How to choose a telescope –

The telescope allows us to leave the Solar System and observe clusters, nebulae and galaxies that can not be resolved with binoculars because they are too weak objects.

  • The opening
  • Types of telescopes: refractors, reflectors and catadioptric
  • Frame
  • The searcher
  • The eyepieces

Image result for telescope parts

To buy a telescope you must fulfill a series of qualities, as well as find a balance between increases, luminosity, portability and cost.

If we compare what the binoculars offer us, the biggest increases in the telescopes allow us to see lunar craters, observe the atmosphere of Jupiter and see the rings of Saturn and even the divisions of its interior. The telescope allows us to leave the Solar System and observe clusters, nebulae and galaxies that can not be resolved with binoculars because they are too weak objects.

The opening

The most important thing when buying a telescope is the opening, that is, the useful diameter of the lens or mirror. The larger its opening, the brighter and more detailed will be the images it will offer us. A 150mm aperture telescope will provide images twice as sharp and four times brighter than another 75mm telescope.

In the case of telescopes, the magnifications are the least important, because they go in the eyepiece. We changed the eyepiece and so we changed the magnifications. The telescopes whose quality comes argued for their increases and nothing else, should be rejected.

Types of telescopes: refractors, reflectors and catadioptric

Los telescopios refractores utilizan lentes para captar la luz.

Refractor telescopes with openings greater than 100 mm are very bulky, heavy and expensive. It is for this reason that the most common refractors have diameters between 60 and 90 mm, which offer clear images of the Moon and the planets, but such small diameters limit their usefulness when observing weak and diffuse objects. Therefore, refractor telescopes are suitable for planetary observation, and not for deep sky. They use lenses to capture light.

Los telescopios refractores utilizan lentes para captar la luz.

Reflectors, also called Newtonian telescopes, do not have the expensive lenses of refractors. They use a mirror to capture light. Therefore, we can find in the market reflectors of 110 or 150 mm for a very affordable price, comparable to that of a much smaller refractor. Being its greater opening, they are a better alternative to observe nebulae and galaxies, that is, for deep sky.

Los telescopios Schmidt-Cassegrain utilizan espejos y una lente correctora.

The retro-reflectors, such as the Maksutov-Cassegrain or the Schmidt-Cassegrain, have quality optics and a great light collection. The tubes are shorter than those of the refractors or reflectors. They use a combination of mirrors and corrective lens.


As important as the optics is also the mount. If it is weak, it causes the images to stop oscillating. There are several types of frames. The horizontal or altacimutal can be moved in the up-down and right-left directions. The best quality have motion controls to achieve more accurate targeting. They are simple to install, but its drawback is that they do not allow to follow the stars with simple movements, but you have to activate the two axes at the same time to keep the object that we are observing within the field of vision. They are most often used in small refractors and in reflectors.

La montura Dobson es tan sencilla de manejar que es ideal para los principiantes.

The Dobson or Dobsonian frames are a type of horizontal frame and are made of pieces of wood that slide on Teflon blocks. They are simple and cheap and their movements are so smooth that to keep the objects centered it is enough to give a light touch with the hand.

Equatorial mounts are more complex. They rotate around a single axis to follow stellar movement. They can take an engine to follow up. There are several types of equatorial mounts, but the most used is the German one.

The fork frames are the most used in catadioptric telescopes. They allow turns like those of the equatorial mounts to follow the stars automatically.

For those who want to dedicate themselves seriously to the celestial observation, the best option is a 150 or 200 mm diameter reflector with a Dobsonian mount. It is easy to install, has a stable frame and an opening more than enough.

Many beginners also start with a 110mm reflector with equatorial mount. Its price is similar to the 150 mm reflector with Dobson mount. The equatorial mount has a more complex installation because it has to be aligned with the north or south pole, but it facilitates the tracking of the objects. In any case it is convenient to find a model with a resistant frame and of good quality.

Some people prefer refractors because they have less maintenance than reflectors. The optics of a refractor between 70 and 90 mm is very robust, does not need adjustments and, when accompanied by a horizontal or equatorial mount, offers quality images. 60 mm refractors do not usually have frames or quality lenses and should be avoided.

If for us it is important that the instrument is portable, a catadioptric is a good choice, but it is more expensive. The Maksutov-Cassegrain 90 mm and the Schmidt-Cassegrain 127 mm are the most frequent models. The most sold are the Schmidt-Cassegrain 200 mm as they are compact and have a large opening.

We can find telescopes in the market with more affordable prices and equipped with electric motors and small computers. After aligning it with two stars, these telescopes are able to point to any object among the hundreds included in its database. They can also be controlled from laptops or from electronic agendas. This offers more precision and reliability if you have a quality telescope with good engines.

The searcher

A telescope must also have a search engine. The search engine is like a small telescope attached to the side of the device and is used to aim. It is essential to locate objects. Some telescopes have a 5 X 24 seeker, but 6 X 30 or 8 X 50 are better, provided the telescope can hold it.

Recently, reflection seekers are also used. You look through a window and you see a red target projected on the bottom of the sky that indicates the exact place to which the telescope is directed.

The eyepieces

When we change the eyepieces we modify the magnifications. The ideal is to have at least three eyepieces: one of 25 mm (for diffuse objects with few increases), another of 8 to 12 mm (to see the Moon and the cumulus with intermediate increases) and one of 7 to 10 mm (for planets) and double stars with many increases). The eyepieces must be of quality, like the Televue Nagler, Vixen or the Plössl, perhaps the best sellers.

The Barlow lens is an excellent alternative for high power eyepieces. This lens doubles or triples the magnifications of any eyepiece, but preserves the ocular relief, making it very comfortable to use.

The magnification provided by an eyepiece is calculated by dividing the focal length of the telescope (this data is usually inscribed in the tube or appears in the instruction manual) between the focal length of the eyepiece (which appears in the eyepiece, in the millimeter). An example, a 20 mm eyepiece placed in a telescope with a focal length of 1000 mm gives 50 magnifications. If the eyepiece was 10 mm, we would have 100 magnifications, and if it were 5 mm, it would give 200 magnifications. We must remember that the greater the number in the eyepiece, the greater the power.

A larger telescope allows us to see more, but it must be made clear that no telescope will ever teach us the nebulae or galaxies with the colors we see in long exposure photographs or computer-treated images. The human eye is not able to perceive the color of weak astronomical objects, even with large telescopes. Nor will we ever see the flags or the tracks left by the astronauts on the Moon: there is no telescope in the world powerful enough for that.

It is practice that gives us skill in telescopic observation. You also have to “educate” the eyes. Sometimes, months of observation are needed for the eyes to get used to the most delicate details. In order to see nebulae and galaxies better, you have to look at them “out of the corner of your eye”, since the central area of ​​the retina is less sensitive. Thus, we can see objects that if we look straight ahead, they will go unnoticed. It is what is called lateral vision. There are also filters to increase the contrast between the nebulae and the sky background.