THE FUNDAMENTAL (in case you do not have time to read the rest)
The power of the telescope is given by its diameter or opening, in mm. It is the main feature of the telescope. Forget about "the increases".
Your telescope can be refractor (with lenses) or reflector (with mirrors).
To begin with, a reflector between 100 and 200mm in diameter, or refractor between 60 and 90mm (around 100-300 €), may be recommended.
If it does not come with the telescope, it is advisable to add an eyepiece of 25 to 40mm (from around 50 €).
If you do not know the firmament well, you will also need a planisphere and / or Sky Guide.
Before buying it ...
If you already know the sky with the naked eye (with the help of a planisphere, a sky guide, software, etc.) then you can start using an optical instrument. If you do not know anything about the sky, the telescope will not help you much. If you give a telescope, it will be a good idea to be accompanied by a planisphere or guide from heaven. There are also computer programs and applications for mobile devices that show objects visible in the sky.
A regular binoculars is a very suitable instrument to start, and it is convenient to explore the sky with them before buying a telescope. The most common and suitable binoculars are the 7x50 (that is, 7 magnification and 50mm diameter in the objectives), or also the 10x50 (10 magnification and 50mm diameter in the objectives). Other increases and diameters can also be worth (try yours!).
In addition, if you place your binoculars on a tripod of those used for photography (in optics usually also sell the necessary adapter to place it), they will become a small "telescope" very suitable for discovering the satellites of Jupiter, the Nebula of Orion, the Andromeda Galaxy and a multitude of other objects. This way you will learn to locate celestial objects, which will facilitate the future use of a telescope.
To choose it ...
Without a doubt, the first thing we will think about is the budget.
Do not think that a moderately good telescope has to be very expensive. From about € 100 you can find a telescope to start, and for about € 200 or € 300 you will surely find a refractor or reflector of sufficient aperture and good quality. The price will depend on the frame you wear, combined with the type of optics. It must also be said that prices vary, sometimes a lot, depending on the establishment where you buy: normally, the best prices are obtained in distributors or stores specializing in astronomical instruments (see below).
Broadly speaking, to buy a telescope you will have to look at two things: the type of optics, and the type of mount.
Let's see a brief description of the types of optics available and the frames that we have in the market.
TYPES OF OPTICS:
Broadly speaking, the "power" of a telescope is mainly given by its diameter or aperture. To begin with, a refractor of between 60 and 90mm, or a reflector of between 100 and 200mm may be fine.
As an orientation we will tell you that a refractor has the power of a double diameter reflector.
Refractor telescopes are composed exclusively of lenses.
Advantage
· Almost no maintenance required.
· Easy to use also as a terrestrial telescope.
· Good for observing Solar System objects (Luna, planets).
Disadvantages
· They are small or medium open.
· If we want a medium power, the tube can be large and unwieldy.
Reflecting telescopes, also called Newton's, are based on the use of curved mirrors.
Advantage
· More opening (and therefore more power) for an affordable price. It will open many possibilities for observation.
· Good for observing both Solar System and weak objects (nebulae, galaxies).
· You can even build it yourself (except maybe buy the parabolic mirror).
Disadvantages
· The tube is open (there is no lens that closes it), so the mirror, which is at the bottom of the tube, needs maintenance: send it to "aluminize" sometime.
· The position to observe sometimes is uncomfortable, because the eyepiece is in the front and can be very high or very low.
Catadioptic telescopes are a combination of lenses and mirrors (Schmidt-Cassegrain type, Maksutov, etc).
Advantage
· Almost no maintenance required.
· Medium or large opening with a short and compact tube, therefore easy to handle and transport.
· Convenient installation and observation.
Disadvantages
· High price.
TYPES OF MOUNT:
A good optics does not help without a firm mount. An unstable mount, that trembles, or that does not allow precise movements will make impossible the observation, since we will despair trying to aim at an object without obtaining it.
The azimuthal mount allows you to directly execute up-down and right-left movements.
Characteristics
· Simple, occupies little space. More economical
· For the reflectors is the Dobson type (almost a simple "box" on the floor, as seen in the photo).
· Does not allow tracking of the stars.
The equatorial mount is used by placing its axis parallel to the axis of rotation of the Earth.
Characteristics
· Allows you to track the star (counteract the rotation of the earth) just by turning a wheel.
· This turn can also be automated with an engine.
· More expensive; sometimes something more voluminous.
· It requires to learn its handling and assembly, although it is quite simple.
Computerized search systems consist of electronic components that allow you to search for celestial objects in a very simple way (GOTO, Autostar, etc)
Characteristics
· Find and follow the objects to observe.
· The price increases, although they are becoming more affordable.
· Requires "orienting" before each session.
· For those who are starting, can prevent us from learning by ourselves.
The question of "increases ..."
The "increases" mean nothing, no matter how much they announce it in the box, since they do not depend on the telescope but on a separate piece: the eyepiece. Normally you have two or three eyepieces of different magnifications, because each celestial object will require certain magnifications to see it well: sometimes it is better to use many, and sometimes it is better to use few.
Example: take a low-resolution photo with your computer and increase it to 400%. Do you see it better? With more increases you will see "closer" but also "more blurred", to the point that you can completely lose the details.
The magnifications must be adequate to the telescope: the double of the aperture in millimeters is usually taken as limit. That is, for a 60mm telescope, as a limit 120 increases (and even then, these will only be used on very specific occasions: lunar observation, very stable atmospheric conditions ...). If you put more increases, you will see almost nothing. If they announce a diameter of 60mm with 425 increases, you can change your mind.
In fact it is very important to have an eyepiece of FEW increases that gives us a large field of vision (eyepieces from 25 to 40mm). Normally it does not come with the telescope and you have to buy it (around 50 or 60 euros). Without it, it is easy for you to think that your telescope "looks bad" or finds it difficult to locate objects.
To know a bit more...
This is perhaps not very necessary to take into account, but if you want to know a little more, another feature is the focal length, related to the length of the tube, and the focal ratio: how many times is the focal length greater than the aperture. For example: Opening 60mm. Focal length 600mm, which is 10 times the aperture: The focal ratio will be 10. (Usually indicated by f / 10). If you like photography, you may be familiar with this concept.
If the focal ratio is "short" (less than 6) the telescope will be luminous, better for weak objects (nebulae, galaxies). If it is "long" (more than 8) it will give good contrast for Solar System objects (Moon, planets). A focal ratio between 5 and 10 is good for almost everything. Outside of that range it is not recommended at least for a first telescope.
The focal length also serves to know how many magnifications an eyepiece will give you: dividing the focal length of the telescope between the focal length of the eyepiece, which is the number that usually comes printed on the eyepiece (all in mm). For example:
Focal length of the telescope = 1000mm, I have a 25mm eyepiece, then it produces 1000/25 = 40 magnifications.
And one last advice:
Do not buy a telescope of poor quality, even if it is cheaper, it will only serve to discourage you. There are many clues to recognize them: the little-known brand (although this is not definitive); the place where it is sold (it is not definitive either); the price; too bright colors in the box or in the telescope itself; and above all, advertise for "having many increases". Or even having in the box the photo of someone observing through the glass of a window.