Buying a telescope? This is where you have to watch out | Ganymedes Amstelveen

Buying a telescope? But what exactly should you pay attention to? We’ve compiled everything we know about telescopes for you and are happy to share it with you so you can find and purchase the best telescope for your needs and/or situation.

The market for telescopes is booming! More and more people are getting hooked on stargazing. In the Netherlands, thousands of people practice the hobby of astronomy. Often they are united in amateur associations. Public observatories can be found everywhere in the Netherlands. Most people who have visited a viewing night once become infected with the virus and want to buy a telescope themselves.
The price of telescopes ranges pretty much from 200 euros to 17,000 euros. So there is something for everyone!

As with any investment, you undoubtedly want to weigh the pros and cons. So in this blog we are definitely going to talk about focal lengths, apertures, diameters and eyepieces, but we will also make sure you start asking the right questions of yourself.

Grab a nice cup of coffee and brace yourself to learn all about telescopes. Do you have a little less time? Then jump directly to the topic you want to learn more about via the anchors below!

  1. How does a telescope actually work?
  2. The history of telescopes
  3. What types of telescopes are there?
  4. Terms you need to know
  5. We know, it’s painful, but adjust your expectations
  6. What do you want to go see?
  7. What place do you want to look at?
  8. Indispensable: a good tripod
  9. How to use a telescope
  10. How far can you see with an average quality telescope?
  11. What exactly is astrophotography and which telescope is suitable for it?
  12. Is a spotting scope also a telescope?
  13. Fun to read!
  14. Purchasing a telescope

How does a telescope actually work?

A telescope is an instrument that magnifies objects and/or can gather much more light than our own eyes. Whereas on Earth we use binoculars to magnify objects, we use a telescope for observations in the universe, that is, outside the Earth’s atmosphere. The first telescope even comes from Dutch soil. It was Hans Lippershey who placed two ground lenses in a copper tube, making all the objects you saw through the copper tube seem closer.

But how exactly does it work? If objects are far away then they are smaller and also darker to the point that they reflect so little light that they cannot reach our eyes.

Thus, to see objects from afar we must:

  1. Having as much light as possible;
  2. Magnifying the apparent object

One way to do this is to use lenses. The light rays reflecting off an object reach the object lens, but as a result they also distort and bend toward each other until they reach the focal point. The only problem now is that the image is perceived upside down, oops! Now the second lens comes in handy, namely the one closest to the eye. This ensures that the light rays are straightened, so to speak, before reaching the eye. Incidentally, it actually makes little difference that objects are inverted for deepsky objects and/or planets.

The other factor that matters is the amount of light. The amount of light that can pass through depends on the size of the objective lens. Of course, the size of a lens has its limitations. There are lenses more than a meter in diameter. A good example is the James Webb Space Telescope with a diameter of about 6.5 meters.

However, there is also a downside. So-called refractor telescopes (which thus work with light computation, or refraction) suffer from “lost light. The light that should pass through the lens is actually reflected. So not ideal.

The solution to this is the invention of the reflector telescope. This telescope does not work with Convex (convex) lenses like a refractor telescope, but with concave (concave) mirrors.


You still need a large mirror but the advantage is that it is easier to make large concave mirrors than large Convex lenses.
Consequently, the largest reflector telescope has a diameter of 10.4 meters!!! In fact, work is currently underway on a telescope as large as 30 meters in diameter and even larger!!!! Wow.

Every invention is made to get around a problem and make life a lot easier. Does this solve all our problems then? Not quite.

Regardless of the type of telescope you use, there exists the phenomenon of atmospheric distortion. If you look at a star with the naked eye, it appears to twinkle. So not very pleasant. That is why many telescopes are placed at height because the air is much thinner there and the effect is less pronounced.

And even better! You’ve probably heard of the Hubble Space Telescope! That was launched into space, allowing it to capture light rays from stars and planets (virtually) unimpeded. Recently, the James Webb Space Telescope was launched as a successor and/or complement to the Hubble Space Telescope.

And how accurate is this telescope, for example, you may be asking yourself! Well pretty accurate. Think of a firefly in Tokyo that you can observe in New York.

However, there are ways to magnify objects other than optically (the same way our eyes work, i.e., with light).

For example, the Spitzer telescope which works with infrared cameras that detect heat sources in space. The Kepler telescope is an unmanned spacecraft developed by Nasa to detect planets near other stars that may be habitable like Earth. The Kepler spacecraft uses a schmidt camera to study 100,000 stars at once.

Enough theory about telescopes! Let’s take a closer look at telescopes that may be right for you. We’ll walk you through the technical details so you can make an informed decision if you want to buy a telescope.

The history of telescopes

The Dutch are true inventors. The invention of the telescope demonstrates this. It is suspected that it was Hans Lipperhey who was the first to invent the telescope. Lipperhey, from Middelburg, was seemingly in a battle with two other men, namely, Sacharias Jansen and Jacob Adriaanszoon Metius. Lipperhey, however, was the first to file a successful patent application.

Yet there are also stories that Jansen was the first to come up with the idea of a telescope. Indeed, Jansen and Lipperhey were both spectacle grinders and were practically neighbors of each other. Rumor has it that Lipperhey went with Jansen’s invention when the latter traveled to Germany to promote his invention. We will never know exactly.

The Italian inventor Galileo Galilei was the first to come up with an improved version of the telescope, which made it possible to study celestial bodies. What he saw and could observe was at odds with scientific studies of the time. For the first time, dark spots on the sun could be observed, a myriad of stars, and he could see craters and mountains on the moon. His invention and studies are considered the beginning of modern astronomy.

The invention of the telescope is an important moment in human history. Today, the invention of the telescope still has many applications. It is used for maritime, military, scientific and astronomical applications. Prince Maurice of Orange immediately put pressure on his archenemy Ambrogia Spinola with the invention of the telescope. In turn, he responded that from now on he would never be safe because from now on he could be seen from afar.

An important feature of a good invention is that it keeps improving over time. So it was with the invention of the telescope.

What types of telescopes are there?

Lipperhey’s original invention is known as the “Dutch viewer”. However, the Dutch viewer had a limited viewing angle. Johannes Kepler developed a variant in which the viewing angle was much larger. Instead of a concave lens, he used a convex lens.

Another important development is the use of mirrors instead of lenses. Lenses also become heavier and more expensive the larger they are. In fact, lenses must be sharpened on both sides. Another phenomenon is chromatic aberration. This is caused by a property that glass has called “dispersion”. Color shifting is a name that might be a little clearer. In fact, light of different wavelengths is not refracted to the same extent at the surface of lenses.

Mirrors do not have these objections, which is why you also see that all the great telescopes in the world use mirrors.

Today, there are three types of telescopes, namely:

Terms you need to know

Aperture/diameter of the lens

This is an important feature you want to look for when buying a telescope. The diameter determines how much light is captured, the maximum resolution and thus the maximum useful magnification (see below). Available diameters can be found from 50mm to 356mm.

Focal length

The focal length is the distance light must travel to converge into a single point (the focal point). The focal length is an important number for telescopes. Partly because of the focal point, the magnification of a telescope is determined. In practice, you will find focal lengths from 250mm to 3910mm.

Many times on a telescope you will find the following D=102 mm F=1000 mm depending of course on the aperture and focal length.

Aperture / focal ratio

You find this number by dividing the focal length by the diameter (aperture). Practical ratios are 2 to 15. This number (f) indicates the speed of the device.

Minimum sensible magnification (x)

The size of a human pupil varies between 2 and 7mm. This maximum is used in calculating the minimum sensible magnification. You do this by dividing the diameter/aperture by 7 (maximum pupil size). It ranges from 1 to 50.

Maximum sensible magnification (x)

This is the magnification limit. It is approximately 2.34 times the aperture/diameter. The maximum is about 840 for available telescopes with an aperture of 356mm. Keep in mind the limits of our seeing, often magnification of more than 300 is not useful because the atmosphere through which we are looking is cloudy.

Maximum resolution

It can be calculated by dividing arcsec by the number of pixels. An arcsec is a number corresponding to 1/60. Practically, this is expressed as a grade 1 through 3. Resolution is important in relation to distinctiveness. High distinctiveness can distinguish 2 stars that are close to each other while with low distinctiveness it is perceived as 1 star.

Boundary magnitude

This is a somewhat theoretical figure. If you assume that a star with magnitude 6 is still clearly visible to the naked eye, then you can calculate the limiting magnitude as follows: 6+5x log (d). It is expressed in centimeters. The limiting magnitude for our telescopes is a maximum of 15.


A coating increases light transmission. A coating is applied to the front correction plate and can provide up to 15% more light transmission. We know the following coatings: Starbright XLT, Fully-coated, STN-coated and Fully-multi-coated.

Telescope type

The first telescopes invented in the Netherlands were refractor telescopes. These telescopes work with refraction of light. Another word for refraction of light is refraction. Hence, these telescopes are called refractor telescopes. Refractor telescopes are still very popular for the consumer market.

Reflector telescopes use mostly convex mirrors. The major advantage over lenses is that mirrors do not suffer from color shifting due to the non-uniform refraction of light. In addition, as lenses become larger, they therefore also become much heavier and more expensive.

Then we also have Schmidt-Cassegrain telescopes. A beautiful name but what exactly is it? These are called Catadioptric telescopes. These use both lenses and mirrors. Light collection here is done by a mirror but a lens is also placed in the front of the viewer to eliminate image errors.

Then we have the Maksutov telescopes. These are telescopes with both mirrors and lenses.
It combines a mirror with a weakly negative meniscus lens. A meniscus lens is a lens with a convex and a concave side (i.e., both a convex and concave lens).
Maksutov you often see in Cassegrain variants hence the name Maksutov-Cassegrain telescopes.

In conclusion. There are particularly many telescope variants. Throughout the telescope’s rich history, designers have tried to correct errors and adverse effects. Today for consumer telescopes, telescope manufacturers naturally look at the manufacturing process and related costs.

Type of optics

We have already talked about mirrors and lenses. Looking further into lenses, a number of lens systems can be distinguished. For example, achromatic. Here the fire distance is equal at at least two wavelengths. The systems are designed to counteract adverse effects such as chromatic and spherical aberration.

In telescopes, you see 2-fold or even 3-fold variants (doublet / triplet).


The eyepiece helps determine not only the magnification, but also the quality of the image. It goes somewhat in concert with optics of the telescope. To explain it easily, I will give the example of a stereo system. You can have a very good amplifier, but if the speakers are nothing you won’t get out of it what’s in it. And vice versa, the same is true. Are the speakers outstanding, but the clock radio is the amplifier, it will never be much. In this example, the speakers are the eyepieces. However, buying better eyepieces always gives an improvement in your image. And because the standard eyepiece size these days is 31.7 mm (1.25″), which is the diameter of the sleeve of the eyepiece that goes into the diagonal, you can always use the eyepiece again on another telescope. They used to use 24.5 mm (0.96″) eyepieces and now the 50.8 mm (2″ ) are on the rise. Standard with most telescope sets is 31.7 mm eyepieces. These are standard eyepieces, so in most cases there are still gains to be made here. It is also desirable to have multiple eyepieces so that you can easily search at the lowest magnification by having a larger field of view. If you have found the object then you can use another eyepiece for stronger magnification.

Field of view

The field of view of a telescope is partly determined by the focal point of a telescope, the longer the focal point the narrower/smaller the field of view is. In binoculars, for example, it often says 123 m/1000 m. This means that at 1000 meters away you have a field of view 123 meters wide. For binoculars, this is expressed in degrees. In stargazing it is less so, but because we are looking over a longer distance we can have the moon fully in view at reasonable magnification. Here it will be about 1 degree.


The diagonal is most commonly used with a lens system and Schmidt-Cassegrain system. With these telescopes, you look at the back of the telescope and it is desirable to be able to look into it from above. The diagonal comes in 45 degrees and 90 degrees. With a Newton system, you always look from the side of the telescope and do not need a diagonal. A telescope set usually includes a 90-degree diagonal. This sets the image straight, only left and right are swapped. In some telescope sets, this has also been adjusted. However, this is done with a prism and it often produces some loss in your image.


With mounts, you see roughly 2 forms, azimuthal and equatorial. Azimutal mounts you can move horizontally and vertically. Equatorial designs have the advantage that it always rotates with the pole star, requiring only 1 movement to track an object (e.g., a star). For performing astrophotography, an equatorial is generally the best choice.

Focuser type

These are the systems that are used. A well-known and widely used system is rack and pinion where a gear moves in a linear gear. The Crayford focuser is a more advanced system which works with a spring-loaded shaft that holds the focus tube opposite bearings and controls its movement.

Focuser size

Found in varieties from 1.25 inches to 3.5 inches. You’ll find the larger ones on the more expensive variants.

Tube material

The material of which the (sleeve of the) telescope is made. Each material has different characteristics in terms of hardness and robustness. The casing is made of such materials as aluminum, steel, plastic and carbon.


Seeing is a concept that as you get further into the field, and talk to other amateurs, is something they always complain about. The seeing, or to throw in a fancy term astronomical vision, is term used in astronomy for the influence (blur, “twinkling”) of atmospheric disturbances (turbulence) on vision through telescopes. Visibility is one of the biggest problems in astronomical observations from Earth.

We tried to talk about the principles of stargazing in our own terms, as we at Ganymedes educate someone about a telescope. There are many other aspects to consider. In another blog, we will discuss in detail the topic of astrophotography.

Actually, it is wise to purchase the first telescope from a company or store where you can see and “feel” the telescope, and where you are actually explained how it works. Get informed and realize what kind of instrument you are bringing into the house. Most telescopes are on a tripod, and this is larger in reality than you can see in a photograph anyway. Also, most telescopes are easy to take apart and store, often in 3 main parts: tripod, mount and telescope.

There are a legion of models available on the market, of any brand at various toy stores or online bookstores. However, there is much, if not a lot of “junk” among them. Don’t be tempted by buying a toy telescope for little money. In that case, buy binoculars so they can at least explore the sky. With a toy telescope you ruin interest more than you generate it. You can see in our store what is cheap, we recommend this, but you can also see what you are buying something with for many fun evenings of viewing pleasure.

I hope that from this text you have become a little wiser in the matter of stargazing, and that you dare to take the step to explore the untold vast night sky. We, Jeroen, Peter and Paul, are here to help you get started. It takes a lot of patience and perseverance, but in return you get something special.

We know, it’s painful, but adjust your expectations

You’ve probably already fantasized what it would be like to see the wonderful world of the universe in detail. Perhaps also made a little crazy by images from the Hubble telescope, among others, you may have gone exploring and made the decision to buy a telescope.

But in all honesty, if you expect this then you will probably come out cold. And fair is fair. Can you expect to match Hublle’s quality with your home telescope? No, you can’t. So be realistic and know what you want to look at. Thus, you will also always purchase the right scope for your situation.

What do you want to go see?

Decide in advance what you want to do. Roughly speaking, there is a distinction between visual astronomy or photographic astronomy(astrophotography).

If you want to observe visually you should do it at night. At night, however, our eyes work differently and are barely able to perceive colors. The telescopes that take the beautiful pictures also have a much larger aperture than our eyes. Photographs also use very long exposure times while your eyes can transmit the light of the moment to your brain. So if you want to observe directly visually, you will mostly see no more than spots of light. But if this is all, is it for me I hear you say! We think so because there is still a lot of beauty to admire. For example, the moon with all the beautiful details that you can’t stop looking at. With a good telescope of several hundred euros, you can also look at planets. After all, you also need a lens with higher magnifications. For deep sky objects, you do need an instrument large enough for these observations. But the biggest problem for good observations is light pollution. After all, it should be as dark as possible. Nevertheless, it is definitely possible to observe other galaxies with your own eyes, this is tremendously cool, right?

With photographic observations, you can expect much more in terms of quality and visible detail. However, these telescopes are also more expensive to buy and you have to exercise some patience to arrive at a nice picture. If you have a system that can track an object you can enter the location of the object on the computer after aligning the mount and then you can enjoy a cup of coffee while the system does its job.

In short: there is plenty to experience and see but above all, know exactly what you want to look at and for what purpose you are going to buy a telescope.

What place do you want to look at?

As mentioned, we need a dark night sky to make the best possible observations. But where do you find those places where it’s dark? Places where you can observe are called observatories. The Netherlands has some good locations for stargazing. The Netherlands also has 2 real dark-sky parks, namely the Lauwersmeer National Park and Boschplaat on Terschelling. Furthermore, it is still really good dark on the black rooster in Friesland, dark moors in Drenthe, the dirty hole in South Holland, the sandy plains of the Veluwe, the Maas dunes in Limburg and the beaches of Zeeland.

A nice website with the most beautiful spots can be found on Outdoorinspiration. Outside the Netherlands famous and unique places are for example the Mauna Kea Observatory – Hawaii, Observatorio del Roque de los Muchachos – La Palma (Spain), Observatoire du Pic du Midi de Bigorre (France) and Observatorio Cerro Mamalluca (Chile).

Indispensable: a good tripod

A good tripod is indispensable if you want to purchase a telescope. A solid tripod ensures that your expensive telescope will not come off the tripod and thus can be damaged. In addition, a relatively slightly heavier tripod also protects against wind gusts causing your telescope to topple over. Of course, a tripod also ensures that your camera does not move unnaturally and therefore interfere with observation.

We sell unloading tripods from Celestron, iOptron, ZWO, Benro and other brands.

How do you use a telescope?

For someone just starting out in the hobby, perhaps the most important question. Therefore, in collaboration with, we have created this handy video full of tips & tricks.

How far can you see with an average quality telescope?

This is a question that is actually not very relevant, in that higher magnification does not always mean better. So more is not always better. What matters, of course, is the quality that can be achieved with a given magnification.

To give an idea of what, for example, the Hubble telescope can do. This telescope is capable of observing a Tokyo firefly in New York. You’re talking about a distance of more than 10,000 kilometers. So that’s pretty impressive.

But we don’t have a Hubble telescope, of course. So therefore something more practical. You can view the moon, sun, planets and various nebulae or galaxies with a medium-quality telescope. So…plenty to observe we think!

What exactly is astrophotography and which telescope is suitable for it?

We have already explained that with a cultivation scope you can observe visually or photographically. Because photographic observations require different requirements, there are special astrophotography telescopes on the market. So astrophotography is actually a specialty within photography that focuses on objects in the (nighttime) night sky.

In any case, you need a telescope that supports an astrophotography camera and the necessary accessories such as filter sets, adapters, filter holders, guidescopes and more.

In any case, be well informed if you want to start astrophotography.

Is a spotting scope also a telescope?

Yes. They are just a lot handier and thus suitable for on-site use. It is therefore widely used by birders to see birds in even better detail. Spotting scopes can also be used in penthouses to see even more of the surroundings or high in the mountains to explore the area. Spotting scopes are available in the range of several hundred dollars to several thousand dollars.

Fun to read!

There are lots of great websites about telescopes and astrophotography. There is a lot of information on the websites of observatories. In the Netherlands there are also many public observatories that organize, for example, evenings where you can watch and ask questions. There are also universities that open their observatories and, of course, can share a lot of information.

The European Southern Observatory (ESO) is the European organization for astronomical research in the southern hemisphere.

In the Netherlands, you also have many museums and planetariums that focus on astronomy.

More info and various links can be found, for example, on the KNVWS website.

Buying a telescope at Ganymedes

Ganymedes has the largest collection of stargazers/telescopes in the Netherlands. Think stargazers from:

To make the choice even easier, we have created a roadmap to purchase the right telescope.
It is important to consider the following.

Your budget

Stargazers can be quite pricey. Therefore, set in advance a maximum amount you want to spend on stargazing. For the beginner, an off-the-shelf telescope is recommended. There is no need to purchase additional accessories for this, which means you can get started on your new hobby right away.

Excellent telescopes with which to start your new hobby are the Celestron AstroMaster telescopes or telescopes from the Celestron StarSense Explorer series.

The tripod and mount

Much more important than is often assumed are the tripod and mount of telescopes. After all, the heavier a telescope is the more solidly it must be shored up by a stable mount and tripod. Again, there are several variations to choose from. Ideally, a thick tripod with a sturdy tripod and a heavy-duty mount. But this is where the budget comes into play again: such a solution for stargazing is on the expensive side.

The following variants are present. An azimuthal (parallactic) mount. An Azimuthal mount does nothing but move left and right and up or down. There are several variants in this, the ordinary hand-held one, possibly a computer-controlled one such as the Celestron NexStar SLT, SE or CPC series. There are also the Dobson series telescopes. Which are mostly praised for their value for money. You get a lot of aperture, it’s a Newton system telescope, azimuthal tripod. However, tracking with such a system is not something for beginners. It is excellent for quickly viewing many objects.

A parallactic/equatorial mount also has 2 axes over which to rotate. But a parallactic/equatorial mount can be adjusted to the latitude of the earth. For the Netherlands, that ranges roughly from 51 to 53 degrees. 51 degrees for Maastricht and 53 for Groningen. We usually suggest adjusting it to 52 degrees. You can now track about 1 axis and follow the correct trajectory that moon and planets also follow or all the other objects in our night sky. These mounts used to have fine motion knobs that allowed you to keep tracking the object so very fine. Today, these mounts are equipped with computer control. Using the date, time and coordinates, the position is determined and an alignment on, say, 2 stars makes the mount know where it is. The 40,000+ objects available makes it possible to see a lot in an evening.

Telescope type

Broadly speaking, there are three types of telescopes. First, there is the Newton system, which is also called reflector or mirror telescope. In addition, you have the lens system or refractor. And finally, there is the Schmidt-Cassegrain system. Which system is best to choose depends on what you are interested in. A lens system is great if you want to study the moon and planets. For deepsky objects, a Newton system or even a Schmidt-Cassegrain system is more useful. Keep in mind that “different rules” apply to astrophotography, these are explained in more detail in our blog on astrophotography.

As with many things, there are advantages and disadvantages with each telescope type. The most important are:

Newton system:


  • Larger aperture vs. lens system.
  • No problems with color errors.
  • Relatively cheaper because the opening is larger.
  • Well suited for deepsky objects such as nebulae.


  • Cooling time of the main mirror.
  • Collimation of catch mirror relative to main mirror.
  • Obstruction in the light path, capture mirror suspension.
  • Less suitable for moon and planets.

Lens system:


  • Full opening and therefore no interference in the light path.
  • Sharp and high-contrast images.
  • Little to no cooling time.
  • Well suited for moon and planets.


  • Smaller aperture compared to the newton or Schmidt-Cassegrain system.
  • Color error in the lens, unless the lens is made of more expensive optics.
  • Relatively more expensive because of the smaller openings.
  • Less suitable for deepsky objects (in the case of visual use).

Schmidt-Cassegrain system


  • Relatively compact construction while focal point is long.
  • No problems with color errors.
  • Very many expansion possibilities.
  • Smaller versions easy to take with you when traveling.
  • Well suited for moon and planets as well as deepsky objects.


  • Longer cooling time due to closed mirror system.
  • Collimation of catch mirror relative to main mirror.
  • Obstruction in light road.
  • Mirror flop, although that was more of an issue in earlier times.

Telescope aperture

The aperture size of telescopes determines how much you can see. This is because a larger aperture captures more light, making the image brighter and allowing you to discern more details. A small aperture is suitable for viewing the moon and planets; a large one for deepsky objects.

We hope that with this information you will at least be better able to ask the right questions so that you have gotten an idea of what is possible.

We also understand if you still have questions. So feel free to stop by our store in Amstelveen. Our staff is ready with advice and assistance!

Already looking at all our telescopes? Have fun!