How do you make large lenses for telescopes?

On Wed, 11 Oct 2000 you wrote:

> Hi I am Jeremy. I am a 7th grade student and I need some help is science. How
> do you make the huge lens in the telescope and what kind is it? Convex or
> Concave.
>

Hi Jeremy,

Most telescopes built these days use mirrors instead of lenses. The
mirrors are concave. Those telescopes that are built with lenses (called
"refractors") use convex lenses.

The mirrors and lenses are made the same basic way as they were three
hundred years ago - they are ground into shape from large chunks of
glass. Molten glass is poured into a mold that is the basic shape as the
desired mirror or lens. Then, various machines are used to carefully
grind off extra glass and form a perfectly-shaped mirror.

If you would like to read how amateur astronomers grind their own
mirrors, try reading the following web pages:
http://users.uniserve.com/~victorp/mirror.htm

Thanks for writing! Please let me know if you have more quiestions.

Sincerely,
Kurtis

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