Come scegliere un telescopio

How to choose a telescope. 2

Which telescope model to choose. 3

Characteristics of the refractor telescope. 3

Reflector Telescope Features. 4

Characteristics of the catadioptric telescope. 5

What does the f/ratio mean in a telescope. 6

Choosing the telescope mount. 7

Equatorial mount. 7

Altazimuth mount. 7

Frame accessories: computerized movement . 7

How to Find the Right Telescope for Your Astronomy Interests 8

Light pollution. 8

Location of observation: Where do you live? . 8

Regardless, for the enthusiast, switching from binoculars to a telescope is a necessary step. For those with this passion, the time comes to decide whether or not to buy a telescope, even for beginners; and for any amateur astronomer, this is probably one of the most important decisions they’ll make.

Making the right choice means having in your hands an instrument that will make many evenings spent exploring the celestial vault enjoyable , while choosing badly usually leads to frustration and disinterest in this activity, to the point of reselling the instrument that nipped this passion in the bud, perhaps through an advertisement in specialized magazines, certainly “in excellent condition” for having almost never used it.

How to choose a telescope

The right decision depends on numerous factors, not only the telescope itself but also a whole series of surrounding conditions that need to be considered . For example, if the Moon and planets fascinate you and you live in the city, perhaps in a fifth-floor apartment with limited space, you will need to choose a telescope that is completely different from the one an amateur astronomer living in the Tuscan countryside, passionate about galaxies and with a warehouse available to convert into a personal observatory, would purchase.

Equally important is to take into account the cost of the equipment , the weight to be transported , and previous experience in observing the vault, with the naked eye and/or with binoculars.

The telescope’s aperture , that is, the diameter of the primary mirror or main lens, is certainly the instrument’s most important characteristic , as it determines the contrast and brightness of everything you can observe. An instrument with a 6-centimeter aperture will never allow you to see faint stars or details like a 15-centimeter telescope. And a 15-centimeter telescope cannot compare to a good 25-centimeter one.

When purchasing , power isn’t the primary consideration , as magnification can be varied simply by using different eyepieces as needed. An eyepiece is nothing more than a removable barrel containing various lenses, and it’s through this that observations are made.

Eyepieces can be purchased separately, and most commercial telescopes include at least one. However, it’s pointless to use high magnifications with small telescopes; the result would simply be an enlarged blur, not any details or features.

To be able to observe images at 200x or higher definition, it is necessary to have an instrument with a large aperture and appropriately mounted: however, the most common uses are limited to lower magnifications, and the lowest power, which still allows for pleasant images to be obtained, is the simplest to use.

In ideal sky conditions, the maximum useful power of the instrument is calculated considering 25 magnifications for every centimeter of aperture: for example, a 15 cm will have a useful power of 300x, even too much!

The advice is therefore to avoid purchasing telescopes advertised for their high magnification capacity : if the manufacturer of a 60 cm telescope emphasizes the “475x magnification” of his instrument, know that he thinks you are ignorant or gullible, or he is trying to hide serious construction shortcomings: emphasizing the achievable magnification to such an extent is often synonymous with a toy telescope.

While aperture is truly important, don’t think that buying a good telescope simply means purchasing the largest aperture within your budget: making the right choice isn’t that easy . Consider, in fact, that you’ll rarely use an instrument that’s difficult to assemble, or one that’s too large for your home.

Even with the same aperture, some telescopes are easier to handle, others offer better contrast, and still others are more affordable. Below, a series of tips that, by guiding you through the many options available, will help you make the best decision.

Which telescope model to choose

• refractors
• spotlights
• catadioptric

There are three types of telescopes to choose from. Each has unique characteristics, advantages, and disadvantages that you should definitely consider to ensure they match your habits and what you want to observe.

Features of the refractor telescope

Light is collected and focused by the front lens of the optical tube, which is long and relatively thin. Among the vast array of available refractor telescopes, the choice can range from toy models to top-quality instruments, perhaps the best on the market. Department stores often sell those that fall into the first category. The low optical quality and usually very unstable mounts make it difficult to aim anything. The advice is to opt for binoculars, at least if your budget limits you to this type of instrument.

If you already own a telescope of this type, resign yourself: Galileo , for example, would have been delighted. And if it gives you a few problems, be patient and, without getting angry, limit your ambitions. Ultimately, it’s your attitude when using the telescope that counts, and remember that many amateur astronomers have used similar instruments for their first experiences: for observing the Moon or other easily identifiable luminous bodies, they might be just right.

Refracting telescopes worthy of the name, on the other hand, are very expensive. Some companies have offered new lens configurations, called “apochromats,” which have led to the emergence of excellent telescopes on the amateur instrument market, among the best and most expensive ever. These new lenses are not to be confused with the “achromats” of common refractors. They generally come with high-quality mounts that perform equally well.

• Advantages:
  Refracting telescopes are essentially robust, require little maintenance, and have sealed tubes to keep dust out and reduce internal air movements that cause image degradation. The better the lenses, the brighter and more contrasty the images produced by a refractor, depending on the aperture, one of the most desirable characteristics for observing planets and the Moon.
• Disadvantages:
  Refractors typically have small apertures, ranging from 13 to 80 cm, and these values ​​are too low for some fields of astronomy: for example, galaxies and nebulae, if observed, will appear very dim, as will other faint objects in the celestial vault. Furthermore, for a comfortable observing position, they often require a 90° mirror placed before the eyepiece: this mirror inverts the left and right images, making it difficult to compare observations with maps. In terms of cost, the price per centimeter of aperture for a good refractor is higher than for other types of telescopes.

Reflector Telescope Features

A relatively large and heavy concave mirror collects and focuses light, replacing a lens. On the side, at the top of the optical tube, is the eyepiece, through which the observation is made. For many years, and for some still today, the reflecting telescope has dominated amateur astronomy. In the diagram illustrated above, and for beginners, “reflector” means Newtonian .

• Advantages:
  For the same price, a reflector is the best telescope on the market. Many people can build their own thanks to the simplicity of the configuration (in Italy, high-quality mirrors are produced by at least two vendors/manufacturers, editor’s note), and it also allows for excellent optical quality. The image is not mirrored but “corrected” because there are an even number of mirrors (two), and there is no problem with condensation due to nighttime humidity, a common annoyance with other configurations. The low, stable mount provides stability and allows the eyepiece to be kept at a comfortable height.
• Disadvantages:
  Reflectors often require greater care and maintenance. The optical tube, being open, allows dust to enter and settle on the optics, even when the instrument is kept closed and covered (however, a modest amount of dust cannot significantly affect performance). The mirror alignment must be adjusted occasionally; the procedure is simple but tedious: simply turn the screws on the mirror mount accordingly. Furthermore, during observation, the air inside the tube can move and create drafts, which can disturb the image, at least until the telescope and surrounding air temperatures are equal. This problem does not arise if the tube is well ventilated.

Features of the catadioptric telescope

This type of telescope uses both lenses and mirrors: the most popular is the Schmidt-Cassegrain model, on the market since the 1970s and still a popular and sought-after instrument, just like reflectors and refractors, which have been in use for centuries. The advantages and disadvantages of catadioptric telescopes are described below.

• Advantages
  : Portability, a modest price compared to other optical configurations, and some of this instrument’s unique features (such as electronic tracking) are certainly strong points of the Schmidt-Cassegrain telescope. For example, a 20-cm telescope fits in a trunk that can be carried with one hand, even with a separate tripod, while a 20-cm reflector telescope is cumbersome and heavy to maneuver. A catadioptric telescope fits in the trunk, while a reflector would take up the entire interior space of a car. Photographing the night sky is made less difficult, but not easy, by the motorization developed for star tracking, which, thanks to the shorter tube, is much more reliable. Schmidt-Cassegrains are therefore excellent for astrophotography: for both photography and the use of CCD cameras, complex electronics have been developed that allow for the control of the guiding of these telescopes’ mounts. Some models are even available that can track the sky completely independently.
• Disadvantages:
  Especially when observing planets, the image rendered by a Schmidt-Cassegrain offers less contrast than that of a reflector with the same aperture. For the same aperture, the catadioptric telescope is more expensive. A diagonal (right-angle) mirror is often used to make the observer’s position more comfortable, resulting in an inverted view. Adjustments, especially the major ones, must be made by the manufacturer, and it should also be noted that the focusing mechanism is delicate and often imprecise.

What does the f/ratio mean in a telescope?

A telescope’s performance depends on its f/ratio, which particularly affects the performance of reflecting telescopes. The higher this ratio, the better the performance. Saying that a telescope has an f/6 or f/15 ratio simply means dividing the focal length, or distance between the objective and the eyepiece, by the aperture.

So a 10 cm lens with a focal length of 1 m is defined as f/10. In complex instruments such as those with a Cassegrain configuration, the “effective focal length” is used according to the same principle, not the actual distance between the two parts. Therefore, for a 12 cm Cassegrain with an effective focal length of 2.5 m, the aforementioned ratio will be f/20.

Image contrast is reduced when the secondary mirror is relatively large, and in these cases the ratio is always lower than f/6 or f/5: near the edges of the field of view, image distortion is more evident and misalignment of the optical system can occur very easily.

It is not easy to make good mirrors with low ratios, and furthermore the smaller the f/ value, the better and therefore more expensive the eyepieces to be used to have contrasty images at the sides and not just in the center of the field.

For these reasons, an f/4 reflector telescope is not comparable to an equally well-built f/8. On the other hand, an f/4 is also more manageable and much easier to transport: it is in fact half the length of an f/8!

A 25cm f/4 reflector is no more than 1.20m long, and can easily fit in the back seat of a car, allowing you to go anywhere and observe the darkest skies. A 25cm f/8 is over 2m long and isn’t as convenient to carry: everything has its advantages and disadvantages.

Choosing a telescope mount

The mount is another important feature, because even the best telescope, on a bad mount, becomes terrible: every slight tremor turns into an earthquake, no matter what magnification you’re using. And it’s not easy to observe during an earthquake. Unfortunately, a flaw of all mounts is vibration, and this is usually due to design errors (or the manufacturer cutting production costs) of one or more nodes, although often the motivation lies in an attempt to make the mount ever lighter, in order to avoid transporting it with a forklift.

Always remember not to compromise on the size and weight of the mount, whichever one you choose. Nothing kills enthusiasm like constant vibration, and only a solid mount that barely moves when touched or when focusing fills the user with joy. There are basically two types of mounts:

• equatorial
• altazimuth

Let’s see together how they are structured, what they are used for and what the differences are.

Equatorial mount

This type of mount was designed to easily follow the motion of the celestial vault due to the Earth’s rotation, which even at magnifications of 75x or 100x would remove observed objects from the field of view in just a minute. The motion of the sky is automatically tracked by the ” clock motor ,” which in most cases is already integrated into the equatorial mount.

Another reason this mount is useful is that, in response to its movements, the eyepiece indicates the north-south and east-west directions: starting from a map, it becomes much easier to locate what you are looking for among the stars. To work, at the beginning of each observation, this type of mount must be aligned with the north celestial pole: however, this operation does not need to be very accurate; it is sufficient that the polar axis points approximately towards the North Star .

Altazimuth mount

Simpler than equatorial mounts, they move in both directions, up-down and left-right: to track celestial objects, you must therefore adjust both axes. For the same stability, altazimuth mounts are less expensive and lighter, and these advantages are well demonstrated by those of Dobsonian reflecting telescopes, giant, low-cost telescopes.

To handle large instruments on mounts of this type, you need proven experience and a high level of knowledge of navigating celestial objects: Dobsonian telescopes are among the best to use for large-aperture enthusiasts.

Frame accessories: computerized movement

Computerized control of the axes’ motorization, combined with a large database of celestial bodies, are the newest options for Schmidt-Cassegrains and some high-quality refracting telescopes. Once the computer has been initialized, with the help of a couple of new stars and a few other appropriate adjustments, these devices automatically orient themselves to the desired objects, as if by magic. Since no knowledge of the sky is required, astronomy in these terms might seem simple.

There are many opinions about these computerized telescopes, which undoubtedly represent the way of the future. Some, now experienced observers, praise them for allowing more time to be spent observing objects rather than wasting time searching for them. Others, like myself, believe that such an instrument is both too expensive and unsuitable for learning, not to mention that electronic control takes away much of the fun.

How to Find the Right Telescope for Your Astronomy Interests

• Planets and Moon
  If your interests focus on objects that require high magnification, good contrast and sharp resolution (such as the planets or the Moon ), the most suitable instruments for you will probably be refractors or reflectors with a high f/ratio value .
• Galaxies and nebulae
  : If, however, your passion is faint objects ( galaxies and nebulae ), you’ll want to choose a large reflector with a very, very large aperture. And you’ll be making the most logical choice.
• A bit of everything
  If, however, you don’t want to specialize but want a telescope that can cover a wide field of view, opt for a 20 cm Schmidt-Cassegrain or a 15-20 cm f/8 or f/6 reflector.

Light pollution

Another factor that can influence the choice of telescope best suited to your needs is light pollution . The Moon and major planets are visible even in the most severe light pollution, while much fainter objects such as galaxies and nebulae can remain invisible altogether. Therefore, the view of the Moon from a city dweller’s fifth-floor balcony, observing the sky with a refractor, could be comparable to the observation of a Tuscan country dweller. However, to both, almost all deep-sky objects will remain invisible.

Location of observation: Where do you live?

A telescope allows us to observe the sky , but the instrument, once purchased, also needs to be stored and transported. To use it, even when non-astronomers like you go to sleep, you’ll have to set it up and disassemble it again. And while your enthusiasm for this passion may initially seem overwhelming, in the long run this could become a challenge, and observations will become increasingly rare.

Too many beginners overlook the problem and find themselves with a “white elephant” on their hands, which they rarely use. So before getting fixated on tempting ads for gigantic instruments, remember that a small telescope, perhaps a 10-cm, if used more, will certainly reveal more of the universe than an unused 40-cm telescope.

The best telescope to buy will therefore be the one you will use the most . In fact, it won’t be the diameter of the instrument you have available that will make you enjoy becoming a good astronomer, but this will be all the more possible the more time you dedicate to observing the sky . When choosing, think about both where you will use it and where you will store it : if there are steps in the middle, think carefully before going beyond 15 cm.

For storage, the ideal location would be a closed porch or a dry, ventilated area like a shed. These locations are preferably close to where you will use it, allowing it to be at the right temperature. This way, when you go from the warm temperatures of your home to the cold outside at night, you’ll greatly reduce the effects of image distortion caused by air movements inside the optical tube, as well as the condensation that forms when you bring it back indoors.

If the location you choose to store your telescope is cold, be aware of condensation that can form with changes in the weather. If condensation is likely, leave the optical tube open to ventilate the interior and wrap the entire telescope in a blanket. Remember, you can only cover it if the interior is perfectly dry.

Will you be avoiding trees and lights when transporting your telescope? If your observing site is permanent , it’s a good idea to install a pedestal so you don’t have to move the tripod back and forth. Planting a telephone pole in the ground or filling a large pipe with sand will provide more stability than expensive tripods. And where you’ll be using it, a shelter or observatory around the telescope is ideal.

Tips for choosing a telescope

Once you’ve narrowed your choice—probably between a 20 cm catadioptric and a 15-20 cm reflector —browse the various manufacturers’ catalogs, comparing their features and carefully considering their dimensions and weight. Almost all reputable manufacturers advertise in industry magazines , both in Italy and abroad, especially in the US, where Sky & Telescope magazine is available . For the best price, inquire with several dealers , but also pay attention to their return and repair policies (which, for some new instruments, sometimes become an unfortunate necessity).

If you purchase from foreign companies (for example, American ones), be aware that in some cases, delivery of your order may take more than a year from the time you placed your order . So, try to insist on a specific delivery date , knowing that American law requires that an ordered product be shipped within 30 days of payment. After this time, the seller is obligated to refund your money or request your consent for a delayed delivery. I believe that seeking good service and prompt shipping is more important than chasing the lowest price.

Many amateur astronomers dedicate themselves to building their own reflector telescopes, and if you’d like, you can also consider this option: by purchasing the mirror already ground and polished, you won’t need any complex tools, just an electric drill. You’ll certainly save money and gain a thorough understanding of how your telescope works.

Checking the quality of the optics

Before purchasing a telescope , if possible, try it out: point it at a star . Or do it immediately afterward if that’s not possible at the time. In fact, if you find a used instrument and therefore out of warranty, this is important to get an immediate idea of ​​the quality of the optics, since this can vary significantly even between apparently identical instruments, perhaps from the same production line.

This happens because mass-produced optics aren’t always of guaranteed quality, not only because each piece is hand-finished, but also because the personnel employed are supposed to be qualified, which isn’t always the case. So you can’t be sure of your purchase until you’ve tried it.

A simple first test to do, very indicative, is the following: with the instrument at the same temperature as the outside (about one or two hours after taking it out of the warmth of the house), and the optics well aligned or ” collimated ” (follow the enclosed instructions), point at a star of magnitude 2 or 3, such as Polaris, which does not even move, using very high magnification.

Then rotate the focus knob to adjust the focus, first one way, then the other, to the optimal focus. The image of the star, as well as the diffraction rings, should appear the same on both sides, that is, with the same shape and the same distribution of light within them.

This test is made more difficult by low ” seeing ,” the distortion caused by the instability of the Earth’s atmosphere. Try again the following nights, hoping for better seeing conditions.

Few telescopes pass this rigorous test without fail. If the blurry images of the star are very similar in both directions, you have a good telescope. If they are clearly different, something is wrong. But before jumping to conclusions, try again the following nights and don’t forget to give the instrument adequate time to cool.
Drop Test
If it fails the test described below, the telescope will always have focusing problems. This problem occurs at high magnifications, when the stars gradually appear to blend together and cross the maximum focus point, quite unlike a good telescope, which instead lets the star “drop” into that point.

The ” drop test ” is less significant than the previous one, but it still allows you to evaluate the instrument. The phenomenon of atmospheric seeing distorts all the stars, but if you have tried a good telescope and one with poor performance you will certainly notice the difference,

The recommendation to give is to always carry out the test at high magnifications , since the imperfections of your eyesight could create misleading effects at low magnifications, regardless of the quality of the instrument and the eyepiece.

If you encounter these defects, consider having it repaired or returning it to where you bought it. The best manufacturers can almost always solve any problems that may arise. Regardless, it doesn’t matter who made your telescope: you have the right for it to at least function decently, so act accordingly.

Trust the experience and opinions of other amateur astronomers to help you choose your telescope: the help and honest advice of local astronomical associations will be invaluable. You might even be able to try out some of their instruments, which will help you decide whether or not it’s worth it to buy a telescope that doubles the aperture you were aiming for, costs four times as much, and ends up with an instrument six times heavier.