Comparison Celestron Astro Fi 130 vs Celestron SkyProdigy 130

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"

The smallest magnification that the telescope provides. As in the case of the maximum useful increase (see above), in this case we are not talking about an absolutely possible minimum, but about a limit beyond which it makes no sense from a practical point of view. In this case, this limit is related to the size of the exit pupil of the telescope — roughly speaking, a speck of light projected by the eyepiece onto the observer's eye. The lower the magnification, the larger the exit pupil; if it becomes larger than the pupil of the observer's eye, then part of the light, in fact, does not enter the eye, and the efficiency of the optical system decreases. The minimum magnification is the magnification at which the diameter of the exit pupil of the telescope is equal to the size of the pupil of the human eye at night (7 – 8 mm); this parameter is also called "equipupillary magnification". Using a telescope with eyepieces that provide lower magnification values is considered unjustified.

Usually, the formula D/7 is used to determine the equal-pupillary magnification, where D is the diameter of the lens in millimetres (see above): for example, for a model with an aperture of 140 mm, the minimum magnification will be 140/7 = 20x. However, this formula is valid only for night use; when viewed during the day, when the pupil in the eye decreases in size, the actual values of the minimum magnification will be larger — on the order of D / 2.

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.

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.

The type of focuser (mechanical unit responsible for focus the image) provided in the design of the telescope. The focus procedure involves moving the eyepiece of the telescope relative to the lens; different types of focusers differ in the type of mechanism that provides such movement.

— Rack. As the name suggests, these focusers use a rack and pinion mechanism that is moved by turning a pinion gear; and this gear, in turn, is connected to the focus knob. The main advantages of rack systems are simplicity and low cost. At the same time, such mechanisms are not very accurate, moreover, they often have backlashes. In this regard, focusers of this type are typical mainly for low-cost entry-level telescopes.

— Crayford. Focusers of the Crayford system use roller mechanisms in which there are no teeth, and the movement of the eyepiece is carried out due to the friction force between the roller and the moving surface. They are considered much more advanced than rack and pinion — in particular, due to the absence of backlash and smooth focus. The only serious drawback of "crayfords" can be called a certain probability of slippage; however, due to the use of special materials and other design tricks, this probability is practically reduced to zero. Due to this, this type of focuser is found even in the most advanced professional-level telescopes.

— Threaded. The design of the threaded focuser is based on two tubes...— one is inserted into the other and seated on the thread. The movement of the eyepiece necessary for focus is carried out by rotation around the longitudinal axis — similar to how a screw moves in a thread. Such focusers are extremely simple and inexpensive, but they are subject to noticeable backlash and require regular lubrication. In addition, they are rather inconvenient for astrophotography: when adjusting the focus, you have to rotate the camera connected to the eyepiece. Therefore, this kind of focus mechanisms is quite rare, mainly in small and relatively inexpensive telescopes.

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.

The size of the “seat” for the eyepiece, provided in the design of the telescope. Modern models use sockets of standard sizes — most often 0.96", 1.25" or 2".

This parameter is useful, first of all, if you want to buy eyepieces separately: their bore diameter must match the characteristics of the telescope. However, 2" sockets allow the installation of 1.25" eyepieces through a special adapter, but the reverse option is not possible. Note that telescopes with a rim diameter of 2 "are considered the most advanced, because in addition to eyepieces, many additional accessories (distortion correctors, photo adapters, etc.) are produced for this size, and 2" eyepieces themselves provide a wider field of view (although they are more expensive). In turn, "eyes" at 1.25 "are used in relatively inexpensive models, and at 0.96" — in the simplest entry-level telescopes with small lenses (usually up to 50 mm).

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.

The presence of a diagonal mirror in the design or scope of delivery of the telescope.

This accessory is used in combination with lens and mirror-lens telescopes (see "Design"). In such models, the eyepiece is located at the end of the tube and is directed along the optical axis of the telescope; in some situations — for example, when observing objects near the zenith — such an arrangement can be very inconvenient for the observer. The diagonal mirror allows you to direct the eyepiece at an angle to the optical axis, which provides comfort in the situations mentioned. However the image usually turns out to be mirrored (from right to left), however, when observing astronomical objects, this can hardly be called a serious drawback. Diagonal mirrors can be both removable and built-in, it can also be possible to change the angle of rotation of the eyepiece.