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17 February 2020

The Most Frequent Doubts When Choosing a Telescope

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The telescope is one of the most amazing instruments in science. In spite of its relative simplicity, it allows us to reveal a large number of objects and celestial phenomena invisible to the naked eye, which remained hidden in the darkness between the stars. It opens a window to other galaxies, planets surrounded by rings, remnants of supernovas, star clusters or mountains on the Moon… these are some of the favourite destinations of a night of observation with a telescope, easy to reach with a little practice. However, for newcomers, approaching a telescope often raises a sea of doubts:

Whose telescope… Newton’s or Galileo’s?

Galileo Galilei was the first astronomer to use a telescope to observe the sky. In the 17th century, glass lenses began to become popular for viewing distant objects. Galileo honed the design until he succeeded in making a 33-power telescope with which he discovered a new universe, although the simplest binoculars available today allow you to see further. This type of telescope is known as a refractor. In such a device, light passes through a set of lenses that deflects, or refracts, the image, something akin to what a magnifying glass does.

A few years later, Isaac Newton designed a new type of telescope, the reflector, applying the laws of optics that he himself had enunciated. Instead of using lenses, his telescope had a parabolic mirror, where light was reflected and then concentrated on another secondary mirror that deflected it to the eye of the observer. This is the same technology that parabolic antennas use to boost the signal.

Four centuries later, these are still the two most common types of telescopes. Choosing one or the other depends on the use to which it will be put:

  • Refracting telescopes, like Galileo’s, are easy to use. It is not necessary to calibrate the optics before working with them, they give sharp images, are small in size and easy to transport. Their main defects are that the manufacturing process of the lenses is expensive, and they only have a few centimetres of aperture, so they capture little light.
  • Reflectors, like Newton’s, allow for larger diameters at a lower price, as mirrors are cheaper than lenses. In contrast, they require calibration before use, are bulkier and the images they deliver are brighter but less sharp.

Size matters, magnification does not

The most basic sales pitch for telescopes is usually the number of magnifications; however, what matters most is the size. The bigger, the better. The more light that enters the telescope, the brighter the image. Accordingly, more objects are captured and are seen better.

If size were the only thing that mattered, there would only be reflective, mirrored telescopes; however, ease of use, sharpness and portability must also be taken into account.

To observe the Moon, planets, double stars and the brightest distant objects in the sky —such as the Orion Nebula or the Andromeda Galaxy— a refracting telescope (small or medium) is usually the best option. It is much more comfortable, and with more than enough imaging capacity to begin exploring the cosmos for the brightest objects, which are also the easiest to locate.

Parts of a telescope. Credit: Borja Tosar

However, if the main targets are deep sky objects —such as galaxies, nebulae or asteroids— larger diameters are needed. In these cases, a reflecting telescope is more suitable. More seasoned astronomers often turn to reflectors, after years of squeezing light from a refracting telescope.

The telescope is only as good as its weakest link

Although it differentiates the two types of telescopes we have seen, the tube with the optical system is not everything; there are other parts that are equally or more important. The mount is the tripod on which the telescope is installed. The most basic is the azimuthal mount; cheap and easy to use, it can be found on the simplest instruments. Another type is the equatorial mount; a little more difficult to use, but with one great advantage: it allows you to follow the movement of the observed object, and in that sense it is more practical than the azimuthal mount.

As important as the optical tube is the eyepiece, which is the lens where the eye is placed. A good eyepiece may be worth more than a mediocre telescope, but it could also be well worth the investment. In fact, amateur astronomers often have two or three eyepieces, so they can vary the telescope’s magnification and field of view, much like the interchangeable lenses on a camera.

Nor should we forget clouds and light pollution, the great enemies of astronomical observation. The best telescope in a bad sky is useless. Although it may seem paradoxical, the better the telescope, the better the quality of the sky must be to take advantage of it.

Options for the impatient and the handy

Knowing the sky and learning the basics of handling a telescope is not a matter of a couple of nights—it takes time and patience. Fortunately, people with less patience can now get fully automated telescopes; it is the most expensive, but certainly the most comfortable option. These telescopes have a control in which you need only introduce the object to be observed, and the telescope automatically locates it without further intervention. In some versions the screen also gives information about the observed celestial object.

Robotic telescope. Credit: MEADE

On the other hand, the handyman can find a new challenge in the construction of their own telescope. It is common to find astronomers who buy separate Newton-type mirrors and build a simple Dobson mount. It’s certainly the cheapest option for a large telescope. Some people even polish the mirrors of their telescope in their workshop; it requires skill, but it is possible for someone who is handy.

Telescope or the naked eye? When in doubt, reach for the binoculars

For those who enjoy heading outside to gaze at starry nights, the telescope is the next step in their exploration of the universe. But it is not essential. You can do a lot of astronomy without a telescope: from recognising dozens of constellations, stars, planets and even the brightest so-called deep sky objects, such as the Andromeda galaxy or the Orion Nebula; or learning to interpret the movements of the stars in the sky—it is even advisable to do all this with the naked eye, step by step, before buying a telescope.

For the undecided, there is a middle ground: dusting off your old binoculars, or getting a new pair. They don’t need to be specific to astronomy; the most basic binoculars, together with a star guide, put dozens of objects within reach, such as the moon, Jupiter’s satellites, the nearest galaxies and nebulae, and even some “seasonal” comets.

Borja Tosar


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