Performance

There are many characteristics that determine the performance of a telescope. They are below with a description.

Chromatic Aberration

This phenomena is caused by the fact that different wavelengths of light bend in materials at different angles (The Refraction of Light Part II). For instance, blue bends in glass with a different angle than red does due to the fact that blue light has a shorter wavelength than red light. This is the main reason that we can see "the green flash", but unfortunately, it also is the main reason for the hated purple fringe that surrounds most bright celestial bodies in the more inexpensive lines of refracting telescopes. In order to correct this phenomenon, either expensive fluorite glass or assemblies that have multiple lenses that bend the different wavelengths of light so that they will have the same focal length must be utilized. A diagram of this problem can be seen below.

Image courtesy of Refraction of Light Part II

Air Turbulence

Air turbulence within a telescope's optical system is the number one killer of optical performance for a perfectly collimated telescope. This air turbulence is caused when a telescope is brought from a warm environment to a cold one or vice versa. When the telescope is brought from the warm to the cold, the air inside the tube is cooling down, but it is cooling down unevenly. Air in the top region of the tube near the sides will get cold and become denser, moving down to the bottom region. This cycle continues and the end result is that, when one looks at a planet or star, it will look blurry and detail-less. One will see "waves" moving across the image, like the heat waves one sees rising up from asphalt on a hot day.

Collimation

If the optics within a telescope system do not align perfectly, then the cone of light that hits the eyepiece will not be aimed dead center. This will cause a certain "fuzziness" to be introduced to the image and could even cause double images in severe cases. It can also cause stars to be "smeared" or "streaked" at the outer edges of the field of view.

Vignetting

Vignetting occurs when the field of view at the eyepiece is not fully illuminated. This can be caused by several factors, the primary of which is that the diameter of the tube just before the light cone reaches the eyepiece is too small. This size limitation clips off the outer edges of the light cone, causing a darkening around the edges of the field of view. (Vignetting Caused by the Limiting Aperature of Various Tubes and Adapters)