Re: ATM About making a glass blank

["Bob May" <bobmay@nethere.com>]

Basically, you really don't need to use optical glass for doing a reflector.
If you want to do a refractor, the best is to go pay the cost for the
optical glass from one of the suppliers in Asia, Europe, US or other places
where it is sold.  For refractors, you really do want the melt index papers
so that you can finish the design with the actual refractive indicies of the
glass you have - for an excellent scope, that additional step will be quite
important.
Thus, for the typical reflecting telescope, the use of ordinary plate glass
will make a sensibly perfect scope without problems.  The 25mm glass will do
just fine for any scope of up a half meter if you grind and polish it right.
Scopes in the third meter class and smaller will be done easily with no need
to worry about grinding or polishing astigmatism into the glass.
The spending of money for anything more pricey in the way of ordinary glass
like opthalamatic glass or BK7 (as mentioned in the post - usually just
refined versions of the plate glass more than anything else when you
actually look at what the glass is made of) is going to be nothing more than
an excess expense which will do nothing for the quality of the mirror.
Going to Pyrex or other of the low expansion coefficient glasses will help a
little for the ability of the scope to handle rapid changes in temp but once
the scope is settled out, the differences are basically null.
My recommendations?  Go find a glass supplier in your area that has 25mm
thick glass avaiable and get a 250mm circle and go have fun grinding it.
You first scope will not have that high of a quality where you will have to
worry about anything else with the glass.  The next step if you want to do a
thick mirror is to go to one of the low expansion glasses like Zerodur and
so forth so that the scope won't care about temp changes as much.  I'll also
note that thin glass will settle out faster than thicker glass will.  It is
mostly the thickness of the glass that determines how fast and how regular
it will settle to the right temp.