[ATM] 20" *glass* tool

[asw45 <asw45@student.canterbury.ac.nz>]

Hi everyone,

I am making a 520mm (20") telescope mirror out of 30mm thick annealed plate 
glass sourced from Sydney Glass, Australia.

I plan on hogging out the blank using a 8" cast iron barbell weight that I 
used for hogging an 18" by 3/4" blank last summer. It still has a nice smooth 
surface and a slight spherical curve on the outer ring. I then have a number 
of options regarding tools and I am hoping I can find some advice. I have the 
following materials:

500mm (19.7") glass 'blank' 3/5" thick 
410mm (16.1") disc of grade 304 stainless steel 3/5" thick
2 bits of 3/5" thick particle board left over from the stand.
1" square porcelain tiles, 50lb of dental plaster, and 17 lb of portland 
cement.

With good temperature management, would the steel be suitable for a pitch lap 
foundation? I'm quite keen to use the glass as a base only and trepan a 10" 
blank out of it later. So, what would be the best way to use these materials 
for tools?  I'm thinking of making a classic tile tool out of dental plaster, 
portland cement, and the "secret sauce" I've heard about and attaching the 
tiles with parrafin wax.

Also, I believe that the only fundamental difference between Dental Stone 
(Hydrostone) and Dental Plaster (HydroCal) is the water/cement ratio. Dental 
plaster is 100% gypsum, or calcium sulphate hemihydrate. Dental stone is 
usually a mixture of 5-10% portland cement and 90-95% gypsum. Apart from this, 
the difference lies entirely in the recommended water/cement ratio. Until 
recently, when clever chemical admixtures became common, the only difference 
between 15 MPa site concrete and 50 MPa concrete used for tall towers was the 
water/cement/aggregate ratio. The company that manufactures HydroCal and 
Hydrostone, specifies Hydrostone as 30ml water / 100g powder. HydroCal, or 
dental PLASTER, is 60ml water / 100g powder. So you should be able to make 
dental 'stone' from a mixture of cheap dental plaster and portland cement 
merely by using a water/cement ratio for Dental STONE. I'm going to try this 
when I make a tool. Any comments on this?

What is a good focal ratio? I don't want to go faster than f/5 as I've never 
figured a mirror before. What is your experience of the benefits/drawbacks of 
f/5 as opposed to, say, f/6 (easier to figure :-) I plan on using the scope 
for visual & webcam use intially and later (when funds allow!) to mount a 
cookbook CCD or similar. I'd also really really like to make an equatorial 
mount with the mirror RTV'd to a 27 point cell. Is this possible?

I'm sorry this is so long. This is my first post here - I'm 19, and currently 
a 4th year physics/engineering student in Christchurch, New Zealand. Anyone 
live near me?

Thanks for your help!!

Alan Williams

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