What is a Telescope Diagonal?

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  • Written By: Jessica Ellis
  • Edited By: Bronwyn Harris
  • Last Modified Date: 15 October 2019
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A telescope diagonal is a type of mirror or prism that allows for easier viewing thanks to the carefully angled reflector. The size, quality of material, and angle of the telescope diagonal can all help improve the reflected image that the user sees through the eyepiece. The telescope diagonal dates back to the earliest days of telescope invention, and is believed to have first been used by Isaac Newton in the mid-17th century.

A diagonal's primary purpose is to reflect light at a specific angle so that the user may view it. Without a diagonal, the only unobstructed viewing position would be directly behind the main lenses of an optical telescope. When looking at high-up vistas, such as stars, the user would essentially have to be below the telescope, or even lying underneath it. To make viewing easier, the image from the main lens or mirror is reflected by the diagonal to an eyepiece, which is often placed on top of the telescope and off to one side.

The Newtonian telescope is often cited as the first to use a telescope diagonal. This reflecting model uses a concave lens as the primary mirror and a flat diagonal mirror to reflect the light at a 90 degree angle, allowing the user to look down into the scope instead of getting behind it. Newtonian telescopes, first used to view the moons of Jupiter and count the rings on Saturn, are still used in modern astronomy.


Today, a telescope diagonal is often an essential component of an optical scope, and has been standardized into a few basic sizes. The standardization allows an optimal relationship between the diagonal mirror and the eyepiece, in order to create a clear, easily viewable reflected image. There are three sizes of diagonal common in amateur-use telescopes: .965, 1.25, and 2 inches (2.45, 3.175, and 5.08 cm, respecitively).

In addition to differentiation in mirror size, the angle of the telescope diagonal may change between models. Telescopes meant for stargazing or astronomical observation typically feature a 90 degree angled diagonal. In other telescopes meant for birdwatching, whale watching, or other terrestrial viewing, a 45 degree diagonal is common. The degree of reflection impacts the placement of the eyepiece.

Because the diagonal reflects the initial light taken in by the front lens, some light will almost always be lost in the process. The material used in making the reflecting lens can help increase the quality of the image, though it cannot replicate it exactly. Special coatings on the lens increase the amount of light that reflects; according to some experts, properly coated diagonals can reflect about 90% of the light received.


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