Why Do Telescopes Use Mirrors? A Very Important Reason

Most people tend not to get excited about mirrors. But astronomers aren’t most people. In telescopes, mirrors have an almost magical power, gathering light which has travelled for thousands, millions, or even billions of years, rendering the invisible visible. 

A large enough mirror can bring even the most remote galaxies within reach. Not all telescopes use mirrors to collect light, but most telescopes make use of mirrors in some way.

In this article, we’ll look at the different types of mirrors used in telescopes and learn how to care for the mirrors in our equipment.

Primary Mirrors

In reflecting telescopes such as the popular Newtonian, the primary mirror sits at the bottom of the optical tube assembly (OTA). These convex mirrors have the all-important job of gathering and focusing light from space. 

Where a flat mirror would reflect the faint starlight back out the tube, the curve of the primary mirror harnesses that light into a cone, the length of which defines the telescope’s focal length and magnifying power. By focusing the mirrored image into a point, the faint light becomes concentrated, producing an image of increased brightness which is further magnified by the telescope eyepiece before entering the pupil of your eye. 

The light-gathering capacity of a primary mirror is a function of its size, which defines the telescope’s aperture. When we talk about an 8-inch reflector, we are saying the primary mirror has a diameter of 8 inches. (Note that the OTA diameter will be somewhat larger.) 

Because image brightness increases with surface area, you get significant gains in brightness as you move up in aperture. For example, a 12-inch reflector will gather more than double the light of an 8-inch. 

Types of Primary Mirrors

Primaries aren’t all created equal. Different curves may be employed in manufacturing the mirror, depending on the type of telescope. 

  • Spherical primaries are the cheapest to manufacture and are often found in low-cost Newtonian reflectors. Their shape does not allow rays from the inner and outer parts of the mirror to focus together, a defect called spherical aberration. However, higher-quality Schmidt-Cassegrain telescopes also employ spherical mirrors, with the aberration being compensated for by a special lens called a corrector plate.
  • Parabolic primaries focus all incoming light precisely and are widely used in Dobsonian reflectors. A parabolic mirror doesn’t guarantee a quality telescope, but it’s a good start. 
  • Hyperbolic primaries are used in more specialized telescopes such as Ritchey-Chretiens and are not often encountered in amateur astronomy. 

Secondary Mirrors

Reflecting telescopes also incorporate secondary mirrors. These smaller, flat mirrors don’t gather any light; instead, they direct the light from the primary mirror to the telescope’s focuser, where the eyepiece is located. 

Because they are suspended over the telescope’s aperture, they create a central obstruction which blocks some of the incoming light and reduces image brightness and contrast somewhat. This is a necessary evil in reflectors and Cassegrain telescopes but is avoided in refractors which don’t require secondary mirrors. 

Newtonian reflectors suspend the secondary mirror on metal vanes (called the spider); the mirror is mounted diagonally to direct light out the side of the tube. 

On a Schmidt-Cassegrain telescope, the secondary mirror is attached to the corrector plate which covers the aperture, and it faces the primary, directing the light back down the tube where it exits through a hole in the primary. Maksutov telescopes, on the other hand, typically use a mirrored spot on the back of the corrector plate rather than a separate mirror.  

Star diagonals

Refracting telescopes don’t have primary mirrors; they instead use glass lenses to refract (bend) incoming light into a cone which points straight into the focuser at the opposite end of the OTA. In theory, this eliminates the need for mirrors. But in practice, looking straight through a refractor would be uncomfortable, if not impossible, because of the steep angle required to reach high elevations. 

Enter the star diagonal, a simple device with two tubes at right angles and a flat, diagonal mirror between them. They serve a very similar purpose to secondary mirrors, but because they are separate devices and not built directly into the telescope, they are treated as a separate category. 

Mirror Coatings

Commercial telescope mirrors are made of glass with a reflective coating applied to its surface. Aluminum is by far the most common material for coating primary mirrors. The reflective coatings are protected with a chemical coating to prevent corrosion. Protected aluminum coatings typically reflect 85-90% of incoming light. 

Enhanced aluminum coatings have a special dielectric layer which offers increased protection and reflectivity up to 95%. Finally, star diagonals may have full dielectric coatings applied for up to 99% reflectivity. These coatings provide noticeably brighter images compared a standard-issue diagonal but are too costly for use in large primary mirrors. 

Professional Telescope Mirrors

Most professional telescopes, including the Very Large Telescope (VLT), the Hubble Space Telescope (HST), and the James Webb Space Telescope (JWST), use primary and secondary mirrors in the same way as backyard reflectors. 

However, these primaries are several metres in diameter and weigh many tonnes. They can be made from exotic materials, such as the JWST primary which is made of gold-plated beryllium to achieve the optimal balance of weight, rigidity, and reflectivity. 

Ground-based telescopes like the VLT typically use active or adaptive optics in their primaries. Such mirrors are divided into segments controlled by actuators which can move each segment independently of the others. 

Active optics maintain the figure of large mirrors which might otherwise deform under their own weight, while adaptive optics perform miniscule adjustments to the figure of the mirror to cancel out turbulence in Earth’s atmosphere. 

Such telescopes use sensors to read the atmosphere and send inputs to the adaptive optics, which deform the mirror at high speed. This technique allows ground-based telescopes to achieve resolution which would otherwise be impossible. 

Mirror Maintenance

Telescopes mirrors are surprisingly efficient at attracting dust and dirt. Many telescope owners are horrified the first time they shine a flashlight down their optical tube assembly. (This practice, by the way, is not recommended.) 

However, telescope mirrors should be cleaned periodically, and only as needed. A light coating of dust has minimal impact on a telescope’s performance. If you need to clean your mirrors more than once a year, either you are being too picky, or you should examine your methods of storing the telescope.

Optical surfaces should be covered when not in use. If condensation forms on your mirrors and lenses, allow them to dry completely before covering. If you need to clean your primary mirror, follow the manufacturer’s directions to remove the primary mirror cell. Many reputable suppliers have online tutorials for cleaning mirrors. 

Use distilled water to rinse the mirror and avoid touching the surface with your bare hands. And don’t forget to check collimation after reinstalling the mirror!

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