How to Choose a Telescope
We are often asked by people, "Which telescope should I buy?" Well, that's a tough question to answer, because choosing a telescope is like choosing a car - one size doesn't fit all! So, we've prepared some guidance notes, which should give you enough information to get started.
So Just How Do I Choose a Telescope?
There’s no question at all that the sky is a wonderful place – there is just so much to see. But unless you know where to look and have at least some understanding of what it is you are looking at, an aspiring astronomer can very quickly lose interest. So our first important message is that it matters quite a bit just how you get your first experience of the sky. Some people’s first real look is just with their eyes or through binoculars, while others might be lucky enough to be able to look through a telescope. Both experiences can capture (or lose) you as an astronomer forever, depending on a number of things which might seem irrelevant at the time, but which can seriously impact how you enjoy your "first time". What you are looking at, what you are using to look at it with, and the knowledge level of the person who is helping you (if indeed you have anyone to help you) are all relevant, and contrary to what you might expect, there are some objects which actually look better through binoculars than through a telescope. The most important thing is to try to share your first experience with someone who at least partly knows the sky, and who can point out things which otherwise you might miss. From the first moment you glimpse that clear, crisp image of Saturn’s rings, or see a close-up of the Moon’s craters, or the majestic glow of the Great Orion Nebula, or the cloud bands on Jupiter, you will know whether or not you are destined to be an astronomer.
Astronomy appeals to different people for different reasons. Some people enjoy "discovering" other worlds with their own eyes, and for others astronomy offers a relaxing escape from the day to day routine of "normal" life. It’s also a great family activity, allowing parents and children to explore new frontiers together, to learn together. If you are the sort of person who enjoys teaching others, then there is tremendous scope for this, while at the same time enjoying your hobby. One of the most rewarding parts of the time we spend teaching people is to see the look of wonder in the eyes of young and old alike as they see for the first time the almost unreal images of Saturn and it’s rings, or Jupiter and its moons, looking as if I have placed a colour slide in front of the telescope. There are amateur astronomy clubs all over the world, and most of them organize regular star parties where you can mix with other enthusiasts and learn from them. So, you've thought about it and have decided that you definitely want to buy a telescope. Where do you start?
Like choosing a house or a car, buying a first telescope is a highly subjective decision. There is no "best" telescope for everyone. The one that's right for you will depend on your lifestyle, where you plan to use it, where you are able to store it, and also on what you wish to do with it in your astronomy. Let's look at some of the most important considerations that might influence your choice of a scope.
Forget about "power." It's diameter that counts.
You will see many telescope advertisements which brag about the magnifying power. Ignore them! A telescope's most important feature is not its magnifying power, but rather its ability to gather light and to display the image in a sharp, easy to see manner. Light gathering power is determined by the diameter (aperture) of the main lens or mirror. The more light gathered, the better you will be able to see the objects when they are magnified. Telescopes advertised on the basis of high magnification ("450x!") are virtually always of inferior quality. In all telescopes, power is adjusted by changing eyepieces, lower powers providing brighter, sharper images. So you can boost the power as high as you like on any telescope, but all this does is make the image larger. If the optical system quality is poor - as it will be in a cheapo telescope - then you will have a poor quality, fuzzy image to start with, and magnifying it will only make it more and more fuzzy. So remember, the sharpness and clarity of an image is usually most dependent on the quality of the optical system. The worse the alignment and quality of the various lenses and mirrors, the worse the image.
How big a scope do you need?
The single most important specification for any astronomical telescope is its aperture. This term refers to the diameter of the telescope’s main optical element, be it a lens or a mirror. The aperture of a telescope relates directly to the two vital aspects of the scope’s performance: its light-gathering power (which determines how bright objects viewed in the scope will appear), and its maximum resolving power (how much fine detail it can reveal). There are other criteria to be considered in selecting a telescope, but if you learn only one thing from this explanation, let it be this: the larger a telescope’s aperture (i.e., the fatter it is), the more you will see. Of course your choice depends on what you want to do. For viewing craters on the Moon, the rings of Saturn, and Jupiter with its four bright moons, a 60mm (2.5 inch) or 70mm (3 inch) refractor or a 3 to 6 inch reflector will do the job. A slightly larger instrument – perhaps an 80mm (3.5 inch) to 100mm (4 inch) refractor or 8 inch to 10 inch reflector will show more planetary and lunar detail as well as glowing nebulae and sparkling star clusters. Going bigger still, under dark, non-light-polluted skies, a scope of 10 inches or more in diameter can produce magnificent images of fainter clusters and nebulae, and start to show some detail in galaxies.
Have realistic expectations and do not be fooled by advertising photographs showing bright galaxies. With the exception of some nebulae and the brighter planets, most astronomical objects are very faint. Generally, the larger the telescope the brighter will be the images and the more detail you will see, but be warned that even in large amateur telescopes most galaxies show up as nothing more than faint, fuzzy objects. But don't buy more size than you can sensibly handle or store. Don’t forget that you must be able to comfortably lift and transport it, and you will also need adequate space to store it somewhere safe and dry. Many people have chosen a large scope only to find that they can’t be bothered to haul it out to use, or that it doesn’t fit in the car, or their aching back prevents them from lifting and carrying it around, or that it is too complicated to put together. So these scopes collect dust in a cupboard until the day that the owner decides to sell them. The correct choice is very important, and a good pair of binoculars might well have been more useful in these cases. On the other hand, be careful not to buy something small and portable, only to be disappointed by what you can see through it. Like I said before in this section, have realistic expectations before you buy.
Refractor, Reflector, or Catadioptric?
There are several different types of telescope: Refractors, Reflectors and Catadioptrics (shown in order below)
Refractors: This is what most people think of when they hear the word "telescope." Its tube is most often long and skinny, mounted on a tripod, with a lens at one end and the eyepiece at the other. Refractors were the first type of telescope invented, and the finest refractors still provide the best images of any design for a given aperture. They are often chosen by observers with a dominant interest in the planets and Moon, because they can provide sharp, high-contrast views at high magnification and are less bothered by atmospheric "seeing" than the other designs. They also require less maintenance than reflectors or compound scopes, and are therefore popular with beginners. The refractor’s good performance at high power and relative insensitivity to light pollution makes it a good choice for a city-based observer, as the design performs best on the objects that are most easily seen from urban or suburban locations. These advantages do not come without a price — literally: refractors are the most expensive telescopes per inch of aperture. Their long focal length sometimes restricts the field of view, making it difficult to take in large extended objects like some clusters of stars. And the long tube, with the eyepiece located at the back end, requires a tall, stable tripod.
The reflector uses a mirror, rather than a lens, to gather and focus light. By far the most common design is the Newtonian reflector, which places a concave (dish-shaped) primary mirror at the bottom end of the telescope tube. A small, flat secondary mirror at the other end directs the focused light out the side of the tube and into the eyepiece. Newtonians offer the largest aperture available at given price, and when well made they can provide sharp, high contrast views which rival all but the finest refractors. A Newtonian’s low centre of gravity and eyepiece location at the top of the tube allow for comfortable viewing with a more compact mounting, which can be made stable with much less bulk and cost than the tall mounting required by a refractor of similar aperture. Big reflectors of 10" aperture and larger on Dobsonian mountings are the most popular telescopes for astronomers who seek to gather "buckets of light" for deep-sky observing. These giant scopes perform best at remote dark sky sites, away from the glare of city lights. The value and versatility of the smaller 4.5" to 8" Newtonians, mounted either equatorially or as Dobsonians, makes them a fine choice for the beginner with general interest. Newtonian reflectors require occasional maintenance. Unlike the lenses in a refractor, the mirrors in a reflector need periodic alignment, or collimation, for best performance. A reflector’s tube is also more open to air and humidity than that of a refractor, and if left uncovered the mirrors can accumulate dust and grime, which will require occasional cleaning. While these maintenance concerns are often overstated, a Newtonian may not be the right choice for someone who finds the prospect of occasional tinkering with the telescope unappealing.
The Schmidt-Cassegrain and Dall-Kirkham are examples of Catadioptric scopes and are the most modern of the three common designs for amateur telescopes. They provide a very sensible option for compactness, light collecting and magnifying power. Refractors provide good image quality but larger sizes start to get quite expensive compared to reflectors, because of the difficulty of making large diameter, high quality glass lenses. Mirrors are much easier and cheaper to make in larger sizes, but because of the way they work reflectors can be rather long and bulky at larger mirror sizes, which makes them tough to carry around. Catadioptric scopes were developed to get around this problem. You can think of them as "folded" telescopes and they work very well, but they are usually more expensive. The greatest advantage of this design is its compactness: the lenses and mirrors "fold up" the light path inside the telescope, reducing large-aperture scopes to a manageable size. If an equatorial mounting is desired, the smaller tube can be carried on lighter and more economical mounts than that required by a Newtonian of the same size. Compound telescopes are most popular with observers who desire both generous aperture and an equatorial mounting in a transportable package. The names Schmidt-Cassegrain, Maksutov-Cassegrain and Dall-Kerkham refer to specific designs of compound telescopes, which use differently shaped lenses and mirrors to achieve a similar result. The Maksutov is often cited as offering better image quality, though there is little in the way of optical theory to support this opinion. In top-quality telescopes from careful manufacturers, all designs can yield excellent images. There are a few drawbacks to all compound designs. As in any telescope which employs mirrors, occasional alignment is required for peak performance. The cost of a compound telescope is higher than that of a Newtonian of the same aperture, though still lower than the cost of a comparably sized refractor. Most significantly for the planetary observer, the secondary mirror in a compound is much larger than that in a Newtonian, and its presence in the light path of the scope reduces contrast somewhat for high-powered viewing. In general, astronomers who desire a highly capable, easily transportable telescope find these worthwhile compromises, and have made the compound scopes very popular.
DO NOT buy a flimsy, cheap scope at the supermarket with the intention of getting a taste of the sky and upgrading later. Many of those scopes are so poor quality and frustrating that they can turn budding astronomers off the hobby for good!
On the other hand, don’t give up on astronomy if the scope of your dreams is financially out of reach at this moment. There are many reasonably priced, high-quality beginner’s scopes that can reveal incredible wonders, while helping a novice define his or her particular observing interest.
Finally, if you are one of the fortunate few for whom price represents little obstacle, think twice before buying the biggest, most expensive telescope in stock. Many of the large, fully featured scopes favoured by experienced observers are also the most complicated, and are too much to grasp for someone still trying to find the Big Dipper!
What About Astrophotography?
Before concluding, here’s a quick word for the beginner who wants to jump right into astro-photography through their new telescope: Don’t! At least, not until you have taken some time to learn the sky and become familiar with operating your scope. Photography of the heavens can be a wonderfully rewarding pastime, but it is hugely more difficult and frustrating to do than you can imagine. It is a combination of art and science with a steep learning curve which can thoroughly put you off it forever if you try to take on too much at once. Of course, if astro-photography is a primary interest there is nothing wrong with selecting a first scope based on its easy adaptability to camera work in the future. While most telescopes can be used for picture taking the most important qualifications for a photographic instrument are a rock solid equatorial mounting, an excellent tracking mechanism and ease of attaching a camera so that it can be focused. For a variety of technical and economic reasons, compound telescopes of 8" aperture and larger are most popular for photography. They also make fine instruments for general observing.