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Buying Your First Telescope
Astronomer Will Gater offers tips and advice on the exciting moment you decide to take the plunge and invest in a telescope.
One of the things all of us at BBC Sky at Night Magazine love the most about astronomy is that you don't need any fancy or expensive equipment to get started. A warm coat, clear skies and a sense of intrigue about what's up there are all you need to begin your adventure in this wonderful hobby.
But there comes a time, naturally, when your thoughts turn to delving deeper into the heavens and when that happens most people start to think about getting a telescope. If you're at that point now, this article will help to guide you through the process of selecting your first telescope, from the things to look for in an instrument, its mount and the essential, and non-essential, accessories.
What about Binoculars?
The best way to begin is not by diving into the world of apertures and eyepieces, but by taking a step back and asking a question. For the purposes of this guide we're going to assume that you're familiar with the naked-eye night sky and can identify many of the bright stars and constellations, but have never used a telescope before. That's important, because the question, which has become something of a cliché in astronomy circles, is: have you considered getting a decent pair of binoculars first?
There are very good reasons why this question is repeated so often that is become a cliché. First, binoculars can open up a great many more objects to observation than the naked eye, from rich Milky Way star fields to star clusters and the brighter galaxies and nebulae. What's more, a good pair of binoculars will often outperform a cheap, poor-quality starter telescope. But the other reason — and one of the key arguments for considering binoculars before a telescope — is that they offer an easy way to learn crucial observing skills that will be useful later in your astronomy career. For example, with binoculars the experience of moving from a naked-eye view to one seen through an eyepiece is easier, as is learning the essential skill of 'hopping' from one star to another, in a magnified view, to track down a celestial target.
You may have already been using binoculars for a while, though, or perhaps you simply want to jump straight to a telescope. In which case there are some other questions to answer before you begin choosing an instrument. Questions such as what type of telescope do you want; what you intend to do with it; and, of course, how much you're willing to spend. Don't worry if you can't answer these straightaway as there are ways to gather the information you need to make an informed decision for each of them. For example, you could visit your local astronomical society, star party or astronomy trade show before you set foot in a shop. At a society meeting you may be able to see some small telescopes in use and speak to people who have used specific models. At a star party, however, you might even get to look through the telescopes, perhaps even the model you're considering.
What will become immediately obvious when you go to any of these events is the huge array of telescope designs, sizes and mountings that are available. So it's well worth getting to know the different types of telescopes and how to decipher the specifications you may encounter.
Types of Telescopes
Generally speaking, telescopes fall into one of three categories. Firstly, there are reflectors, whose defining feature is an arrangement of mirrors that collect and focus light; then there's refractors, which use glass lenses to do the same things; and finally there are catadioptrics, the telescopes that use a combination of lenses and mirrors to do the job. Within these categories there are numerous permutations and, of course, designs that vary from one manufacturer to another.
Refractors tend to be what most people imagine when they think of a telescope: a lens, or group of lenses, mounted in a long metal tube with the eyepiece (the bit you look through) at the bottom end. Refractors often come with a 'diagonal', an accessory that reflects the view 90° up off the telescope's axis to make your observing easier.
The two most common catadioptric designs are the Maksutov-Cassegrain and the Schmidt-Cassegrain. A Maksutov-Cassegrain telescope uses a curved lens and mirrors mounted in a relatively short, stubby tube while a Schmidt-Cassegrain has a large, glass corrector plate holding a small secondary mirror at one end with the primary mirror mounted at the other end.
When it comes to reflector telescopes, many beginners gravitate towards Newtonian models. In telescopes of this design light is collected by a main mirror housed at the bottom of a long tube. Once it hits this primary mirror the light is reflected back to a smaller secondary mirror that bounces it out at a right angle through the eyepiece. For this reason you look through a Newtonian by standing by the top end of the telescope, rather than the bottom end, and peering through an eyepiece that's mounted on the side of the tube.
The Newtonian design uses the same optical configuration as the other popular type of reflector, the Dobsonian. The difference with a Dobsonian is that the telescope's tube is mounted on a simple rotating base and not the more complex type of mount that Newtonians are typically found on? Which brings us onto the matter of mounts.
If a telescope's lens is its eyes, then the mount is its neck. When you look at something, you use your neck to tilt your head up or down and turn it left or right to point your eyes in the right direction. Telescope mounts do exactly the same — they allow the telescope to be moved up and down and turned to the left or right. Astronomers call that upward or downward tilt altitude, and left or right rotation azimuth. So, for instance, Dobsonians sit on a basic altitude-azimuth (more commonly known as altaz) mount, while many Maksutov-Cassegrain and Schmidt-Cassegrain telescopes are attached to one or two computer-controlled arms that are essentially just advanced versions of an altaz mount.
With an altaz mount you need to adjust both the altitude and azimuth settings in order to track an object across the sky and keep it in view. This is because Earth's rotation makes it appear as if the sky is moving, so frequent manual adjustments are required to keep your target centred. That is unless you have an advanced (often expensive) motorised and computer-controlled altaz mount that will take care of the adjustments for you.
But there's another type of mount that gets around this issue in a simple way. The equatorial, or EQ, mount also moves on two axes, but instead of having altitude and azimuth axes, equatorial mounts have a right ascension (RA or polar) axis and a declination axis. These two axes refer to an astronomical coordinate system for navigating the sky that's similar to the latitude and longitude system that's used for navigating on Earth. An equatorial mount is built in such a way that when the RA axis is aligned with Earth's rotational axis, changes need only be made to that one axis to match the sky's movement.
To align an equatorial mount to Earth's rotational axis, the mount's RA axis needs to be pointing precisely towards the north celestial pole — the same point in the sky that Earth's rotational axis points towards. In the northern hemisphere this point is very close to the star Polaris. It's for this reason that many mid-range equatorial mounts come with a small 'polar scope' within the RA axis that has a reticle (targeting crosshair) for precise polar alignment.
Manual or automatic?
Whatever type of mount you end up with it has to be rock-solid. If there's any instability in the tripod's legs or play in the mount's fixings and controls then you'll find observing can become a frustrating affair — another reason to try any telescope before you buy.
While you can track the moving sky manually, it's far more efficient to use an electronically driven mount. These come in several forms from simple, motor-driven equatorial mounts, all the way up to Go-To mounts that, with the aid of a small computer handset (and sometimes GPS), take care of all tracking. Go-To mounts allow you to point your telescope towards a target simply by typing in the object's name or New General Catalogue (NGC) number.
Modern Go-To systems are a superb tool for observing but can often add a lot of money to the price tag of a beginner telescope; money that might be better spent on larger optics sitting on a simpler mount. After all, it's the telescope's aperture — the size of its main mirror or lens — that is perhaps the most important specification. The larger the aperture, the more light can be gathered. Hence most beginners are usually better off going for a good reflector, such as a Newtonian, since refractor telescopes tend to be more expensive than reflectors of the same aperture.
Eyepieces and Finderscopes
After the telescope's main body and the mount, the other key component of any stargazing setup is the eyepiece. The eyepiece is the glass lens that you look through to see whichever celestial body you're observing.
On the side of the eyepiece you'll find a measurement given in millimetres. This is the focal length of the eyepiece and it's by using eyepieces with different focal lengths that you change the magnification of the view through the telescope. The longer the focal length of the eyepiece, the less magnified the view through it will be. It's a good idea to have one or two good-quality eyepieces — one with a short focal length, perhaps around 10mm, and another maybe in the region of 25-30mm — rather than a whole range of cheap ones. And you can completely ignore any marketing hype boasting that a scope can provide hundreds of times magnification — this isn't the measure of a good instrument or a guarantee of superior views. A poor-quality telescope can still magnify many hundreds of times.
Finally, on top of the telescope you'll often find a miniature, low-magnification refractor telescope. This is a finderscope and it's used for centering a celestial object in the main scope's eyepiece. Finder scopes — or their cousins the illuminated red dot/reticule finders — are extremely useful when it comes to tracking down celestial targets.
Needless to say, there's certainly a lot to consider when you buy a telescope. But, then again, a good first telescope will last you many years and be a joy to observe with throughout that time. Choose wisely and your scope will take you on a thrilling journey of discovery that no other pastime can offer.
Which Telescope is best for you?
Answering these questions should help you choose the right first telescope for your needs
What do you want to look at?
Sounds obvious right? The night sky! If you're keen to focus primarily on observing faint galaxies, clusters and nebulae, then it makes sense to go for something like a Dobsonian with as big a mirror as you can afford. A smaller aperture Newtonian on a fancier mount will be more suited to closer, brighter objects.
Which telescope has the right size, weight, portability for you?
A good large-aperture Dobsonian will provide superb views but it may hardly ever be used if you've got to lug it downstairs to observe with. Trade shows are useful for examining many different telescopes in detail to gauge their true size, portability and, to some extent, their build quality.
Is the telescope right for your future aims?
It's worth considering early on where you think your interests may develop in the future. If you think you might eventually want to do more imaging, for example, you may want to choose a telescope with a mount that you can easily upgrade to a more advanced model at a later stage.
Do you need 'feature X' when you could get a better 'feature Y'?
The allure of electronic gadgetry or a computerized mount can be strong when buying your first telescope, but do you really need all that extra tech? You may find that your budget would go much further on a simpler setup with larger optics that'll probably give you better views.
ABOUT THE WRITER
Will Gater is an astronomy journalist and presenter. Follow him on Twitter at @willgater.
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