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Sent: Saturday, April 15, 2006 11:00 PM
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Subject: ATM Digest, Vol 28, Issue 16
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Today's Topics:
1. Re: Test (hermit)
2. A much belated update on various projects! Glass casting and
other things. (Thomas Janstrom)
3. Flex mirror - have you made one, any failures, any successes,
any hints? (Gil McFarlane)
4. Re: wedge in lenses (vladimir sacek)
----------------------------------------------------------------------
Message: 1
Date: Fri, 14 Apr 2006 23:12:40 -0400
From: hermit <hermit@outofoptions.org>
Subject: Re: [ATM] Test
To: Thomas Rutherford <rutherfordt@earthlink.net>
Cc: ATM List <atm@atmlist.net>
Message-ID: <444064A8.7020409@outofoptions.org>
Content-Type: text/plain; charset=ISO-8859-1
Earthink is the latest to bounce our emails. :(
The disgusted Ken....
Thomas Rutherford wrote:
>Test. I haven't received any traffic on the list for 10 days-- is it
>me
>or the list?
>
>
>Tom Rutherford
>
>
>_______________________________________________
>ATM mailing list http://www.atmlist.net/
>
>
------------------------------
Thomas and all,
Hello. I would very much like to know if you are using hydrostone for
your molds? If not, what are you using for your mold and how are you
dealing with expansion issues if any. I am in the process of grinding my
first cast blank...20.5 inch 2" thick. I had several issues and my end
goal is to make a mold from that first blank with generated curve.
Any help is appreciated. You would not believe what I did to get this
first blank and I don't want to do it again...ever. Almost ruined my
kiln.
Thank you.
Miguel
Message: 2
Date: Sat, 15 Apr 2006 17:15:34 +1000
From: "Thomas Janstrom" <thomas@moiler.com>
Subject: [ATM] A much belated update on various projects! Glass
casting and other things.
To: "'ATM List'" <atm@atmlist.net>
Message-ID: <000001c6605c$6d4935d0$0b01a8c0@Mylaptop>
Content-Type: text/plain; charset="windows-1250"
Hi All,
Well as some of you might remember Early last year I embarked on a
journey that will ultimately lead to my being able to cast my own
cellular blanks. Well at this point I have finally been able to start
work on the kiln, not the one I REALLY wanted to build, but big enough
to prove the concept and hopefully enable the recovery of some of my
costs. Ok maybe not!
The kiln that I'm building will be able to house a mould large enough to
cast a 16.5" mirror blank, at present I have the refractory bricks and
mortar on hand, and over the next two weeks I will be taking delivery of
the heating elements, computer temperature control system, and ancillary
bits and pieces.
So all things being equal I should be able to cast my first trial blank,
a 10" pre-cored Cass blank (non-cellular) out of soda-lime glass. I will
be under taking this task in about three to four weeks time. A short
time to work out what worked (and what didn't!) and I should be ready to
have a try with Pyrex, probably using the same design etc.
Now for some other things, firstly my vacuum chamber/home aluminizer is
slowly nearing completion, my 8" DK bino-scope is on hold till the
coating rig is up and running as that's where I'm up to on that project.
I have various other things on the go, but this will do for now, I will
post some pics of the kiln project if there is any interest in seeing
what I'm up to;)
Cheers, Thomas.
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------------------------------
Message: 3
Date: Sat, 15 Apr 2006 00:26:38 -0700 (PDT)
From: Gil McFarlane <gilboastro@yahoo.com>
Subject: [ATM] Flex mirror - have you made one, any failures, any
successes, any hints?
To: ATM <atm@atmlist.net>
Message-ID: <20060415072638.20240.qmail@web31102.mail.mud.yahoo.com>
Content-Type: text/plain; charset=iso-8859-1
I've neglected www.flexmirror.com (that I set up for
Bill Kelley) for some time - partly due to my personal failures, and
partly because it was being hosted by a free server that was very
difficult to link to.
Anyway...
I'd like to include any pictures, stories, hints,
helps, anyone would like to share about their flex
mirror trials. I'll give you credit, thanks, and put
it on the site. Send anything to
gmcfarlane@sbcglobal.net or gilboastro@yahoo.com
Thanks,
Gil
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------------------------------
Message: 4
Date: Sat, 15 Apr 2006 16:45:21 -0400
From: "vladimir sacek" <vla@toast.net>
Subject: Re: [ATM] wedge in lenses
To: <atm@atmlist.net>
Message-ID: <002601c660cd$84283eb0$0d2c0242@Handsome>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original
vladimir sacek wrote:
>
> For any Fraunhofer doublet,[...]. This gives the general
>RMS wavefront (coma) error as w~tD/14.6F^2 (for "D"
> in inches, F the objective F# - f.l./D - and the wedge angle
> "t" in degrees), and t~(14.6wF^2)/D. For "w" in units of
> the 555nm wavelength, it would come to t~(wF^2)/123D.
>
Not quite. I took another look at it, and found a couple of
things to add and correct. The good news is that the tolerance
is more forgiving, by about a factor of 2. For the wedge
angle "t" in degrees it is approximated by:
t~(wF^2)/64D
with "w" being the RMS wavefront error induced (in units
of the 555nm wavelength), "F" the objective F# and D the
aperture diameter in inches. The difference vs. initial result
is on the account of using (proper) balanced coma RMS
(50%), and the rest an error in calculating aberration
coefficient.
For the linear wedge "T" in inches, it is approximated by:
T~(wF^2)/3667
For any f/15 Fraunhofer doublet and 1/28 wave RMS error
(0.95 Strehl/MTF degradation factor) limit, the lens wedge
can't exceed 0.0022". For an f/10, it comes to 0.001".
Strictly talking, this is the tolerance for the two inner surfaces,
for which the sensitivity is highest and nearly leveled. For the
front surface it is nearly 10 times loser, and for the last one
nearly 5 times loser.
Since this is based on a particular Fraunhofer configuration,
(radii proportion 1 : 0.58 : 0.59 : 2.43), variations in radii
proportion will result in the tolerance level variation. While
it is not likely to be more than 15%-20%, it can be easily
adjusted by applying a correction factor, as given by
c=(1.03fR1/R3^2)^2,
where "f" is the objective f.l., R1 the front surface radius and
R3 the third surface radius (assumed the one about equally or
slightly more sensitive than the second surface). So, better
approximation of the limiting wedge "T" in inches is:
T~(wF^2)/3667c
In my checkups, it comes within a few percent from
raytracing results. Normal glass indici variations can be
neglected. Other than Fraunhofer doublet configurations will
likely have different wedge sensitivity. The Steinheil, which
for all else equal has the radii roughly 1/3 shorter, has
roughly twice the sensitivity of the Fraunhofer.
A quick look at the wedge caused color separation in the focal
plane. For the linear wedge "t" in inches, it is given by
s=18,900dt
in Airy disc diameters, with "d" being the index differential.
With commonly used glasses, the F-C differential is ~0.008
for the crown, and ~0.017 for the flint element. For an f/15
objective with 0.0022" wedge at the 3rd surface, it would
result in 0.7AD F/C separation (blur centers). That should
be acceptable for achromats, in general, although I can
imagine one could prefer somewhat less of spectral
separation.
Vlad
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End of ATM Digest, Vol 28, Issue 16
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