What is a Reflecting Telescope?

by Alexander Sutton

A reflecting telescope is one of two kinds of optical telescopes, refracting and reflecting. A reflecting telescope uses a single or combination of curved mirrors which reflect light to form an image. The reflecting telescope was invented in the 17th century as an alternative to the refracting telescope. The reflecting telescope was made to correct the server chromatic aberration that was part of working with a refracting telescope. Reflecting telescopes also create other aberrations, but they allow for a very large diameter which is helpful with research telescopes. The reflecting telescope comes in many designs which improve image quality. Reflecting telescopes are often referred to as “catoptric” telescopes.

There are many technical considerations to be aware of when using a reflecting telescope. Various aberrations are caused by the reflecting telescope, such as that the inner and outer zones of the telescope may not share the same focus. Spherical aberration can be eliminated with non-spherical mirrors.

Other aberrations are also common with reflecting telescopes. A coma may appear which is when a variation of telescope magnification may cause radial smudging which worsens at the edge of the visual field. Astigmatism may also occur around the focus of the aperture. Distortion of the field of view does not affect objects, but may harm sharpness. The best image plane is in general curved. This may not correspond to the detectors shape and may later cause focus errors. There are other reflector designs referred to as catadioptrics that correct some aberrations.

The majority of research-grade telescopes are reflectors. This is because, unlike a lens, the mirror does not need to be free of imperfections; it merely needs to be polished. Also, light of different wavelengths travels through a medium other than a vacuum at different speeds. The reflectors also work in a wider spectrum of light because certain wavelengths are absorbed when passing through lenses in comparison to reflecting off of them.

Alex Sutton has worked in the telescopes profession for nearly 11 years. For more information please visit telescopes

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What is a Refracting Telescope? by Alexander Sutton

A refracting telescope uses a lens as its objective to form an image. The refracting telescope was designed for used in a spy glass, but is also used in other devices such as binoculars and telephoto camera lenses.

There are two principles to a refracting telescope, an objective lens and an eyepiece. These two principles gather more light for the human eye and helps to focus and present it in a brighter, clearer, and magnified virtual image. A refractive telescope bends light to cause parallel light rays to converge at a focal point.

Galilean Telescopes are refracting telescopes. Galileo came up with the design in 1608 and uses a convex lens and a concave eyepiece lens. The Galilean telescope magnifies objects up to thirty times. The Galilean telescope was the first telescope to see the planet Jupiter and its moons.

The Keplerian Telescope, invent by Johannes Kepler in 1611 improves upon Galileo’s design. The Keplerian Telescope uses a convex lens in opposition to Galileo’s concave lens. This allows for a broader field of view and greater eye relief. However, the view is introverted.

An achromatic refractor is a refracting lens which was invented in 1733. The design over came the need for very long focal lengths and used two pieces of glass with different dispersion to limit the effects of chromatic and spherical aberration. Each side of each piece of glass is ground and polished, at which point the two pieces of glass are assembled together. Achromatic lenses bring two wavelengths together and focus them on the same plane.

Alex Sutton has worked in the telescopes profession for nearly 11 years. For more information please visit telescopes

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