How to choose a telescope
What device will serve as an excellent gift for a child, expanding his horizons? What purchase can be the beginning of a hobby for a person of any age, gender and income? What activity, at the same time, requires attention and perseverance and encourages trips to nature? As you might have guessed from the title, these questions relate to telescopes and amateur astronomy.
So, first it should be emphasized that a telescope is such a thing that is not very useful without proper knowledge. In this case, a star map will help, which can exist both in electronic form and in classic paper form. I must say that modern astronomical programs allow you to print maps on paper so that they can be used in nature. And with good telescopes, a license for such an application can come as a gift.
Having a map, you can find out what objects, in principle, can be observed in the sky. Further, we recommend studying their properties, which will help arouse interest in astronomy itself, because it is interesting precisely because of the scale of the studied celestial bodies.
Knowing the varieties of celestial objects, you can proceed to the difference between telescopes as such. Like any technical device, there is a set of characteristics here that allows you to understand what advantages and disadvantages this or that model has.
It is this characteristic of the telescope that is the main one, and not the magnification, as one might think. Why?
The fact is that any object observed in an optical telescope is a source of light, reflected or its own. In this case, if the object itself is bright enough to see it with the naked eye, then its details will be less bright.
Plus, there are objects that emit light in insufficient quantities for our eyes.
Thus, a telescope, or similar optical instrument, is an «amplifier» of the light entering our eye.
Therefore, the main characteristic of a telescope is the aperture diameter, that is, the diameter of the objective. The larger it is, the more information we get with it.
Equal to the ratio of the focal length of the lens and the focal length of the eyepiece. The magnification determines the angle of view of the telescope, that is, high magnifications are good for viewing the details of the moons and planets (point objects), and weak ones are good for viewing nebulae and other extended objects.
In addition to magnification, the field of view of the eyepiece affects the angle of view of the telescope, so if you want to “widen the view” of the telescope, it may be worth just matching it with a different eyepiece.
Resolution magnification (maximum useful magnification)
Equals the lens diameter in millimeters multiplied by two. To clarify: for example, you want to see the rings of Saturn through a telescope. To do this, you need to look specifically at the resolving magnification, that is, the larger the diameter of the lens, the more details you will see. Mere magnification does not define this possibility.
Lens focal length
The aperture ratio of the lens depends on this characteristic, which is equal to the ratio of the diameter to the focal length. Aperture, in fact, affects the camera settings for astrophotography.
At the same time, an increase in aperture ratio leads to the appearance of optical distortions — aberrations. As always, you need to strike a balance between aperture and focal length, depending on the planned tasks.
Types of telescopes by optical device
As for the telescopes themselves, they consist of several detachable parts, thanks to which they can be transported disassembled. The parts themselves can be interchangeable for different models of telescopes, and it is their properties that determine the properties of the telescope as a whole.
Naturally, all amateur telescopes are optical; we will not consider radio and X-ray telescopes.
As we know, the main elements that may be present in an optical device are lenses, prisms and mirrors. Depending on their presence and location, telescopes can be divided into several types:
These telescopes are based on refractive optical elements — lenses, which is why they have an elongated shape.
Telescope refractor Celestron PowerSeeker 70 EQ
In fact, this is the oldest type of telescope, historically it appeared the earliest. And it is far from the worst, it has a number of advantages over other types of telescopes. These are the benefits:
- Durability. As you know, optical lenses retain their properties for several decades.
- Tightness. The closed design prevents dust from getting inside the telescope.
- High image quality.
- Ability to create a direct image when using inverting prisms.
At the same time, the refractor telescope also has disadvantages:
- A small relative aperture, due to which little light enters the telescope.
- Long length reduces the convenience of carrying and storage.
- Chromatic aberrations. Manifested in the appearance of colored borders along the edges of objects.
What is the best use of this type of telescope? The refractor is suitable for observing objects with high brightness — stars, planets and satellites. If you want to watch the rings of Saturn or dust storms on Mars, this is the way to go.
However, at night there are objects that cannot be seen with the naked eye, not because they are too small, but because they have low brightness. These are various nebulae, galaxies and so on. In such cases, the next type of telescope will be better for your money.
In telescopes of this type, mirrors are used as the main optical elements. More precisely, the main concave mirror, which is located at the back of the telescope and reflects light onto a small mirror in the center of the structure. Thus, at the same focal lengths, the reflector is shorter than the refractor. That is, if refractors could be called a “pipe”, then reflectors are more like a barrel.
Telescope reflector Celestron AstroMaster 130 EQ
Reflectors have the following advantages:
- High luminosity. In simple terms, the large diameter of the reflectors contributes to the entry of a large amount of light into the telescope, and as a result, distant objects of space that have low brightness can be seen through it.
- Small length.
At the same time, this design has certain disadvantages:
- Open design. Due to this shortcoming, reflectors suffer from dirt entering the housing.
- Coma is a type of aberration in which images of light sources at the edges receive light trails.
- Durability of mirrors
- Large flat mirrors are more susceptible to deformations with temperature changes and displacements under mechanical stress. Ideally, when you buy a reflecting telescope, you should be able to adjust the position of its mirror.
So, thanks to the large diameter of the mirror and aperture ratio, with the help of a reflecting telescope, we can observe objects with low brightness. These are mainly objects outside the solar system, the so-called deep space objects. However, reflectors are suitable for everything else.
As the name implies, they have both mirrors and lenses in their device, that is, they are a kind of hybrid of the first two types. Their advantages:
- Sealed construction.
- Absence of aberrations. The mixed design allows you to compensate for the shortcomings of the lenses with the help of mirrors and vice versa. Due to this, both chromatic aberrations and coma are absent in mirror-lens telescopes.
- Small sizes.
- The complexity of the design entails a drop in contrast.
- High price.
So, the device of mirror-lens telescopes allows to reduce the size, in comparison with the first two schemes. This property can be used for two cases:
- The most compact telescope with average characteristics designed for travel is being developed.
- An expensive amateur telescope is being developed with the maximum characteristics for this class, which is ahead of simple refractors and reflectors due to the large focal length and aperture.
Telescope Veber MAK 1000×90
Telescope Celestron NexStar Evolution 9.25
Mirror-lens telescopes often have a motorized mount, which allows you to entrust automatic search and tracking of space objects.
Telescope mount types
A mount, in relation to telescopes, is nothing more than a device that holds the main tube on a tripod or stand and allows you to control its direction.
Speaking of mounts, the following question should first be covered.
The fact is that the starry sky constantly revolves around the Earth, and the axis of rotation does not coincide with the axis of rotation of our planet. In the process of observing with a telescope, objects move, and over time, you have to adjust its direction. The convenience of this correction is determined by the device of the mount.
So, there are two main types of telescope mounts:
- Alt-azimuth. Everything is simple here — the telescope rotates around two perpendicular axes — vertical (azimuth) and horizontal. The displacement of objects in the starry sky has to be compensated by movement along two axes at once, which complicates the work with the telescope.
- Equatorial. The rotation is also carried out around two perpendicular axes, but one of them is located at an angle to the horizon and is directed to the so-called celestial pole, that is, the point around which the firmament rotates. The telescope can be set in such a way that it will be possible to follow the object by rotating only one axis, which is more convenient than with the first type of mount.
Telescope azimuth mount
Celestron FirstScope 76 Azimuthal Dobsonian Telescope
Telescope with equatorial mount
As already mentioned, there are so-called motorized mounts. In the simplest case, they are a motor with a gearbox that allow you to rotate the telescope at the speed you need to track the object.
More advanced mounts can work in automatic mode, pointing the telescope at the specified object of the starry sky by name or coordinates.
The telescope cannot be used without the appropriate optical accessories, which allow more flexible adjustment of this device for specific tasks. Let’s consider them in order of importance.
An eyepiece is a part of an optical instrument that serves to form an image, facing the observer’s eye.
Eyepiece for telescope Veber Pluto 25mm PLOSSL 1.25″
In the case of telescopes, the eyepieces are interchangeable. The main characteristic of an eyepiece is the focal length, which affects the magnification of the telescope, as mentioned. The smaller the focal length of the eyepiece, the greater the magnification of the telescope. However, when choosing an eyepiece, you should not exceed the maximum useful magnification.
When looking at photographs of telescopes, we can notice a small optical tube that is attached to the main one, parallel to it. She is called a seeker.
Optical finder Sky-Watcher 8×50
It is easy to guess that the seeker serves to point the telescope, having a wider field of view.
Most often there are finders with magnification and focusing, but there are also models with the so-called red dot, that is, made according to the principle of a holographic sight.
Red Dot Finder Synta Sky-Watcher
Also, the seeker can be equipped with a laser beam that is visible in the atmosphere and allows you to orient the telescope properly.
This accessory is a lens that is placed in front of the eyepiece and multiplies the focal length of the lens. The magnification factor is the main characteristic of the Barlow lens.
Objective inserted into a Barlow lens
Theoretically, one Barlow lens doubles the number of possible magnifications of a telescope with eyepieces. For example, if you have two eyepieces, there will be four possible magnifications with one Barlow lens.
In addition, the use of a Barlow lens increases the eyepiece eye relief, that is, it allows you to use a greater distance between the eye and the eyepiece when observing.
But, like any additional element, the Barlow lens introduces certain distortions into the image.
Some Barlow lenses have the additional function of a camera adapter. To do this, they have a special T-thread on the body.
Inverting prisms and diagonal mirrors
A prism is another accessory that is mounted in front of the eyepiece and serves to ensure that the visible image becomes straight, that is, not upside down or mirrored.
Inverting prism for telescope
Diagonal mirrors work in a similar way, the image in them becomes not inverted, but remains mirrored horizontally, unlike prisms.
Both of these types of accessories are useful when observing terrestrial objects.
Optical filter — glass that transmits light with certain characteristics. Telescope filters are mounted on the eyepiece.
Optical filters for telescope
Let’s list what filters for telescopes are (the functions of many of them are clear from the name).
- Colored (green, orange, red, yellow, purple).
- Deep Sky — filters. As a rule, they transmit light in a narrow range. Used to observe deep space objects.
Thus, amateur telescopes are a modular device, the capabilities of which can be expanded with accessories.
Astronomy is not the most common hobby. This is due to the fact that this is an occupation for enthusiasts — despite the technical simplicity of telescopes, there are many nuances that require great knowledge of the subject.
In addition, in our time, people are not as eager for space as, for example, 50 years ago. Discoveries in the field of astronomy extend into the field of local problems and very distant objects. It is already clear that there are no unique resources, and, moreover, life, in near space.
A significant role is played by the fact that astronomy is little studied at school.
However, we think that this science and working with telescopes can «hook» anyone, and you should check it out. And, oddly enough, amateurs have the opportunity to notice something new in the sky.
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