Field of view 1 km away
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
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.
Min. focus distance
The smallest distance to the object under consideration at which the telescope is able to fully focus on it — that is, the minimum distance at which the image in the eyepiece will remain clear.
Spotting scopes were originally designed for viewing distant objects, so focus problems can occur if the distance is too small. Thus, manufacturers indicate this parameter in the characteristics. However, even in the most powerful and "long-range" models, the minimum focus distance is about 25 m — at this distance, the naked eye is often enough. Therefore, you should pay attention to this parameter only in cases where the ability to work normally close is of fundamental importance — for example, if the pipe is used at a shooting range, where the distance to the targets can be different, including pretty small.
Lens diameter
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
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.
Eye relief
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.
Changeable eyepiece
The ability to remove the telescope eyepiece and replace it with another one.
The eyepiece is one of the key parts of an optical device, which determines not only comfort for the user, but also quite practical working moments. So, the magnification factor depends on the characteristics of this part (see above) — up to the point that in some telescopes the magnification change is carried out exclusively by changing the eyepieces. Some allow the installation of different eyepieces — with a fixed or variable magnification. In addition, eyepieces can also differ in eye relief, diopter correction range (see above for both) and other practical characteristics.
Note that this feature is typical mainly for premium-level models. At the same time, not all eyepieces available for this model are usually supplied in the kit. When choosing an “eye”, it should be taken into account separately that many manufacturers do not produce eyepieces for all pipes of their brand, but for individual series or even single models; so the issue of compatibility must be approached very carefully.
Body
Material and general construction of the telescope housing.
Most modern models use rubberized cases, in which a solid base (metal or durable polycarbonate) is covered with rubber. This provides not only strength, but also increased shock resistance — even in the case when shock resistance as such (see above) is not claimed in the characteristics. In premium models, magnesium alloy cases are found — it is characterized by high strength combined with low weight.
A separate category are the Galilean pipes (see above) — they are usually produced in a "retro" design and use appropriate materials, such as copper and a high-quality leatherette cover.
