Testing
Once the mirror is fully polished and shiny, it is ready for
testing. Though it is still clear glass, it is still a partially reflective
mirror (see the image of the moon through
un-silvered telescope). This is the point where the worker determines how
good or bad a job he or she is doing. In the past, telescope makers mounted
their mirrors in a telescope, aimed them at a bright star and interpreted the
fuzzy diffraction spot surrounding the star. Based on the image, one could
subjectively guess what might be wrong or right with the mirror.
Fortunately, the modern testing apparatus is far more
accurate and far easier to set up. A light bulb is placed at the center of
curvature of the mirror, but slightly to the side. Ideally, if the mirror was a
perfect spheroid, all rays from a point light source will bounce back to the
center of curvature. This can be explained by the fact that every ray
originating from a center of a circle will intersect the circle perpendicular
to the tangent (see the diagram).
The
testing apparatus has a knife-edge that is used to cut light rays. If the
mirror is a spheroid, as described above, the light will converge to a single
point. If the knife-edge cuts across this point, all of the rays will be
extinguished at the same time. Therefore, if one looks at the mirror behind
this knife-edge, he or she will observe a uniform darkening across the mirror.
In reality, the process is not instantaneous because a true point of
convergence is never obtained.
If one obtains the
perfect spheroid and has a relatively small mirror diameter, one may stop at
this point and be satisfied with a decent mirror; however, rarely by chance can
someone create a perfect spheroid. There are usually imperfections of some sort
or another, but the worker need not dismay. For one thing, a small deviation
from the spheroid, called a paraboloid is desired (See paraboloid section).
If the mirror is not
a spheroid, the rays do not all converge to one point. Some converge in front
of the radius of curvature and some behind. If the knife-edge cuts the light
rays in front of the convergence, light from one side of the mirror is blocked.
If the knife cuts behind the convergence, light from the other side of the
mirror is blocked (See an illustrated explanation).
If the mirror surface deviates from a spheroid, the rays converge from different
parts of the mirror at different places. This creates a shadow pattern visible from behind the
knife-edge.
One must go back to
polishing to correct all imperfections and to obtain the exact paraboloid. The
correct paraboloid will have a shadow pattern that changes as the knife-edge
moves closer and farther from the mirror. There will be an inner depression
apparent that grows and shrinks with movement of the knife-edge. If one
measures the knife-edge movement to one thousandth of an inch, this corresponds
to mirror measurement of millionths of an inch.
|
|
|
|
|
|
|
|
