Wasn't it Jellby who wrote:
>Among other things, sascha wrote:
>
>> I was refering to another effect which makes objects appear blurry in a
>> telescope. I'm not sure what causes this effect, but I think it is
>> bacause the mirror (a paraboloid) will focus only parallel rays (the
>> object to be viewed must be either a point or ininitely far away). A
>> planet is neither of both, so not all the light from the planet will be
>> focused an the same plane, resulting in a blurry image. It gets better
>> if you use a larger mirror (I guess that's the reason why they use 8
>> meter mirrors in the VLT :-)
>
>The main reason for using large telescopes (with large mirrors) is they 
>receive more light and so allow astronomers to study fainter and more 
>distant objects.

But there is also the effect of diffraction limited resolution.
Diffraction can cause two closely spaced light sources to appear to be a
single source. It's purely a wave effect, so a ray tracer (even one that
fires photons) won't naturally simulate the effect.

By the time the light from Saturn gets here, the wave fronts are
effectively planar. The wave fronts associated with two different points
on the surface of Saturn are at a slight angle to each other. To a first
approximation (further approximations get quite complicated[1]) for a
perfect mirror, if the angle is insufficient to cause the waves to be in
phase at one edge of the mirror and half a wavelength out of phase at
the opposite edge, then the two points cannot be resolved.

With a wider aperture, the angle between the waves that gives a half-
wavelength difference is smaller and so closer objects can be resolved.

[1]<http://www.pha.jhu.edu/~jlotz/aoptics/node2.html>

-- 
Mike Williams
Gentleman of Leisure

Post a reply to this message