Re: Optics question
From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 06/17/04
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Date: Thu, 17 Jun 2004 20:59:09 GMT
On Thu, 17 Jun 2004 20:09:29 GMT, "Craig Franck" <craig.franck@verizon.net>
wrote:
>In 4.2.2 they discuss coma and attribute it to "the intersection of
>rays not being symmetrical." Shouldn't "off axis light" come into the
>telescope in a symmetrical fashion when confronting an evenly distributed
>light source? If one were to rotate the lens or mirror, would the coma
>rotate as well?
In most cases, the optics in a telescope are rotationally symmetric. That means
that rotating elements does not affect the image. The direction of the coma is a
function of the position of the off-axis source.
>WRT diffraction spikes, if the secondary mirror and struts are not in the
>plane of focus, why would the diffraction effect occur where the mirror
>itself is not visible? And if it's an "edge effect," way doesn't the edge of
>the telescope tube diffract the light as well in a way that is visible?
It does. In a Newtonian telescope, there is diffraction from three sources: the
vanes or stalk, the central mirror, and the outside of the aperture. The latter
two are visible in the form of diffraction rings around a point source- what is
commonly referred to at the Airy disk. They are perfectly visible, but may go
largely unnoticed because they are rotationally symmetric with the star. You see
them in an unobstructed telescope from the aperture alone. The diffraction
spikes from the stalk or spider are much more obvious because they are oriented.
>Also, (and this may sound silly to those more knowledgeable than I)
>isn't there a compound glass that has the property of being neutral to light
>in one direction and reflect it 90 degrees from the other? It would be like
>a two-way mirror with the mirror embedded into the prism at an angle.
>It's such a simple solution to having an obstruction that this type of
>prism must present insurmountable problems.
I'm not sure what you are thinking of here- a material that passes light in one
direction but reflects it in the other? It is worthwhile to remember that
outside the quantum domain, optical materials are very symmetric. That means
that you can trace a ray through a system in either direction and the results
will be the same. Another way of thinking of this is that if you look at a ray
trace, there is no way of determining which way the light was traveling.
In reality, the "problem" of an obstruction is greatly overstated. In most
cases, the effects of an obstruction are nearly impossible to detect. The few
cases where an unobstructed design provides better results correspond rather
nicely to cases where size of aperture isn't too important, and unobstructed
designs are practical.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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