>
> Turned out O.K.?  This mirror is the best piece of glass on the planet!!!
> Except for Adler's of course.
>
> I made a relatively conservative scope, 8" f/7, with a 1:9 thickness
(.87").
> For this size, the wedge tolerance was .009 or something.  I ground the
> wedge out when I smoothed the curves (made 7 of these mirrors), and kept
it
> to .001 by using accented pressure on the first few wets of each grade,
then
> smooothing wets.
>
> I cut a 9" steel lap on my lathe to fit the pregenerated curves, and
started
> with 320 grit. I like the steel lap, and was very interested in the dump
lap
> before I got the lathe working.  They work fast and all my mirrors are
> within .1" radius. I found I could get rid of .001 wedge in a few wets.
> Probably best to get rid of the wedge asap and keep it out.  At 12 micron
> grit, it's slow work to get rid of .001 glass, especially a 10".
>
> I read that someone had made a 10" f/6 flex.  {at} stellafane?
>
> I'll be glad to keep in touch on this project with you.  I'll bet the pull
> on this mirror is slight with the reduced thickness.
>
> Colin
>
> -----Original Message-----
> From: Ian McKernan [mailto:mckernan{at}central.murdoch.edu.au]
> Sent: Wednesday, May 21, 2003 10:13 PM
> To: optic{at}gwi.net
> Subject: Flex mirror
>
>
> Hi!  Saw a posting on the ATM list where you said you had made a flex
> mirror that turned out ok.  I'm starting on my first one at the moment.
> 254mm diam, 17.5mm thick, f 6  / 6.2 , flat back, plate glass. I made a 10
> inch mirror last year from the same glass and parabolised it, turned out
> pretty well.
>
> Have almost ground to a sphere at 220 grit (according to the sharpie
> test)  I took a plaster casting off my last mirror, covered it in steel
> washer dumps and am grinding with this (works really well if the tool is a
> good sphere to begin with).
>
> Anyhow, any advice or comments from someone who has done it would be
> appreciated. Any pitfalls in making the mirror or cell that you found that
> i may ba able to avoid??  Ive got a copy of Alan Alders article from S&T
> and have finally gotten FLEX to work (had to run win2k in safe mode!!) but
> as i found when making the last mirror there is no substitute for advice
> from someone who has done it. Particularly interested in how you dealt
with
> wedge?? For this mirror the tolerance is only 0.0031".
>
> Seeyahs!
>
>
>
> *********************************************************************
> Ian McKernan
> Plant Biology Technician
>
>          c/o Murdoch University
>          Division of Science and Engineering
>          School of Biological Sciences and Biotechnology
>          South St Murdoch  6150
>          Western Australia
>          tel: + 61-08-9360 2206
>          fax: + 61-08-9360 6303
>
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