Telescope Diagonals: A Beginner's Guide

Telescope diagonals (angled mirrors or prisms) make looking at the night sky user-friendly by moving the sky image to a place where it is easier to examine. This beginner's guide shows what diagonals are, why they are used and how to choose the best type for your scope. Most diagonals look like two pieces of well-machined metal pipe put together at an angle. Many entry-level telescopes come with a diagonal already in place, but dedicated amateur astronomers often choose a diagonal or two to meet specific sky-gazing needs.

A diagonal is almost essential. Without it, an amateur astronomer might have to lie on the ground to see the celestial object he or she is examining. Telescope diagonals, sometimes called star diagonals, work by transferring the image to an eyepiece, where a person can study the view comfortably from a chair.

Mirrors and prisms are the two main types of diagonals. Neither is better than the other, but each is better for certain telescopes, as Gary Hand explains in this Cloudy Nights article. Users should choose the style that best suits their respective telescopes and viewing styles.

Focal length, objective and focal ratio

The most important factor in choosing a diagonal is focal ratio. The light that enters a telescope is collected at the objective. In a refractive telescope, the objective is a lens that bends the light rays inward. In a reflector telescope, the objective is a curved mirror. The focal length is the length over which the light rays converge into an image. The focal ratio is the ratio of the focal length to the size of the objective. It is crucial in selecting a star diagonal.

A telescope with a long focal ratio is ideal for a prism diagonal. This system will be slower and give better contrast. It is outstanding for observing bright objects like the moon, other planets and double stars.

A scope with a short focal ratio is more suited to a mirror diagonal. This is a faster system with a larger and brighter field of view, according to 'Through the Telescope' by Patricia Barnes-Svarney. Such telescopes work well for looking at distant nebulae, galaxies and star clusters.

Prism diagonals

Prism diagonals use a prism to bend the image 90 degrees. This can cause distortion in the currently popular fast telescopes. One advantage of a prism diagonal is that the image arrives correctly oriented, both horizontally and vertically, assuming an Amici roof prism is used. The orientation of the image matches the sky or the sky map. On the other hand, some prefer to use a simple 90-degree angle prism, which reverses left and right in the image in order to lose less light inside a more complicated prism.

When used in an instrument with a short focal length, a prism diagonal can introduce chromatic aberrations. This is because the prism focuses different wavelengths of light to slightly different points. Images might appear to be fringed with unexpected colors.

Mirror diagonals

Mirror diagonals are the most commonly used. A mirror in the diagonal is set at a 45-degree angle to the rear cell of the scope, reflecting the image to the eyepiece. The image arrives in its correct orientation vertically, but left and right are reversed by the reflection of the mirror. This reversal can make it harder to compare the image to a star chart until a viewer learns to make the adjustment.

Newer mirror diagonals with dielectric coatings that resist deterioration have long, useful lives. Though they tend to scatter light more than prisms do, the difference will not be visible to most viewers.

A quality mirror diagonal loses little light, does not change the color of the image and costs less to produce. Many amateur astronomers choose a mirror diagonal, favoring a fast, modern scope and a diagonal that is durable and relatively economical.


Other factors, such as collimation, can influence image quality. Collimation means lining up the scope so that the image passes though the telescope and the diagonal correctly. If the scope and diagonal are not correctly aligned, the quality of the diagonal will not matter. Many users collimate their own scopes by using simple equipment and instructions from the manufacturer.

Telescope diagonals make viewing a more comfortable experience. With a diagonal in place, the amateur astronomer is freed to fully enjoy the blazing glory of the night sky.

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