All About…Reflector telescopes

The reflector telescope is an amazing instrument. We take a look at their history and how they work…

The great 17th Century scientist Sir Isaac Newton is credited with the invention of the reflector telescope, although there were others who came up with a similar idea for such a device at around the same time.

The only type of telescope in use by astronomers in the early-1600s was of course the refractor which used glass lenses in a tube in order to gather and focus light. Several scientists were aware, however, that there was another way to achieve this, using a mirror. In 1668, Newton produced a small telescope which used a spherical mirror made of polished metal that bounced the light reflected from it up the tube to a much smaller flat mirror at an angle of 45 degrees. This in turn reflected light through a small hole made in the side of the tube where it could be focused and viewed through an eyepiece lens. This type of telescope soon became known as the Newtonian reflector and it is still very much in use today although its size and method of construction has taken a great leap from Newton’s first production. However, the problem with making metal mirrors, made from a material called ‘speculum’, an alloy of copper and tin which can be highly polished, meant that they did not become popular for nearly another 100 years when the technology was improved such that the mirrors could now be made of glass.

It was quickly realised that reflecting telescopes had many benefits including less optical problems, known as aberrations, than refractors at the time. And, probably the greatest advantage of all, the fact that mirrors could be easily made much larger than lenses. As construction methods and technology improved, mirrors and therefore telescopes, became larger. This in turn meant that fainter objects could be discerned and detail, known as resolution, could be greater. Because it is cheaper to manufacture mirrors of a given size than lenses of the same size, reflectors also have an advantage on a cost/performance scale. Due to this and some of its inherent optical advantages, Newtonian reflectors are popular for astronomers wanting to study deep sky objects which are, by their nature, faint. Newtonian reflectors don’t hold all the aces, though. Due to the secondary mirror effectively blocking some of the light entering the tube, contrast in images can be affected, although this is usually minimal. It can be enough though, to make a difference to planetary and lunar studies where contrast and detail can be critical.

Over time the Newtonian reflector was joined by other designs of telescope, some of which tried to combine the advantages of both the reflector and the refractor. The ‘compound’ telescopes now come in many guises and can be useful for certain types of observation and study, but the Newtonian reflector is still ubiquitous, being used by both amateur and professional astronomers the world over.

Pros and cons

Newtonian reflectors make great amateur telescopes as you get a good aperture for your money. They are versatile so can be used for viewing the Moon, planets and deep sky objects. However, they do take a little more maintenance than say a refractor as the mirrors have to be aligned in the tube, with each other and with the focuser in a process called collimation. Although this can seem daunting at first, providing the user is careful and methodical it is usually straightforward and with practice, quite quick to perform and only needs doing once in a while. Because the telescope tube is open to the sky mirrors can become tarnished and dirty; they can be cleaned, or every few years re-coated professionally. This is relatively inexpensive and is like having a brand-new telescope once the mirrors are reinstalled. Therefore the first time purchaser needs to consider carefully if this is the right kind of telescope for them.

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