Lens material

Hello!
I'm new and enjoying my 4.5 inch reflector,but its been cloudy of late, so I'm researching my next scope. I may have a bit of aperture fever but I'd really like to know the answers to these:
1. Does a lens have to be glass? In a reflector it gets covered with aluminum so why not use something cheaper?
2. Why not use aluminum shape it and then polish it?
3. Pyrex seems to be the glass of choice. Why not permanently borrow Mom or the Mrs. pie plate and grind that?
I'm sure there are reasons or people would be doing it, but I'm new(and cheap.. er frugal) so I had to ask. Thanks, Dean.

Dean Rouncville said:

Hello!
I'm new and enjoying my 4.5 inch reflector,but its been cloudy of late, so I'm researching my next scope. I may have a bit of aperture fever but I'd really like to know the answers to these:
1. Does a lens have to be glass? In a reflector it gets covered with aluminum so why not use something cheaper?
2. Why not use aluminum shape it and then polish it?
3. Pyrex seems to be the glass of choice. Why not permanently borrow Mom or the Mrs. pie plate and grind that?
I'm sure there are reasons or people would be doing it, but I'm new(and cheap.. er frugal) so I had to ask. Thanks, Dean.

Glass is easy to work and takes a good polish. If you use Pyrex, it has a low coefficient of expansion, and so changes shape very little with temperature changes. Before glass, mirrors were made from a mix of copper and tin ("speculum" mirrors). These mirrors tarnished fairly quickly and had to be redone often.

Folks have made mirrors of aluminum. I believe it is more work, and aluminum has a very high coefficient of expansion.

Someone did make a mirror from a Pyrex pie plate. A possible issue is incomplete annealing. Good Pyrex blanks are precision annealed to remove stress from the blank. This prevents it from distoring with temperature changes. Pie plates are unlikely to be precision annealed, although any Pyrex material has been annealed to prevent the internal stress from breaking the dish with tempertature changes.

Folks have even made mirrors from Obsidian.

Clear skies, Alan

1) The telescope mirror could be made out of
anything, it does not have to be glass. But
opticians are very good at working glass, it
is cheap, and it polishes well. Why use anything
else?

2) Aluminum is very difficult to grind and polish.

3) It needs to be thick (diameter to thickness
ratio 6 to 10 range) for rigidity. Mom's pie plate
would bend too much under its own weight. It also
needs to have internal stresses relieved (annealed)
so it does not change shape over time.