How do I buy a telescope

LACERTA GmbH | Vienna | Linz
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  5. Before you buy (guide)


Good question, bad answer: It all depends on the intended use. The types of telescopes all have their advantages and disadvantages.
- Lens telescope: + Upright image (nature observation), maintenance-free, easy viewing, child-friendly.
- Reflector telescope: + a lot of light for the money, especially with Dobsonian mount, pure color, both low and high magnification.
- Maksutov / Schmidt-Cassegrain: + very compact, upright image.

For purely astronomical use, the most important factor is that the telescope collects a lot of light. The rule is simple: the bigger the better! Because lens telescopes are astronomically expensive with large openings, the reflector telescope has the edge, and also because of its relatively universal applicability. The cheapest than Dobsonian. However, one wisdom says: the best telescope is the one that is used the most. And that doesn't necessarily have to be the most expensive or the biggest, and certainly not the hardest.


Whenever you get into a personal conversation with newcomers and interested parties, frequently recurring questions about this fascinating hobby arise, the answers are often not to be found across the board, but the most important questions and answers are taken up and answered.

Basically, as a beginner, you have to choose the mount: Parallactic mounts facilitate tracking, that is, the observed object no longer moves out of the field of view at high magnification by compensating for the rotation of the earth (by means of a motor or by hand). The azimuthal mount is not aligned with the sky, it has two axes for moving up-down and left-right. The Dobsonian mount takes a different approach, these mounts, which are very simple in terms of structure, have the advantage that they are comparatively stable (even in the low-priced segment) and that a large part of the costs flow into the larger optics, with equatorial mounts this weighting is exactly the opposite and you have to make sure that the telescope does not get too big in order to have enough stability reserves.

Do I need automatic tracking?
Since, as I said, the earth unfortunately rotates, the telescope has to track the moving celestial objects. This is done either manually with shafts on the axes or with motors that automatically take over the tracking. This is of course a pleasant thing if you have always centered the observed object nicely and can devote yourself to the observation in peace. Automatic tracking makes observation a lot easier, especially at high magnifications. With an equatorial mount, only one motor / shaft is required on the axis that points to the pole; with azimuthal mounts, two motors / shafts are required. Tracking is also absolutely necessary for taking photos, for planet / moon recordings with a webcam, a simple motorization is sufficient because of the short exposure times, but if you want to go in the direction of long exposure for galaxies and star clusters, the tracking must be extremely precise so that the stars really become points . Long-term photography is the supreme discipline of astrophotography, which requires good equipment and a lot of experience, not least when it comes to image processing on the computer.

Do I need automatic object positioning?
Pleasant, but be careful: some believe that buying a so-called "GoTo" telescope will completely save you the work of getting to know the sky and adjusting the objects. Most of the time, however, the mount has to be precisely aligned with the Pole Star, and then one, two or three stars have to be set manually to tell the electronics in which direction the telescope is "seeing". Often you see the beginner struggling with the expensive electronics instead of admiring the sky. Even inexpensive GoTo mounts, which do not have to be aligned to the north, need such a calibration procedure. The telescope then points to the correct position in the sky, but often nothing is seen on the telescope because the object aimed at is too faint for the instrument used. In addition, the device cannot be moved by hand because the motor is firmly coupled. It is questionable whether instead of an electronics with 40,000 pre-stored objects (of which only about 100 objects are bright enough to be seen with the small telescope) it would not be better to invest in a larger telescope or a more stable mount. There are already completely self-adjusting devices, but these are very expensive and again only equipped with small telescopes.

Am I only using my telescope visually in the sky?
In that case you should consider the Dobsonian mount. The simple rocker box (IKEA feeling) is stable and costs practically nothing. The money used for telescopes is invested almost exclusively in optics. As an example we take a 200mm Dobsonian telescope, which costs about the same as a 100mm refractor on an EQ3 mount (about 450 euros). The advantage: The larger Dobsonian collects 4 × as much light as the lens telescope, the disadvantage: No operating convenience, such as manual fine movement etc ... However, it is a wallet-friendly solution if someone wants to get to know the sky and visually explore faint objects or planets.

Do I also use my telescope for nature observation?
The azimuthal mount is usually used for nature observation. But there are enough "oversized" photo tripods (Safari, AZ3, AZ4, CVN-II, Porta-2, Merlin, AllView ...) on the market that can keep the instruments stable enough even at higher magnifications or for photography. Thanks to a uniform prism rail system, the same telescope can be attached to both astronomical and nature observation mounts.


What you can see with a telescope depends on a number of factors. In the first place, the opening should be mentioned, i.e. the diameter of the lens or the mirror. The opening determines the amount of light that can be collected, which is particularly important with the large number of so-called deep-sky objects (nebulae, galaxies, etc.), in smaller telescopes these appear much weaker or cannot be seen at all. The opening also determines the resolution, i.e. the larger the opening, the finer details can be perceived. All of this is then also subject to the quality of the mirror or lens. A first-class look can show more than a terribly bad one, even if it is a bit (!) Larger, but often the quality is also placed too much in the foreground. For example, a first-class 4-inch refractor cannot show nearly as much as an average 8-inch mirror, despite all its excellence, but within the scope of its opening, the high-quality device shows significantly more than a simple model.

What can you see now? Let's start on our doorstep:

Moon is definitely the first destination for all beginners. Even for the untrained eye, our Trabant shows a lot of details, but even experienced amateurs can always find something new to observe. The most interesting is the shadow border: this is where the shadows are longest and the lunar relief is the clearest. It moves away every day and thus shows new formations again and again.

Mercury, Venus, Mars, Jupiter and Saturn are easy to see with the naked eye. On Mars, Jupiter and Saturn, structures of the surface and the atmosphere can be observed in the telescope, while on Venus and Mercury the phase shape. Even in small optics, a lot of details can sometimes be seen. On the other hand, the planets are not always visible and the size of the planetary disc varies greatly, for example Mars is only close enough (in opposition) every two years to be able to make out details - and not every opposition offers the same good conditions.

Single and double stars
An important fact is that all stars (with one exception - our sun) remain point-like no matter how high you zoom in. But what can be seen very nicely with the naked eye on many stars is the different colors of the stars. Just look at the obvious difference between two stars in the constellation Orion, on the one hand the red supergiant Betelgeuse on the left shoulder and the blue Rigel on the right foot of Orion. Color differences are particularly noticeable in the case of double stars. One of the best known and also the most beautiful is certainly Albireo in the Swan. Double stars can also be a very rewarding target for the smaller telescope.

Globular clusters
Globular clusters can also be called the satellites of our Milky Way (our home galaxy). They are usually very far away from us. Globular clusters consist of hundreds of thousands of stars arranged in a spherical shape and they are all very ancient objects. At over 12 billion years old, they are only marginally younger than the universe itself. In small telescopes, the brightest representatives can be seen as nebulous, round "cotton balls". In larger amateur instruments, they are broken down into single stars, a sight that is breathtaking and that you won't soon forget.

Planetary nebula
Planetary nebulae owe their name to their round shape, which resembles a planetary disk. They arose from the repulsion of the gas and plasma envelope of a dying star. In contrast to globular clusters, they are young objects, usually only a few thousand years old. The extent of the planetary nebulae is rather small, therefore one has to enlarge these objects very much in order to distinguish them from a star. The best-known representative of its kind is probably the M57 ring nebula in the lyre, whose ring shape can even be seen in smaller telescopes under good conditions.

Galactic nebulae
These nebulae are gas and dust masses that are excited to shine by nearby stars and thereby emit light (emission nebula) or only reflect the starlight falling on them without radiating themselves (reflection nebula). They mostly consist of hydrogen and a few other elements (mainly oxygen, carbon, helium). There are some quite bright representatives of this group of objects, the best known is undoubtedly the Orion Nebula M42. In order to be able to observe most galactic nebulae well and in detail, a certain telescope opening is required and sometimes also the use of nebula filters, which only detect certain wavelengths, namely exactly the lines emitted by the nebulae (H-alpha, H-beta, O- III, etc ...) and thus increase the contrast.

This group of objects, which is particularly interesting for many, are islands of the world, just like our Milky Way is one, consisting mostly of many tens of billions of stars. At least one galaxy can even be seen with the naked eye on many nights, our largest neighbor - the Andromeda galaxy M31 "only" about 2.5 million light years away. M31 can also be seen in smaller telescopes, but details such as spiral arms and the like are visible. only visible with much larger instruments. In the southern sky one can admire two very close irregular galaxies with the naked eye, the small and the large Magellanic Cloud.

3.a. What can I not see with the telescope?

Visual observation with the eye shows us the universe more modestly than on many astrophotos that we find in books or on the Internet, because, unlike a camera, the human eye only "exposes" a fraction of a second and no subsequent image processing takes place. Nevertheless, there are objects that can look more beautiful in the telescope and can be more detailed than in photos, because the eye can take full advantage of short moments of calmness on the moon and planets, for example. The ability to observe can be trained by drawing, and with experience one will be able to see much more than one initially thinks possible.
"You must have seen it with your own eyes!"


First of all, it is important to observe a few requirements: The telescope must be focused, the zenith mirror may be missing on the lens telescope, observation through window panes may be convenient in winter, but unfortunately the windows have no optical quality and do not allow a sharp image . The problem that most often plagues beginners is that they can't find anything in the night sky. You need patience, a little practice and a good star atlas, then you will soon be successful.

How do we find an object in the sky?
Planets quickly reveal themselves through their brightness, nebulae and galaxies, on the other hand, are mostly invisible to the naked eye, so how do we go about it? It is similar when we go on vacation and enter the forest for the first time. You explore the main path, and then more and more the branches and secret paths. So, first of all, it is important to familiarize yourself with the brightest constellations. A planetarium program with which you can simulate the view in the sky and print it out is helpful, but under the starry sky you are often blinded by the screen - the rotating star map and a red light lamp are a good alternative to rough orientation. Once it is clear which constellation is above us, you can go on and look for the best objects for observation with the telescope in a star atlas and determine their position in the sky relative to the bright stars. A nearby star is sighted with the telescope finder - it is best to keep both eyes open, the star will initially not be visible in the finder. But you can see the crosshairs and with the other eye the star. The telescope is then moved towards the star, and then it appears in the viewfinder and can be centered. It should then be visible in the telescope (beforehand you should have placed the viewfinder parallel to the telescope so that they point in the same direction, e.g. at a distant church tower). More stars are now visible in the viewfinder than with the naked eye, and we become familiar with the section of the sky that it shows (like binoculars). It is best to set a few prominent stars, e.g. Orion's belt, and look through the viewfinder and search for the stars in the star atlas and find matches. Most cheap viewfinders are reversing, the atlas must then be rotated 180 °. Once that has been mastered, you go to look for bright objects near bright stars in the atlas and try to identify them in the viewfinder. In winter e.g. the Orion Nebula in Orion, in summer the globular star cluster M13 in Hercules, in autumn the Andromeda Galaxy in Andromeda, in spring preferably the open star cluster Presepe in the inconspicuous constellation Cancer, not far from Gemini.
After the first experiences, this method will be expanded and objects will be sought that are no longer visible in the viewfinder, but only in the telescope. Small triangles are formed with stars or other figures visible in the viewfinder and crosshairs are placed on an apparently empty space in the sky, but since we always compare with the surrounding stars in the star atlas, we know that there must be something there. Sometimes you have to hang several such figures together to get to the goal, this is called star hopping. Getting to know the sky in this way, discovering the objects yourself, so to speak, is also part of the observation and can be a lot of fun.

I see details that others don't describe, why?
Seeing, just like hearing, must also be trained. When someone looks into a telescope for the first time, they do not yet know the typical procedure for looking at astronomical objects, such as indirect vision, dark adaptation, calm posture etc. Just as a good conductor hears the smallest dissonances in his orchestra, which untrained ears remain hidden, a trained eye can perceive subtle details. In association circles it is expressed as follows: "The biosoftware of a beginner between the eye and the brain is in the 1.0 version. However, it is upgraded every night of observation."

Drawing next to the telescope ...
... is the best exercise to expand our perceptual limits "not seeing - ancestry - guessing - seeing". A restless image (which beginners often simply classify as "blurred") is mostly caused by external effects. Telescopes that are not tempered (suddenly brought out into the cold from a warm apartment), restless atmosphere (e.g. wind in the upper troposphere, cold front), but also homemade seeing, such as open windows, co-observers speaking in front of the telescope (warm air rises upwards !!!) or concrete warmed up during the day or warm exhaust fumes from the street, even the warmth of the roofs can completely ruin the picture. These disruptive factors can only be eliminated with a certain amount of experience (looking for an optimal observation site, etc.).


Of course, but only with suitable safety measures otherwise there is a risk of immediate blindness! Eyepiece solar filters belong directly in the garbage can.They should be screwed into the eyepiece, but they will withstand the immense heat of the sun's rays bundled by the lens for a short time and then may burst or melt! Unfortunately, incomprehensibly, they can still be found every now and then in telescope offers. A simple, cheap and safe method of observing the sun is to make a solar filter with the visual filter film, a Din-A4 sheet costs about 25 €, but there are also prefabricated filters in a holder for many telescopes.

In white light, even with smaller telescopes, you can beautifully observe sunspots on the surface of the sun and, with the appropriate quality and conditions, also torch areas and the granulation of the sun at high magnifications. Another possibility to observe the sun safely is the sun projection, for this purpose simply a sheet of paper or a projection screen is stretched about 30 cm away from the eyepiece and the image of the sun is projected onto it. However, this type of sun observation is not entirely harmless, as there is no sun protection in front of the lens!

Another and particularly attractive way of observing the sun is in the H-alpha light. A suitable (i.e. under 1 Angstrom) H-alpha filter or telescope only allows light to pass in a very narrow range, that of the ionized hydrogen. The sun appears in the H-alpha light in red tones with many fine filaments on the surface and protuberances that can shoot up hundreds of thousands of kilometers from the sun's surface within a few hours.


The question may sound strange, but often beginners can't do that much with the new, unknown technology at first. Many questions about the operation are not explained in the instructions because they are taken for granted by their authors.

What are the eyepieces used for?
Since our eyes have a lens in front of the retina, the light from the lens has to be processed. The eyepiece can be understood as a kind of magnifying glass with which you can view the focal point. The stronger the magnifying glass, the higher the magnification. To change the magnification you have to use a different eyepiece, this is placed in the focuser and clamped with a screw (with the Newtonian telescope this is on the side of the tube, with lens telescopes and SC / Maksutov telescopes at the rear end of the tube). Now the image has to be brought into focus with the fine adjustment of the focuser, thereby bringing the focal points of the lens and the eyepiece into alignment.

Which magnification do I get with which eyepiece?
A simple but very important calculation, the magnification is obtained by dividing the telescope focal length by the focal length of the eyepiece. For example: A 102/1000 refractor has a focal length of 1000mm, if you now use a 25mm eyepiece the calculation is 1000: 25 = 40 - we thus achieve a 40-fold magnification. The smaller the focal length of the eyepiece, the higher the magnification, as a rule of thumb you should never go over twice the value of the aperture - i.e. for a 4-inch (102mm) telescope not over 200 times magnification.

What is the star diagonal used for?
This is routinely used in lens telescopes or compact Maksutovs to redirect the light. You can see through these telescopes "from behind" and the best sky is always above your head. The telescope will then point straight up. In order to look through, you would have to lie on your back under the telescope without the star diagonal. With many lens telescopes, the zenith mirror is included in the beam path, so that you cannot focus without it.

(Text: Benny Hartmann and Tommy Nawratil)