Max. useful magnification
The highest useful magnification that the telescope can provide.
The actual magnification of the telescope depends on the focal lengths of the objective (see above) and the eyepiece. Dividing the first by the second, we get the degree of magnification: for example, a system with a 1000 mm objective and a 5 mm eyepiece will give 1000/5 = 200x (in the absence of other elements that affect the magnification, such as a Barlow lens — see below). Thus, by installing different eyepieces in the telescope, you can change the degree of its magnification. However, increasing the magnification beyond a certain limit simply does not make sense: although the apparent size of objects will increase, their detail will not improve, and instead of a small and clear image, the observer will see a large, but blurry one. The maximum useful magnification is precisely the limit above which the telescope simply cannot provide normal image quality. It is believed that, according to the laws of optics, this indicator cannot be more than the diameter of the lens in millimetres, multiplied by two: for example, for a model with an entrance lens of 120 mm, the maximum useful magnification will be 120x2 = 240x.
Note that working at a given degree of multiplicity does not mean the maximum quality and clarity of the image, but in some cases it can be very convenient; see “Maximum resolution magnification"
Lens shielding (by diameter)
The diameter of the space in the field of view of the telescope, closed by any structural element.
Shielding is found exclusively in models with mirrors (reflectors and
mirror-lens, see "Design"): the features of their device are such that any auxiliary element (for example, a mirror that directs light into the eyepiece) is certainly located in the path of light entering the lens and covers part of it. Diameter shielding is indicated as a percentage of the telescope lens size (see above): d/D*100%, where d is the shield diameter, D is the lens diameter. This indicator is also called "linear shielding factor".
A foreign object in the field of view can interfere with observation — for example, in the form of a dark spot when pointing the telescope exactly at the light source. However, a much more serious drawback is the noticeable decrease in contrast associated with the diffraction of light around the screen, and, accordingly, the deterioration of image quality. The linear shielding factor is the main indicator of how much the screen affects the quality of the “picture”: values up to 25% are considered good, up to 30% acceptable, up to 40% tolerable, and shielding more than 40% in diameter leads to serious distortion.
Lens shielding (by area)
The area of space in the field of view of the telescope, closed by some structural element.
Shielding is found exclusively in models with mirrors (reflectors and mirror-lens, see "Design"): the features of their device are such that any auxiliary element (for example, a diagonal mirror, see below) is certainly located in the path of light entering the lens and covers part of it. A foreign object in the field of view can interfere with observation — for example, in the form of a dark spot when pointing the telescope exactly at the light source. However, a much more serious drawback is the noticeable decrease in contrast associated with the diffraction of light around the screen, and, accordingly, the deterioration of image quality. At the same time, the larger the screen, the stronger the impact on the quality of the “picture”.
Area shielding is indicated as a percentage of the total lens area: s/S*100, where s is the screen area, S is the lens area. This parameter is used in fact much less frequently than the screening by diameter described above, because the dependence of image quality on the screen area is described by more complex formulas, and the area itself is more difficult to determine. Also note that some manufacturers or sellers may use area screening data for marketing purposes. For example, for a telescope with 30% shielding in diameter, the shielding in area will be only 9%; the second digit creates a deceptive impression of a small screen...size, while in fact it is quite large and already noticeably affects the contrast and image quality.
Eyepieces
This item indicates the eyepieces included in the standard scope of delivery of the telescope, or rather, the focal lengths of these eyepieces.
Having these data and knowing the focal length of the telescope (see above), it is possible to determine the magnifications that the device can produce out of the box. For a telescope without Barlow lenses (see below) and other additional elements of a similar purpose, the magnification will be equal to the focal length of the objective divided by the focal length of the eyepiece. For example, a 1000 mm optic equipped with 5 and 10 mm "eyes" will be able to give magnifications of 1000/5=200x and 1000/10=100x.
In the absence of a suitable eyepiece in the kit, it can usually be purchased separately.
Enlightenment coating
The presence of an antireflection coating on the surface of the lenses, and sometimes also the prisms of the telescope. Such a coating creates characteristic coloured reflections or iridescent stains on the glass surface.
The meaning of enlightenment is clear from the name: this feature improves the overall light transmission, thus providing a brighter, clearer and higher quality image. For telescopes, this is especially important, since such instruments are used mainly at night and deal with very little light. The general principle behind AR coatings is that they reduce the reflectance of a lens/prism, allowing more light to pass through. In fact, this is implemented as follows: light passes through the coating to the main glass, is reflected from it, but instead of being scattered, it reaches the boundary between the coating and air and is already reflected from it, turning “back” in the original direction. Similarly, it is possible to reduce light loss by reflection from 5% (uncoated lens) to 1% with single-coated and 0.2% or even less with multi-coated; at the same time, due to the microscopic thickness, such coatings do not introduce geometric distortions in the visible image.
Usually, the type of enlightenment is additionally specified in the manufacturer's documentation, and sometimes directly in the characteristics. There are 4 main types in total, here are their main features:
— Single layer (C). One layer of coating on individ...ual (not all) optical elements, and most often only on the outer surface of the lens. This is the simplest and most inexpensive option, used mainly in inexpensive models that are not designed for serious tasks. This is due to the fact that, in general, single-layer enlightenment acts only on a part of the visible spectrum, which is why it is inferior to a multi-layer coating both in terms of efficiency and colour fidelity (sometimes colour distortions can be quite noticeable). And in this case, such a coating is also not applied to everything, but only to individual parts of the optical system. So although single-layer enlightenment is better than none at all, it is suitable mainly for entertainment applications.
— Full single layer (FC). Single-layer coating applied to all optical elements of the telescope. It gives the maximum efficiency available for such coatings in principle. However, since this type of coating is effective only for a relatively small part of the visible spectrum, the quality of colour reproduction is still lower than in multilayer systems.
— Multilayer (MC). Coating of several layers with different refractive indices applied to one or more optical elements (but not all). The number of layers can be different — from 2 – 3 in relatively inexpensive solutions to 6 – 8 or more in high-end telescopes. However, even relatively simple multilayer coatings cover almost the entire visible spectrum and are several times superior to single-layer coatings in terms of reflection reduction. So if good brightness and reliable colour reproduction are important to you, then this option will be more preferable than even full single-layer enlightenment, not to mention incomplete. On the other hand, such optics are more expensive than solutions with a single layer of antireflection coating.
— Full multilayer. The most advanced type of coating: a multi-layer coating applied to all elements of the optical system. This option provides extremely high light transmission and true colour reproduction, but it comes at a cost. Therefore, it can be found mainly among high-end telescopes; and it’s worth looking specifically for a model with such enlightenment when both the brightness of the picture and the reliability of colours are of fundamental importance to you.
Total weight
The total weight of the telescope assembly includes the mount and tripod.
Light weight is convenient primarily for "marching" use and frequent movements from place to place. However, the downside of this is modest performance, high cost, and sometimes both. In addition, a lighter stand smooths out shocks and vibrations worse, which may be important in some situations (for example, if the device is installed near a railway where freight trains often pass).
