Choosing a telescope can seem a challenging task for the newcomer to astronomy. There is a bewildering number on the market, with many different names, sizes and descriptions.
Essentially, however, they all do the same thing. They act like a large eye to collect lots of light from the distant object being observed and then they magnify it.
There are other instruments that observe different forms of radiation from the universe, such as radio waves, but the observing tools that we are interested in are optical telescopes.
Despite the vast choice in the marketplace today, such telescopes come in two basic types – the refractor and the reflector. Variations of the basic forms include hybrids that combine elements from both.
The Refractor
In a simple refractor, or refracting telescope, the light-collecting part of the instrument is a large curved lens called an objective.
As parallel rays of light from an object deep in space pass through this lens, they are refracted, or bent, along the telescope tube towards a point where they converge, forming an image. The distance of this point from the lens determines the focal length that you may see mentioned in its description.
The second vital element of the refractor is the eyepiece, which is simply a magnifying lens, or set of lenses, that is used to enlarge the image produced by the main lens. Astronomers normally use a range of eyepieces so that they can magnify their observing targets by different amounts to suit different situations.
The Reflector
The basic reflector, or reflecting telescope, uses a curved mirror rather than an objective lens, to collect light from whatever is being observed. It was invented by Sir Isaac Newton and is sometimes also known as the Newtonian reflector.
You could look at it as a rather sophisticated version of the shaving mirror. Light travels into the telescope tube and travels the length of the tube before hitting this mirror, known as the primary.
It is then reflected back up the tube to a much smaller flat mirror, positioned at a 45-degree angle, called the secondary. This sends the light out through a hole in the of the side of the tube, where the rays converge to form an image.
As with the refracting telescope, the eyepiece is then placed at this position to magnify the image produced.
Why size matters
Whether you choose a refractor or a reflector, the size of your objective lens or primary mirror will determine how much light you collect from the object you are observing. As a rule, the more light you receive, the more you will be able to magnify it.
Which telescope type should I choose?
Both main types of telescope have their fans. Refractors are especially convenient as smaller, portable telescopes. The smallest useful size is usually considered to have an objective lens 60mm in diamater.
You can find both sorts of telescope from reputable manufacturers via the Skymania shops which are powered by Amazon. US visitors should click here to find suitable models and UK visitors should click here for their store.
Reflectors are cheaper to build when it comes to larger instruments and some amateurs are today working with mirrors 20 inches or more in diameter, although six or eight inches is a much more common size.
Refractors suffer an effect where light passing through the objective gets refracted to slightly different points depending on its colour. This is called chromatic aberration and leads to objects being observed showing colourful fringes around them.
Telescope manufacturers attempt to counter this failing by using a combination of lenses rather than one alone to form the objective. The most successful, and expensive, are called apochromatic refractors which show little if any signs of fringes. The problem does not affect reflecting telescopes.
Don’t forget that binoculars are good-value telescopes too! Celestron offer a SkyMaster 15×70 model that makes a useful addition to the armoury of any astronomer!
The photos are of popular Sky-Watcher brand telescopes and are courtesy of Optical-Vision.
