Re: ATM thermal expansion coefficient of plate glass

[3D@taconic.net]

On 5 Jun 2002 at 18:08, Ken Hunter wrote:

> Not so much "off topic" as it is unobtainable. IMHO
> Where are you going to get a piece of FLAT glass 150 Meters long?
> You'd have to make it inplace and have it ruled inplace etc.etc etc...
> You're certainly not going to be able to ship it UPS even if you
> downsize to only 50 meters long.
> Your dream may be perfectly reasonable theoretically, but I don't see
> it happening practically.
> AND... If you look farther down the page, the MMT (Multiple Mirror
> Telescope) on Mount hopkins has been redesigned with a single mirror
> because they couldn't make it fuse the images together. It's problems
> were trivial to what you are proposing.

I'll have to start somewhere. Personally, I can't put my hands on plate glass larger 
than commerically sold sizes. I have some cast off pieces from a construction site 
that are 1 x 2 meters. These are not gratings, just plain old glass. I have a 10'  length 
of Spectrasheen grating sent as a sample from Spectratek in Los Angeles 
(www.spectratek.net). Their actually production runs have reached 50,000 feet x 5 
feet, and the stuff costs about $10/sq yard. Obviously, if I can get this stuff to work, 
Ken's opinion on attainability will be further humbled. One thing I can say about these 
embossed gratings is that look to be quite efficient at grazing incidence. They are 
floppy and not intrinsically flat. Pressing them between two pieces of plate glass 
caused pockets of trapped air (bubbles), and the top glass limits the exit angle -- 
which for my purposes must approach 90 degrees from the normal. I am thinking 
about other ways to put these very large replicas into service such as stretching on a 
frame or over the plate glass as a backing. 

Meter square gratings on plate glass can be made holographically. In 1988 I used 
one made by Rudie Berkhout, an artist, to demonstrate the optics behind my 3D 
camera. See

http://www.drillamerica.com/redshift/page6.html

Rudie uses a homebrew bench with a sandbox on tires. His laser is a 35 mw HeNe 
which he keeps going with a fan and a prayer. Rudie is a genius, and I am not saying 
that I can do what he did, but I have seen it done in Rudie's case on what amounts to 
an amateur's budget. He exhibits his stuff in art galleries, museums and lately 
architectural sites. 

http://rudieberkhout.home.mindspring.com/

Rudie's gratings are in silver halide which does not permit grazing incidence, but 
there are further developments pioneered by Michael Metz at ImEdge Technologies 
that make grazing incidence practical for conventional holography. I am trying to get 
ImEdge to loan me a tiny sample grating so I can show the principle of operation at 
this year's Stellafane using old Sol as my star.

The segmentation of a parabolic mirror such as the MMT is more difficult than the 
segmentation of a diffraction grating, since in the case of a grating each module is 
exactly the same as every other module. The segments permissably can be spaced 
apart by any distance just so long as the secondary receiver is in line-of-sight of their 
various re-radiated wave fronts. All that is lost in such sementation schemes is some 
efficiency. Of course, all segments must all line up with each other, but remember, 
once set at declination, nothing moves. A static pose overcomes some of the hurdles 
that the MMT could not clear. There will be adjustments, of course. Adjusters based 
on micrometer mechanics are accurate enough to make the requisite tweaks in an 
alignment procedure that uses a laser, something like Howie Glatter's collimation 
device. It is easy to contemplate active piezo adjusters that further dynamically adjust 
for such things as atmospheric refraction and various temperature stresses. Piezos 
might help in areas of mounting instability anticipated by ATM writer, Jeffrey Rowe, 
who thinks that mounting problems might prove to be the Achilles' heel of the device. 
Again, remember it seems from my calculation (as yet unchallenged by ATM's 
congenital skeptics) that unlike a mirror, the tolerance for flatness is in multiple wave 
lengths for sub Angstrom spectral resolving power. This makes my target for quality 
easier to hit than parabolic mirror tolerances which are typically sub wave length for 
image formation. Come to think of it, perhaps crappy parabolic secondary mirrors 
would work to take spectragraphs in this configuration. Egads, yet even another topic 
for me to analyze mathematically.

Everyone is telling me to put up or shut up. I got this rebuff in spades a couple of 
self-described amateurs doing spectroscopy and exo-planet searches. They didn't 
deign to so much as discuss my disclosures. Mrs. Ditto, who you may know as the 
painter, Beverly Botto, while steering me to ATM also warned me at the onset that 
my idea wouldn't get much traction until I had a model device. I hear you and hope to 
show you what I'm up to in August in Springfield. Stay tuned for that. However, so 
much of this must be done in theory even for that venue in the "innovation" category, 
that I'm likely to go on with my musings. I appreciate your correspondence.

Tom