How to choose a telescope?
Now is a fantastic time to become an amateur astronomer. Never before has such a range of telescopes and accessories been readily available to novice observers of the stars wishing to pursue their hobby. Of course, this leads to the careful and long selection process before choosing the right telescope: the incredible variety of telescopes on the market makes it difficult for an informed consumer to choose the right one for them.
Whether you are seriously considering buying your first telescope, or whether it is still just a daydream, this guide will help you narrow down the field. First of all, we will explore the types of telescopes available, and then discuss their key features – the size of the primary lens or mirror, the type of frame, portability, computerization, and accessories. We will also analyse each telescope’s compromises, because any tool always has its advantages and disadvantages.
Before buying anything, you need to determine what’s important to you. What do you want to observe most? How dark is the sky in your area? What is your level of experience?
How much are you willing to spend? What space do you have, and how much weight do you want to carry? Answer these questions, familiarise yourself with what you find on the market, and you will be well on your way to choosing the right telescope that will accompany you for many years to come.
Before examining the different telescopes available, it is good to know the basics of their operation.
Magnification is not everything
It might surprise you, but the opening of a telescope does not determine its magnification (“power”). When choosing a telescope for the first time a beginner may ask, “How much does it magnify?” The answer is, “As much as you want.” Any telescope can provide an almost infinite range of magnifications, depending on the lens that is inserted inside.
However, don’t think that a very high power is the best. Two main factors limit the power that shows a decent view with a given tool: aperture (still) and weather conditions.
There are only limited details in the image created by the main lens or mirror of a telescope, so it is advisable that you find the optimal magnification to see them – without distributing the precious light of the subject you are observing too much, making a soft body too soft to observe, or transforming a luminous body into a confusing blurry mass.
This is why observers generally use lower powers to observe dimmer bodies such as galaxies and nebulae, and medium-high powers for bright objects such as the Moon and planets. Just as enlarging a photograph too much would only show the grains of the film or the pixels of the chip, too much magnification would make the object you are observing out of focus.
But how much power is too much power?
There is a simple rule to finding the most useful magnification: 50 times the aperture of your telescope in inches, or twice the aperture in millimetres.
However, this also is dependent on if the telescope has perfect optical components and the night air seems to be unusually stable.
This means that a high-quality 4-inch (100mm) telescope should not be pushed beyond 200x. In perspective, even a small instrument that has good lenses will show you the rings of Saturn or the main belts of Jupiter, because the latter can be observed at a magnification of 75x.
On the other hand, if you come across a small 60mm telescope from department stores that promises a “300 power!!!”, you will know that it is only advertising to attract, and that you should wisely go elsewhere.
Calculating the magnification of the telescope
Now you know the maximum practical power for each type of tool, but how do you get it? What do those numbers on the eyepiece mean about the magnification they provide?
Each telescope has a focal length, which is effectively the distance from the primary lens, or the primary mirror, to the image they form. (This is not always the same as the length of the tube, since, as we will see later, some telescopes internally “bend” the path of light.)
The focal length is that large number that you will often see printed, or engraved, on the front or back of the telescope, usually anywhere between 400 and 3,000 millimetres, depending on the aperture and type of telescope.
Even the eyepieces have a focal length – 25mm or 10mm, for example. Just divide the focal length of the telescope by that of the eyepiece. For example, a telescope with a focal length of 1,000mm used with a 25mm eyepiece has a power of 1,000 / 25 = 40 (or 40x).
Why is the Moon blurred?
Even with the best telescope, you will notice that it is possible to distinguish smaller lunar or planetary details on certain nights rather than others.
Sometimes, the sharpness of the view can even change from one second to the next. At a high power, you will see that the planets and stars shine and cloud on most nights. The fault lies not within the telescope, but with the turbulent atmosphere of the Earth, and sometimes with local conditions such as hot air rising from the asphalt of the nearby highway which has collected solar heat all day. Astronomers refer to turbulent nights as “bad observing experiences”.
Large openings allow observers to see the weakest objects and the smallest details on the Moon and planets, however, regardless of the opening, the better the view, the better the visual experience. Given that calm air is important, larger telescopes – even those that fall into the category of telescopes from 25mm and above – are often limited to 250x or 300x during all nights, except in really calm ones.
Every experienced observer would tell you that, with practice, you will see more detail in an image – not only because your eye will be more trained, but because the longer you look, the more likely you are to capture moments of unusual atmospheric calm.
Are bigger telescopes always better?
So why choose a 250mm telescope if the weather conditions limit you? Large openings are often chosen by observers who want to collect as much light as possible to observe the most blurred things: galaxies, nebulae, and groups of stars. These so-called “deep sky” objects are generally observed at much lower powers than those used for the Moon and planets, so the quality of the atmospheric view is not a problem.
In addition, larger apertures generally lead to shorter exposure times for astrophotography enthusiasts, especially when combined with a short focal length.
Even if a bigger tool falls within your budget, there is always the question of portability.
A very large amateur telescope would require a permanent observatory, so that you never have to move it, or charitable friends who will help you transport and assemble it at each observation session, and then to disassemble and transport it again.
Clearly, a compromise must be found between convenience and yield – and everyone will have their own vision of what is “portable”. It is easy to get caught up in the “opening fever”, in which you have the urge to buy the largest telescope possible. The bad thing, though, is that, too often, this leviathan will be confined in the cellar or in a closet, being unsuitable for regular use. Remember, the telescope you use most often is actually the one that will show you the most things.
Pay close attention to the weight of the telescope you want to buy, usually indicated on the instruction sheet. Take a barbell or a trunk with the same weight and put it on the scales. Carry it behind you. Bring it back and forth from the place where you will store the telescope to the one where you will use it. Are there stairs in between? How often are you willing to do all of this at the end of a long day?
Telescopes of all shapes and sizes
Having got an idea of the main optical principles that govern the performance of a telescope, and having come to the compromise between yield and portability, we can now explore the different types of telescopes available on the market.
You will probably think there are an infinite number of them, but you are wrong, because they are all ideas that advertising has ingrained within you. Instead, in essence, although there are many different shapes and sizes, telescopes can be divided into three classes: refractors, reflectors, and compounds.
A refractor is the stereotype of the telescope par excellence – a long, sparkling metal tube with large lenses in front and an eyepiece behind. The front lenses (the lens) concentrate the light to form an image on the back. The eyepiece is a small magnifying mirror with which you will look at this image.
Planetary and lunar observers who value their crisp, high contrast images that can withstand very high magnifications are often looking for high quality refractors. In fact, if done well, a refractor can provide images of the best quality obtainable with a given aperture.
Another advantage of the refractor is that it is generally more robust than other telescopes, because its lenses tend to misalign less often. For this reason, the refractors are suitable for those looking for a tool “to take away” or who do not want to tinker with optical components. However, these excellent features come at a price.
A good quality and very large objective lens is a piece of art that requires a special type of glass and must be made by hand. In fact, the refractors of any aperture are often the most expensive tools. Furthermore, in their most commonly encountered forms, the lengths of the tubes of a refractor can prove to be uncomfortable.
A 100mm refractor can also be 1 meter or more long. And, since, the eyepiece is located at the low end of the tube, if you want to observe objects standing very high, you must also buy a very high tripod. A similar tripod must be very solid to prevent wobbling at high powers, so it could be heavy and difficult to handle, not to mention expensive.
For deep sky observers, a refractor may not have a sufficiently developed light capture capability to observe dim objects, and the field of view may be narrow. Modern optical designs have led to shorter and more handy refractors, but at a corresponding higher cost.
It is done with mirrors.
The second type of telescope, the reflector, uses a mirror to collect and concentrate light. Its most common form is the Newtonian reflector (invented by Isaac Newton), with a specially curved concave primary mirror (in the shape of a plate) at the lower end of the telescope. Near the top, a small diagonal secondary mirror directs the light from the primary one to the side of the tube, where it meets a strategically positioned eyepiece.
If you are looking for the best aperture to price ratio, then the reflector is the right telescope for you. If well-constructed and maintained, a reflector can provide sharp and contrasting images of all types of celestial bodies at a much lower cost than a refractor with the same aperture. Furthermore, a Newtonian’s tube is considerably more manageable.
Its length rarely exceeds the diameter of the primary mirror by eight times, and often less. This means that a 200mm Newtonian can be stored inside a tube that is rarely more than a metre long, comfortably entering the back seat of a car for transport to dark and country skies. Combine all this with the center of gravity well below the Newtonian eyepiece, and you have an instrument on a compact and stable frame that presents the eyepiece at a perfect height for almost any orientation of the sky.
Another advantage is that a reflector is the only type of telescope that shows a “normal” image that isn’t mirrored. This is particularly important when you are trying to compare what you see in the eyepiece with what is on a star map. The best, in terms of quality to price ratio, is certainly a peculiar type of reflector called Dobsonian. It is a Newtonian with a very simple and robust frame.
These extremely popular instruments are available with openings from 100mm to more than 760mm, and represent the best choice in terms of convenience for amateur observations.
Like all reflectors (there are many other types, but we will leave them out because beginners rarely buy them), a Newtonian will require occasional maintenance. Unlike the firmly mounted lenses of a refractor, the mirrors of a reflector can easily misalign, and therefore will require periodic collimation (adjustment) to ensure maximum performance, especially if the telescope is moved frequently.
Once you get carried away, it will no longer be a problem, and the mirrors of a normal Newtonian will probably require no more than one adjustment per month. But for those who are not inclined for mechanical work, having to collimate a Newtonian reflector, even if only occasionally, could prove frustrating.
The open tube of the reflector accumulates more dirt and dust on the surfaces of the optical components, even if you are wise enough to cover the tube when storing it, and this will lead to having to clean it every now and then. In addition, the aluminised surfaces of the mirrors of a reflector may need to be covered every 10 or 20 years – more frequently if you live in an urban area with polluted air or near the sea.
The best of both
Finally, there is the third category of instrument; the catadioptric, or compound, telescope. These were invented in 1930 to combine the best features of refractors and reflectors: they use both lenses and mirrors to form an image. The best feature of these instruments is that, in their most common forms (the Schmidt-Cassegrain and the Maksutov-Cassegrain), they are very compact. Their tubes are only two or three times longer and wider, an arrangement allowed by the “optical fold” of light.
The smaller tube can use a lighter frame, and is therefore more manageable. The advantage is that it is possible to obtain a wide-opening telescope with a wide length that is, in any case, easily transportable.
However, some clarification is needed. Like the Newtonian, the Schmidt-Cassegrain requires an occasional optical collimation which diminishes its attractiveness for those who are not inclined to undertake any mechanical work. Their fields of vision can also be quite narrow. In terms of cost, opening by opening, the reflex reflector is placed between the reflector and the refractor.
Like a Newtonian, the popular forms of compound telescopes have a secondary mirror in the instrument’s light path, and this slightly degrades the performance of important lunar and planetary observations. In any case, if well constructed, a Schmidt-Cassegrain or Maksutov will provide images of a wide range of celestial bodies.
In common with the refractors, the reflector tubes are sealed to exclude most of the dust – a big plus for an instrument that you will take out of town. But, if you live in a place where frequent dew formation takes place (practically almost everywhere), a sort of collar or extension to prevent fogging of the corrector plate exposed on the front of the tube is a must.
Telescopes of this type also tend to be the most highly technological, with many options such as computerised aiming and photographic adaptations. In short, these are excellent telescopes for general observations that can use a wide variety of accessories.
The telescope mount
The best telescope in the world is useless unless it rests on a solid, stable, perfectly-functioning mount, one that allows it to be directed towards the desired portion of the sky and follow a celestial body both fluidly and precisely while the Earth turns under it.
In realistic terms, a “stable” mount is one that, when using moderately high power, does not vibrate for more than a second after moving the tube. In particular, the view cannot wobble so much that it prevents you from finding the right fire. And, when you let go of the focus knob, the aim doesn’t have to suddenly move to one side. This completely eliminates all those “department store” toy-like telescopes.
Although there are variations on the theme, you will encounter two types of mount: altazimuth (or alt-az) and equatorial. An alt-az works like the pan/tilt head of a tripod, moving the telescope up and down (in altitude) and to the right and left (in azimuth). The equatorial mount also has two axes, but they are inclined so that one is aligned with the rotation axis of the Earth.
If you are thinking of using a small telescope for occasional observations or for daytime use (for example, bird watching), you will be comfortable with an alt-az mount. Perfectly constructed mounts of this type will be equipped with finely threaded slow-motion controls that will allow the telescope to be moved fluidly even in small amounts, which is particularly important when using high powers.
The value of these finishes will be clear to you once you follow the path of a star or planet at high magnifications. The Dobsonian is an alt-az mount form. Inexpensive materials such as chipboard and Teflon appear in its construction, giving a low-cost frame with a low centre of gravity that (ideally) flows fluidly on both axes which can be controlled with the fingertips. A Newtonian reflector mounted in this way is not only extremely easy to set up and intuitive to use, but also has a great price.
For a telescope suitable for astronomy, and for which photography is a future prospect, a certain form of equatorial mount that counteracts the Earth’s rotation should be considered. It is much easier to trace a celestial body with a telescope mounted in this way, since the only concern you have would be to turn the telescope about an axis – not two simultaneously, as in alt-az. When an equatorial mount is mounted correctly, turning the slow-motion controls of its polar axis is all that is required to keep an object in sight.
More sophisticated mounts, including modern high-tech alt-az mounts, have incorporated electric motors to carry out this operation, making you free to concentrate on observation. So, is one type of mount better than another? Not really, because each has its strengths. For occasional observers who want an easily transportable telescope that can be mounted in different places, an alt-az – especially a Dobsonian – is preferable.
An equatorial one, virtually mandatory for most forms of astrophotography and critical observations of the Moon and high-powered planets, needs to have its polar axis aligned with the rotational axis of the Earth. Although polar alignment is not particularly complicated and becomes a routine practice, it can take a long time at the beginning of an observing session if you want to do it in a truly precise way (necessary for photography but not just for observing).
Currently in vogue, there are computerised robotic telescopes that appear on the market in different guises. These have mounts controlled either from a built-in computer or remotely with an external PC. This allows you to direct the telescope towards any object saved in the computer database.
At first glance, these “GoTo” units would seem to be the answer to a novice’s dreams, given that they take on all the work of finding elusive bodies such as distant galaxies, agglomerations of stars, and asteroids. “Hey,” you might think, “I don’t have to memorise all the star maps!” In fact, that’s not quite the case.
Without a doubt, if well-constructed (very expensive), these robotic telescopes are very fun to use, since they almost magically sift through the skies in search of whatever you have typed, immediately finding the target and presenting it inside the eye.
However, this technology is still maturing to the point where these telescopes will orient themselves as soon as they are brought into the field and turned on. Almost all GoTo systems will ask you to enter the geographical position of your observation site (or the nearest city) and the date and time at the beginning of each observation session.
This allows the computer to calculate the position of any object you want to observe. Often, you will also need to level the telescope tube, point it north (or south in the Southern Hemisphere), and then start an alignment procedure that uses two bright stars (which you must know by name) to synchronise the co-ordinates of the telescope with that of the sky.
It is true that this setup routine can be easily mastered with practice, but it takes time. And, for someone who still does not know the sky well, the vast majority of the current group of robotic telescopes has the potential to prove particularly stressful, at least in the beginning. However, the technology is evolving.
The latest GoTo telescopes produced include their own Global Positioning System (GPS) devices that at least warn you (and also warn the telescope) of your exact position and time, making the process of adjusting the settings slightly easier. Then, there is also the question of how accurately the mechanical parts point the telescope where the electronic brain is pointing.
At astronomical magnifications, there is no room for even the smallest mistake – therefore, not even for cuts in design and manufacturing costs. A poor GoTo telescope will not work, no matter how sophisticated the electronic components are. Here is one last tip to keep in mind: the money spent on the mount of a GoTo electronic telescope could be invested in a larger mount of a traditional telescope.
When used at medium to high power, a telescope shows you only a small portion of the sky. This makes the aiming of a target a frustrating process, unless the telescope has a finder.
As the name suggests, a seeker assists you in locating celestial bodies. All telescopes, except the smallest ones, need it. The most common is a miniature telescope attached via a holder next to the main telescope’s eyepiece. It has a low degree of magnification and therefore a wider field of view, and is equipped with a viewfinder.
Once correctly aligned with the main telescope, centering an object in the viewfinder places it in the field of view of the latter. You need a high-quality, relatively large finder. Look for one that has an aperture (front lens) larger than 25mm and looks well made. Tiny and practically useless finders are too common in cheap telescopes.
A popular alternative is a red dot, which projects a point or ring (or rings) of light into the background when viewed from behind. Many prefer this intuitively simple option, however, it is limited to objects observable with the naked eye, because this type of finder has no magnification, and has an aperture with light collection capacity no larger than a pupil.
You can, however, “jump” from targets with the naked eye to objects belonging to the deepest sky by using the main telescope at its lowest power if you have sufficiently detailed sky maps.
Can I photograph what I see?
Assuming you have purchased a new telescope, it is almost inevitable that you will want to use it to capture the beauty of a planetary image on film or to emulate the wonderful photo galleries of space that adorn magazines such as ‘Sky & Telescope’. In the beginning, there is no reason why you should not be able to, given the necessary equipment with the right inclination at the right time.
But it is always wise to first get used to using your new telescope visually and explore the sky to get to know it better before embarking on the adventure of astrophotography.
Space photography can prove to be incredibly rewarding, but it is as much an art as a science. The learning curve can be steep, the equipment can be expensive, and obtaining the perfect photo can be very time consuming. While any telescope would allow you to photograph the Moon, for anything else you will need a telescope with a well-made, rigid, and precisely-oriented mount.
Each telescope has its price
Resist the temptation to buy the cheapest telescope available. Most of them have very low optical, mechanical, or both, quality, and will disappoint you. If you have a budget of less than $200, consider purchasing a good binocular instead.
That said, quality tools can also be obtained second-hand from a more experienced member of your local astronomy club. Or are you considering building one yourself? If you have good manual skills and you like to work with wood, you can buy optical parts and build a top quality Dobsonian reflector. Again, older members of your local club may be able to help.
Even if you’re willing to spend a lot of money, don’t buy the largest and most expensive telescope you find. Start with the smallest and most manageable. If you are still learning to identify constellations, many of the advanced features of a very expensive tool will be of no use to you. And remember not to take anything too heavy to assemble, transport, and store.
Finally, remember that you will need more than glass and metal. Make sure to save some of your budget for additional eyepieces to expand the telescope’s magnification range, a very detailed (essential!) Stellar atlas and good guides, and any other type of other accessories – especially if you plan on becoming an astrophotographer.
So, is there a perfect telescope out there waiting for you? In truth, there is – it is what you will use most often!
A refractor with superb optical qualities but massive will not be useful at all if you cannot take it out, and the largest of the Dobsonians will not show you the most distant galaxies, if the only place where you can use it is a parking lot polluted by the lights of a city.
Consider carefully what your primary observation interest is, where you could perform your sessions, and what “portable” means to you when choosing a telescope. Lifting weights would do you good, but not everyone could appreciate it. Contact your local astronomical club, which may have organised observation nights where you can try out different telescopes and chat with their owners. Do not be shy. Your local club wouldn’t have entered our database if it didn’t want your call.
A telescope represents a great investment for many, and the universe certainly does not run away. So, take the time you need to decide. Use binoculars to familiarize yourself with star charts and guides to flush out distant and hidden wonders.
By doing this, you will develop exactly the knowledge and skills needed to use a telescope. At the actual time of purchase, you will tend to make a decision that you are really happy with, and you will have a real key to unlocking a life of cosmic wonders.