Re: ATM An $18.00 Null Test

[iss@pvtnetworks.net]

Brad Davy writes

>Could a
>Fresnel lense (such as from an overhead projector) be used to produce a
>parallel light source to test smaller mirrors? 

A Fesnel lens is a plano-concave lens cut up into tiny annular
rings and 'collapsed' along the optical axis to reduce the
thickness of the lens, thus making it lighter. All of the optical
power (light bending) of a lens occurs at the surface, not inside
(unless it has a gradient refractive index).

Fresnel lenses are very crude optically, usually made of
plastic moded against a lathe cut metal mold, and are
mostly used as magnifiers and light condensers, as in
overhead projectors, big screen tv's etc. They are
useless as imagers and would be several orders of magnitude
too crude to produce a collimated beam suitable for null testing
mirrors or lenses.





Andy Saulietis / ISS Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designed mount/telescopes & Machine work
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