Re: ATM thermal expansion coefficient of plate glass

[3D@taconic.net]

FROM: Bob May
DATE: 06/05/2002 09:17:24

> When you look at expansion coefficients, you'll find that they're
dimensionless numbers!  Thus you can say inches/inch or 
whatever you wish! <

I copy. So I can expect something in the ball park of 10 
microns expansion per meter per degree C for some types of 
plate glass, right?


> I might also note that when you replicate a grating, you also end up
replicating it's flatness.< 

I take your point. Also a lot depends on getting a flat 
coating down onto the receiver layer. Optometrics wasn't 
divulging any trade secrets when I was inside their shop. I 
have seen spin coating work pretty well for electronics 
applications but that stuff was never meant to be tested to 
the wave length of light. 


> I'd suggest that you go and use a grating and test your theory on the sun
(suitable for a small telescope and moderate sized grating) or 
on one of the
bright stars that is up at this time of the year to find out 
how things are
going to be. <

Yes, I know. I'm discussing getting my hands on a 1 inch 
square grazing incidence hologram from ImEdge Technologies for 
the sun test. This will prove that the Dittoscope can make a 
spectograph with no moving parts, but that grating's tiny size 
will do nothing to dispel concerns about specifications for 
heat expansion and surface flatness in gratings that are 10's 
of meters long.

> We can discuss how to do this for years but until you actually put some
equipment into the field, we'll never know if it will work.<

It may seem like years, but actually I've been annoying ATM 
for all of two weeks. Given that primary collectors haven't 
changed in 350 years, this doesn't seem like a major chunk of 
discussion space. The motivation is to get a fix on the design 
so I can make both plans and demonstrations. I do experiments 
in advance of asking for help. I've satisfied myself that the 
grazing incidence mode can be nearly as efficient at normal 
incidence. That was a pleasant surprise. 

Tom
"Make me clever long enough, and I'll move the earth."
             apologies to Archimedes