Re: [ATM] Blank question

[Mark Holm <mdholm@telerama.com>]

The exit pupil objection to huge, low power visual telescopes is 
completely valid.  The only way around it is to replace your eye with a 
detector and view the images indirectly.  For a whole bunch of purposes, 
this is the best way to go.  I think, though, if you want to give Jane 
Q. Public some of the thrill and feel of astronomy, you still are best 
off to get her looking through a real telescope out under the night 
sky.  Pictures in books, or images on computer screens do not convey 
even 1% of the reality of actually being out under the sky, looking up, 
and then peering through an eyepiece to see actual photons that traveled 
millions of miles, light years, whatever to get to you.  Understanding 
involves a lot more than looking at abstract images and reading words.  
Breathing the night air, feeling the chill, seeing how tiny Mars looks 
even when magnified in a rather powerful telescope: these are the things 
that build up real understanding.

The various material suggestions that started off this thread are all 
obvious and have been investigated.  There are very real problems that 
prevent each of them from working.  Although I certainly do not wish to 
imply that replacements for glass are impossible (Peter Chen has come 
very close with carbon fiber and resin) I want to go on record for 
beginning and low experience atms.  You should stick with glass.  It has 
a whole bunch of favorable properties for optics that are not duplicated 
by any other known material.  Here are a few.

1. It is non crystalline.  This makes it easier to get a good polish and 
helps keep it isotropic and homogeneous.  All your rocks are full of 
large crystals with nasty huge grain boundaries and all kind of 
inclusions.  Ever wonder why you only see large, natural, single crystal 
specimens in museums behind substantial barriers and none of them is 
large enough to make even a middling atm mirror?

2. It is isotropic, and can be produced in large pieces with controlled 
shape and good homogeneity.  These are not properties to be taken 
lightly and are not that common.  Actually, even glass is not always as 
isotropic as we need.  That is why good mirrors and lenses usually need 
careful annealing.

3. It is rather chemically inert.  This makes for very good aging 
properties, makes processing relatively easy, and makes cleaning, 
coating, coating removal, etc. much easier.  Try removing an aluminum 
coating from a speculum mirror without messing up the speculum.  100 
year old, and older, mirrors and lenses are still producing fine images 
routinely.  How many 100 year old chunks of metal do you know that still 
have highly polished surfaces and are not made of gold or platinum.  
Seen any 100 year old concrete?  Close up, it usually looks like hell.

4. It works well in high vacuum and tolerates fairly high temperatures 
and fairly energetic beam processes.  These properties are a big help in 
coating.

5. Compared with many of the alternatives, it is pretty cheap.

6. Thermal expansion, though not always as low as we would like, is 
still lower than most materials.  If you don't believe me, look it up.  
Metals, for instance are a good factor of 10 higher.

7.  It is mostly non toxic.  Some of the speculum recipes, for instance, 
include a healthy dose of arsenic.  The materials needed to work glass 
are also mostly non toxic.  What toxicity problems there are are easily 
avoided in atm work if you know a few things and don't insist on using 
non recommended materials.

8.  One of the most important for atm's: Glass is easily shaped to high 
precision using almost unbelievably crude materials and methods.  In 
other words, you can make a good, or even excellent, mirror in your 
basement shop without a bunch of expensive stuff.  Excellent mirrors 
have even been made on dining room tables, though the cost of divorce 
lawyers sometimes has to be counted against this.

9. Compared with a lot of possible alternatives, glass's density is 
actually fairly low.

10. We have reliable, inexpensive methods for putting a high 
reflectivity, long lived, uniform thickness, low scattering coating on 
glass.

What are glass's bad points?

1. It is on the brittle side.  Don't whack it.

2. Thermal conductivity is low, though this is partially offset by the 
low thermal expansion.

3. Stiffness isn't as good as we would like.  It would sure be dandy if 
the modulus were another factor of 10 higher.

4. Density, although not awful, is not as low as we would like.

-- 
Mark Holm
mdholm@telerama.com


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