Mirror Design

A Newtonian telescope mirror is designed out of a slab of circular glass, such as Pyrex, or a circular slab of ceramic material. Pyrex glass is more inexpensive, however it often suffers from expansion and contraction problems due to thermal equalization. The ceramic material, however, is not as susceptible to temperature changes. Hence, in cold environments, the ceramic mirror is obviously the best choice since it will hold its figure more precisely over drastic temperature changes.

As for the figure, a Newtonian mirror is typically formed in the shape of a parabola. This makes perfect sense, as a parabolic mirror would have a single focal point that is located directly over the center of the mirror. To see a diagram of how the light is reflected internally in a Newtonian telescope, refer to the optical paths section under "Newtonian Telescope". For reflection, the mirror is coated with a layer of aluminum oxide or silver that typically has a sulfur dioxide coating for protection (The Meade UHTC Group). The mirror requires this protection because the aluminum is applied to the top surface of the mirror, as opposed to the protected aluminum-backed mirrors we are accustomed to in our bathrooms. Without this top coating, the aluminum would simply rub off, yielding a worthless mirror.

Performance

There are various characteristics that determine the performance of a parabolic primary mirror, as is seen in the Newtonian Reflector, Schmidt Cassegrain, and Maksutov Cassegrain telescopes. One of these aspects is, of course, how parabolic the mirror actually is. If the mirror is a perfect paraboloid, then all of the light that hits the mirror will be reflected into the eyepiece in the shape of a perfect cone. However, living in a real world with many imperfections, the shape of the mirror can't be a perfect paraboloid. Instead, sometimes the mirror is closer to the shape of a sphere or closer to the shape of a hyperbola. This results in what is termed under correction and over correction, respectively. A diagram of this phenomena and a specific description can be seen below.

In the under corrected situation, the rays that hit the more spherical portion of the mirror form a new focal point that is behind the parabolic focal point (closer to the mirror). In the over corrected situation, the rays hit the more hyperbolic region of the mirror, causing those rays to make a new focal point farther away from the parabolic focal point of the mirror.

Another issue with mirrors of this type is what happens to the glass while it is cooling down. While the mirror is cooling down, it is actually cooling down unevenly. This causes the mirror to contract differently in varying regions. For this reason, mirrors are typically created with a slight amount of under correction. (A 10'' Newton Telescope).