From: Alfio Puglisi (puglisi@arcetri.astro.it)
Date: Tue Apr 08 2003 - 04:58:12 MDT
On Mon, 7 Apr 2003, Mike Lorrey wrote:
>
>--- Adrian Tymes <wingcat@pacbell.net> wrote:
>>
>> ...it's a *lens*. A rather obvious type, it seems to
>> me. Yes, lenses have all kinds of uses; this is just
>> putting a lens inside a carefully-created flat sheet.
>> I fail to see the significance over what we already
>> had, for instance how it can significantly improve
>> performance over presently available lenses in any of
>> the areas you mentioned. Could you explain what I am
>> failing to grasp, please?
>
>The reason that most astronomy today occurs with reflector telescopes
>rather than refractors is that the result of light passing through
>different thicknesses of glass at different points in lenses, as well
>as the curvature of the lenses themselves, causes reductions in
>resolution. Note how a prism bends light. Blue wavelengths bend less
>than red ones. As a result, in a refractor telescope, bluer wavelengths
>have a longer focal length than redder wavelengths.
This problem can be corrected with additional optics, as it's routinely
done on small refractors with acromatic lenses.
What really limit refractor telescopes is the lens size and weight, as far
as I know no refractor over 1m has ever been built, because a bigger lens
would crush under its own weight.
The weight problem (this time of the glass mirror) also affected reflector
telescopes around 5 or 6 meters, and in fact that was the biggest size for
a telescope for many years. More recently, segmented mirrors and active
optics (a thin mirror suspended by actuators) allow to build bigger
telescopes: most of the new ones are about 8 or 10 meters, and there are
designs for 30, 50 and 100 meters.
Ciao,
Alfio
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