Cassegrain Optical Design

An astronomical telescope uses a collection of optical components--mirrors and lenses--to collect light from faint, distant sources (such as stars and planets) and focus that light so that it can be viewed by an observer or measured by an instrument.

Many different configurations of these components have been designed over the centuries; each has its own strengths and weaknesses.  Most medium and large telescopes used for research--including the 0.8-meter telescope--are configured as Cassegrain. 

As with other reflecting configurations, the Cassegrain uses a concave mirror as its light collection agent.

Light from a celestial source reflects off the mirror and converges at a position somewhat in front of the mirror.  This position is called the "prime focus", and the focal length of the mirror is the distance from the center of the mirror to the prime focus, along the optical axis.

Now, an image of the light source is formed at this location. Theoretically, an observer could view this image; however, this is not altogether practical as the observer would end up blocking much of the light in the process.  So the image is of little use at this location.  Thus, the image needs to be transferred from the prime focus to a location where it can be viewed.

A cassegrain telescope accomplishes this with a second mirror, somewhat smaller than the primary mirror, that will intercept the light before it reaches the prime focus and direct it back down the telescope.

This secondary mirror is slightly convex, which diverges the light such that the image is transferred to a location behind the mirror.

Ironically, the image cannot actually form now because it is blocked by the primary mirror.

This allows the light to go through the back of the primary mirror, and it finally converges to the focal point.

It is here where the image is formed, and here where we would place an eyepiece, or camera or other instrument to record the light.