Optics question

"Craig Franck" wrote in message .. .

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?

Coma is caused by the geometry of optical surfaces, resulting in the
bundles of parallel (normally) off-axis light being focused
differently by different zones of one or more optical surfaces. In
general, only the very center of the surface focuses such bundle of
rays into an on-axis point (talking geometrical optics); every next
concentric zone on the optical surface focuses it into an off-axis
centered circle. Both circle diameter and its off-axis shift increase
with the zone hight, reaching the maximum for the outer edge of the
optical surface.

For a concave mirror, the circle radius is given by hr^2/16F^2, and
the off-axis shift of its center is twice as much, hr^2/8F^2, with "h"
being the off-axis distance of a point in the focal plane, "r" the
aperture radius normalized to 1, and F the F#. The entire length of
comatic blur is 3h/16F^2. For example,
a 200mm f/5 parabola would have comatic blur length 1mm off-axis of
0.0075mm; central area of the mirror would focus at the axial point;
concentric zone at half the mirror radius (so r=0.5) would focus into
a circle of 0.000625mm radius, with its center shifted off-axis by
0.00125mm. And the edge zone (r=1) would focus into a circle of
0.025mm radius, shifted off-axis by 0.05mm.

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 is not an "edge effect", and not a result of "ray bending" etc.
Those are popular misconceptions. What causes diffraction can be
illustrated by a converging wavefront, whose every point emits waves -
so called wavelets - in all directions. If you replace those wavelets
by "raylets", you see that all raylets coming to the focus point have
identical path length, regardless of from what point on the wave front
they arrive (this is because the focal point is a centar of the
wavefront sphere). Since the path length is identical, all these
raylets meet in phase, resulting in maximum wave interference and
highest light intensity. For points slightly off-axis in the focal
plane, the raylets don't have identical path lengths, and don't meet
in phase. The interference and intensity weaken, dropping to zero at
the Airy radius (first minima), then partially recover through the
first bright ring, hit the second minima, and so on, producing ever
fainter rings.

Any obstruction placed in the light path will alter net interference
of the raylets in the focal plane by blocking out portion of the
wavefront.

Vlad