Comparison Yukon 20-50x50 WA vs Celestron LandScout 60

The magnification of the image provided by a telescope. Roughly speaking, this parameter describes how many times the object seen in the eyepiece of the pipe will be larger than when viewed from the same distance with the naked eye.

Multiplicity — the first number (numbers) in the digital marking of optical instruments: for example, the designation 25-75x50 corresponds to a multiplicity from 25x to 75x. Note that most modern telescopes have variable (adjustable) magnification. This allows you to choose the mode of operation depending on the situation: to search for the desired object, it is more convenient to reduce the magnification, providing a wide field of view, and having found it, increase the magnification and examine in detail. However in some models, to change the magnification, you need to replace the eyepiece (see "Replacement eyepiece").

High magnification, on the one hand, makes the tube "long-range" and makes it easy to examine small objects at considerable distances. On the other hand, the angle of view decreases in this case, which makes it difficult to observe moving objects and even aim the optics at the target. In addition, with an increase in the magnification, the diameter of the exit pupil also decreases (see below) and the aperture ratio of the tube; you can compensate for this moment by increasing the lens, but this accordingly affects the price. So it makes sense to specifically look for powerful optics with a high degree of ma...gnification only when such capabilities are fundamentally important.

The type of optical system used in a telescope.

The optical system is a set of lenses and other elements responsible for processing the image entering the eyepiece. Such systems may be of the following types:

— Lens. She's a refractor. Optical systems based solely on lenses. Such systems are relatively simple, inexpensive and at the same time quite functional. Image quality, however, turns out to be somewhat lower than in mirror-lens systems, and with a magnification factor of more than 60x, it deteriorates even more; therefore, lens systems usually have relatively low magnification. In addition, they are longer and heavier. On the other hand, optical devices of this design are quite unpretentious in handling and resistant to shock and shock (although this is still better to avoid); and the mentioned high multiplicity is rarely required in fact. Thus, most modern telescopes use this type of optic.

— Mirror lens. This category includes optical systems built on the basis of concave mirrors (which provide the main increase) and corrective lenses designed to eliminate the distortion that inevitably occurs when using mirrors. One of the key advantages of such systems over lens systems is a clearer image, even at high magnifications — the magnification of telescopes of this type can reach 200x without compromising image quality. In addition, with the same focal length, the body o...f the device can be made much shorter, more compact and lighter. At the same time, mirror-lens systems are expensive and turn out to be rather fragile (however, the latter can be partly compensated for by a rubberized housing and other methods of shock protection). Spyglasses of this type are relatively rare, it is believed that they are better suited for observations at high magnification (including for use as an impromptu telescope).

The field of view of the telescope at a distance of 1 km to the objects under consideration, the so-called "linear field of view". In fact, this is the width (diameter) of the space that falls into the field of view when observed from a distance of 1 km.

This parameter is widely used in the characteristics of telescopes along with the angular field of view (see below): the linear field of view data is more visual and closer to practice, it allows you to evaluate the capabilities of a telescope without resorting to special calculations.

For models of variable magnification (the majority of them), the linear field of view is indicated in the form of two numbers — for the minimum and for the maximum magnification.

Angle of view provided by a telescope.

If you draw two lines from the centre of the lens to two opposite points along the edges of the field of view of the pipe, the angle between these lines will correspond to the angular field of view. Accordingly, the larger the angle, the wider the field of view; however, individual items in it will look smaller. Conversely, an increase in magnification is inevitably associated with a decrease in the viewing angle. And since most modern telescopes have a variable magnification, the angular field of view is also variable, and in the characteristics this indicator is indicated in the form of two numbers — for the minimum and for the maximum magnification.

The presence of diopter correction in the design of the telescope (usually in the eyepiece of the telescope).

This function is intended for those who have vision problems and wear corrective glasses with "plus" or "minus" lenses. It is not very convenient to look into the eyepiece with glasses — in particular, the distance to the eye may be greater than the eye relief (see below), which degrades the quality of the visible image. Contact lenses can be an alternative, but they are not for everyone. Another, more convenient option is just diopter correction: it allows you to set the required number of diopters (to “plus” or “minus”) right in the eyepiece of the device and look into it with the naked eye, seeing a clear image. However the adjustment range (see below) is often relatively small, and for severe vision problems, this function may not provide the desired degree of correction. However, even in such cases, a person in need of glasses will find it much more comfortable to look through a "corrected" eyepiece; the image will be, although not perfect, but clearer than with optics settings for healthy vision.

The range over which the telescope can make diopter adjustment (see above). If the characteristics of the glasses fall within this range, the wearer will be able to see a clear picture in the eyepiece (correctly adjusted) even without glasses. If the glasses are stronger, you will either have to look into them, or take care of contact lenses, or come to terms with the fact that the visible image may not be very clear.

The diameter of the objective is the front lens of the telescope. The term "aperture" is also used for this characteristic.

The lens diameter is one of the most important characteristics of an optical system: the amount of light entering the lens and, accordingly, the image quality (especially in low light) directly depend on the aperture. From the point of view of optical characteristics, we can definitely say that the larger the lens, the better, especially at high magnification (for more details, see “Exit Pupil Diameter”). On the other hand, large lenses significantly affect the size, weight, and most importantly, the cost of telescopes. Therefore, manufacturers usually choose the lens size taking into account the magnification, price category and the specifics of the use of a telescope — especially since at low magnifications and good lighting, even a relatively small aperture may well provide a high-quality image. For more information about these patterns, see "Exit Pupil Diameter". In addition, it is worth noting that the features of the "picture" are affected not only by the mathematical characteristics of the optics, but also by the overall quality of its components.

Exit pupil diameter of a spyglass.

The exit pupil is the projection of the image "seen" by the tube that appears just behind the eyepiece. A person sees an image in a telescope precisely due to the fact that the exit pupil is projected onto the eye.

The exit pupil diameter corresponds to the size of the lens divided by the magnification (see above for both). For example, for a pipe with an aperture of 50 mm, operating at a magnification of 25x, this size will be 50/25 = 2 mm. At the same time, it is believed that in order to ensure the most bright and comfortable image, the exit pupil should be no smaller than the pupil of the observer's eye — and this is 2-3 mm in the light and up to 8 mm (in the elderly — up to 5-6 mm) at dusk. This is the reason why for comfortable work at high magnifications and/or in low light conditions, a telescope must have a fairly large lens. However, most of these optical devices are designed for daytime use, and for this, an exit pupil of 1.33 mm in size is sufficient.

For most modern telescopes, the exit pupil diameter is indicated by two numbers — for the minimum and for the maximum magnification.

Removal of the exit pupil of a telescope.

About the exit pupil itself, see above. Here we note that the offset is such a distance from the eyepiece lens to the observer's eye, at which the size of the visible image from the lens corresponds to the visible size of the eyepiece lens. In other words, the observed "image" in this case occupies the entire space of the eyepiece, without vignetting (darkening at the edges) and without "spreading" beyond the edges of the eyepiece. In this case, the overall image quality will be the best.

When looking down the pipe with the naked eye, the observer usually has no problem getting into the offset distance, and this parameter can be ignored. Problems can arise if the user wears glasses and the diopter adjustment (see above) is not sufficient to comfortably view without glasses. In such cases, it is desirable to use models with eye relief of at least 15 mm: although such a distance will not provide the highest image quality when viewed with glasses, it will allow using the device without any special difficulties. However, in modern telescopes, this parameter can reach 18 mm or even more.

Also note that eye relief may decrease somewhat with increasing magnification; in such cases, two numbers are indicated in the characteristics, corresponding to the removal at the minimum and at the maximum magnification.