This is the fourth of a series of articles about astronomy equipment to help you decide what you might like. If you have questions about a specific piece of equipment, and how it compares to alternatives, send a note to me at and I'll see what I can find out.

Now that we have a telescope with lenses, let's focus on the Star Diagonal. Most refractors and compound reflectors need a separate device to turn the light 90 degrees from the tube of the telescope. (This is part of the design of standard reflectors.) There are two basic types of star diagonals those that use a mirror, and those that use a prism. Which kind should you get? This is not a simple question to answer. There appears to be very little data. Here are some rules of thumb that seem to make sense.

- Rule #1: Mirror Star Diagonals of similar quality from the same manufacturer tend to cost significantly more; almost double the price, than a comparable Prism Star Diagonals.

Rule #2: Larger more expensive telescopes tend to come with Mirror Star Diagonals.

- Rule #3: Buy quality optics the best optics you can afford. Quality varies widely from one manufacturer to the next. Whichever type you decide to buy, buy a good one.

- Rule #4: Light being redirected by a mirror has only one surface to interact with. With a prism, the light has to interact with three surfaces. Mirror surfaces are usually ground more precisely than are prism surfaces.. Prism Star Diagonals lose light transmission.

- Rule #5: A mirror scatters more light than a prism. Scattered light is lost light and that is not a good thing. So be sure to buy quality. A dielectric coating is far better than a standard aluminum coating on a mirror.

- Rule #6: The size of a prism affects the amount of chromatic and spherical aberration that it produces. Bigger is better. Prisms can cause serious problems for telescopes of F/8 or less. Prisms are less of a problem for F/9 and greater.

- Rule #7: I have not done a comparison myself; these comments are from others who have! "When I switched to a mirror diagonal, the light scattering glow around the planets was almost entirely eliminated and contrast was significantly enhanced." "My dielectric coated mirror diagonal gives me whiter and brighter images than my standard coated diagonal."

Taking all of this into consideration, our ideal telescope should probably have a high quality mirror star diagonal.