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Re: [ATM] 25" mirror possibilities (Chapter 2)
Don't touch the figure of that mirror! It can be warped into a perfect
parabola with stress! Not only that, you can probably shorten the focal
length with a different kind of stress. However, you must not disclose to
the U.S. Department of Homeland Surveillance, Interrogation, and Detention
the amount of stored energy in that stress!
----- Original Message -----
From: <dmikk2@netscape.net>
To: <atm@atmlist.net>
Cc: <chukundur@simoivanov.com>; <simoivanov@gmail.com>; <simo@pobox.com>
Sent: Tuesday, April 05, 2005 2:11 PM
Subject: [ATM] 25" mirror possibilities (Chapter 2)
> Hi everyone,
>
> Late last year I asked the list for some advice on how to best put to use
a 25" full thickness mirror I purchased on Ebay. The mirror is from a huge
"Wright Schmidt" collimator made by Tinsley Laboratories for Boeing. I
greatly appreciated the comments and offers of help. At that time, I was
not even able to safely set up the 180 lb mirror to test it. I've been
reading, thinking, lurking on the list, and progressing VERY SLOWLY, within
the constraints of a "day job", family, etc. Since everyone was so helpful,
I thought I should at least give a progress report.
>
> I've finally gotten a combination storage/shipping crate and test stand
made and just did a very basic Foucault test on the mirror. The results are
something like:
>
> Focal Length: 160 inches, f6.4
>
> Figure: oblate ellipsoid, SC about 1.8
>
> I say "something like", since I really didn't have a suitable location to
test the mirror. I did a basic test with the mirror in one corner of my
garage, facing diagonally across the garage, to a point about 5 feet outside
of the garage door. The thermal characteristics of the mirror, garage
floor, garage air and outdoor air over a 26 ft. 8 inch. path tend to make
the image look more like flames in a fire place than a Foucault test. After
trying the test at various times of the day and night, I finally hit a time
one morning when the inside and outside air temperature rose to the point
where they approximately equaled the mirror temperature, and I got a few
usable views. Comparing what I saw at different knife edge positions with
the images from Michael Lindner's Foucault Simulator program lead to my
"guesstimate" of SC = 1.8. I'm at least reasonably confident the value is
between 1.6 and 2, so it really is an oblate ellipsoid.
>
> Based on the responses to my original post, it appears that the most
practical way to put this to use is to have the mirror refigured to a
paraboloid. However, with a focal length of 160 inches, a conventional
Newtonian would still have the diagonal about 12 feet above the face of the
mirror. In a Dobsonian, the front of the mirror would probably have to be
at least a foot above the ground. At the zenith that puts the eyepiece at
least 13 ft above the ground. Even a very tall person would have to stand
on the top of a 7 ft ladder to view at the zenith. This does not seem to
be safe or practical.
>
> So...what about some sort of folded Newtonian? Some very rough numbers,
that allow reasonable illumination of a 35mm size film or CCD are:
>
> Case 1:
> 6" flat secondary mirror, tilted 15 degrees from the optical axis, 32
inches from the focal plane. This reflects the light path back toward the
side of the "tube" at a 30 degree angle from the optical axis. The focal
point is then outside of the tube, 16 inches from the optical axis of the
primary, and about 28 inches closer to the primary, than the secondary is.
This reduces the height of the focal plane above the primary from 144 inches
to 100 inches. Also, the ladder height decreases from about 7 ft to about 3
ft, with a central obstruction of 24%.
>
> Case 2:
> 8" flat secondary mirror, tilted 10.5 degrees from the optical axis, 45
inches from the focal plane. This places the focal plane 42 inches closer
to the primary than the secondary is, and reduces the height of the focal
plane above the primary to 73 inches. The ladder height needed by a tall
person at the zenith would now just be about 1 foot, however, the central
obstruction is 32%.
>
> Are there any major problems with such folded Newtonians, such as problems
with baffling? Also, both of these possibilities have fairly large central
obstructions. How "bad" would a central obstruction of 32% be?
>
> Case 3:
> The alternative to a folded Newtonian seems to be to actually regrind the
mirror to a shorter focal length, say f4 at a much higher cost and more
coma. For an f4, a conventional 5" diagonal 16" from the focal plane would
give a similar size illuminated field with a 20% central obstruction, and a
required ladder height of about 2 feet.
>
> Any comments or suggestions would be appreciated.
>
> Thanks,
> Dennis
>
>
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