Different types of telescope

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 decriptions. 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. The photos are of popular Sky-Watcher brand telescopes and are courtesy of Optical-Vision.

How to choose a telescope

Regardless of the delights of simple sky-watching with the unaided eye, there comes a time when everyone wants to look through a telescope. Saturn is a brilliant yellow 'star' with the eye alone but even a small scope will reveal it for the ringed wonder it really is. If there is an astronomy club in your locality then joining them on an observing night might be enough to satisfy your telescope curiosity. But once hooked on the sky, the chances are you will want a telescope of your own. If you are careful in making your choice, there has never been a better time to buy one - they are cheaper than ever in real terms. Some, including many from the major players in the market, Celestron and Meade, have computerised mounts meaning that in theory they will find your targets for you. Find some great independent telescope reviews at the Cloudy Nights site. When you come to buy a telescope, you can purchase models from these dealers and others 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. Don't forget that you can also find a bargain telescope or accessories such as eyepieces on eBay, though you should remember the adage "buyer beware" and research the model you see advertised and take appropriate precautions before handing over your money. A useful guide to shopping safely online can be found here. Don't forget that binoculars are a great value observational tool too - essentially they are a pair of low-power telescopes. The array of telescopes on offer may seem bewildering but they boil down to two basic types - the refractor and the relector, plus so-called catadioptrics which marry the two forms. The refractor - or refracting telescope - uses a lens to collect light from a star, planet or whatever you happen to be looking at. The reflector - or reflecting telescope - collects the light with a curved mirror. The bigger the lens or the mirror, the more light is collected from your target, and the fainter the objects you can see. You can read more about the different types of telescope in our article here. The light that is collected by the lens or the mirror is directed to a smaller lens (usually a group of lenses) called an eyepiece which magnifies the image and is adjusted to bring the object you are observing into focus. The path through a refractor is shown in the cutaway diagram above and through a reflector in the cutaway below. On astronomical telescopes you will usually have a selection of interchangeable eyepieces so that you can view objects at different magnifications. The distance from the lens or mirror to the point where the image comes to focus is called its focal length. sometimes you will hear mentioned the telescope's focal ratio or f-ratio - this is the focal length divided by the diameter of the main lens or mirror. An f-ratio of 4 or 5 is preferred by amateurs observing deep sky objects such as galaxies an nebulae as the observing field is wide and bright. Such telescopes are often known as Rich Field Telescopes . Planetary observers prefer long focal lengths, with corresponding narrower fields of view, because this allows them to see more detail on their solar system targets. When choosing a telescope, bear in mind your personal circumstances. Apartment-dwellers might go for something light and portable such as a small refractor or Meade's ETX range. If you have a big garden and can leave your telescope outside permanently, you could go for a large reflecting telescope. For more extensive, excellent advice, read this article by Robin Scagell, a leading amateur astronomer in the UK and vice-president of the Society for Popular Astronomy. He has written a classic book, Stargazing With A Telescope, that is a mine of information and comes highly recommended. You can buy it by clicking the links to one of our Amazon-partnered stores above.