Comparison Celestron Astro Fi 5 vs Celestron ExploraScope 114 AZ

The design characterizes the general principle of the optical system of the telescope.

— Lens (refractors). As the name implies, in such telescopes, a system of lenses is responsible for building the image. Their main advantages are simplicity of design and use, as well as unpretentiousness to shaking, shock and adverse weather conditions (which facilitates outdoor use, including in the cold season). On the other hand, this scheme of operation requires the use of long tubes, which accordingly affects the dimensions of the structure, and the diameter of the lenses (see below) for refractors as a whole is noticeably smaller than for reflectors. In addition, lenses are subject to various distortions, in particular, chromatic aberrations, which lead to the appearance of colour halos and reduce image quality. However, modern telescopes often use various design tricks to neutralize these distortions. Refractors are well suited for observations of relatively close objects such as the Moon or planets, as well as panoramic observations at a relatively low power. In addition, this option is considered optimal for beginner astronomers, including children.

— Mirror (reflectors). In telescopes of this design, the role of the lens is played by a concave mirror, which provides the main magnification of the image. The simplest and mos...t popular reflex scheme — Newton's telescope — involves a combination of a concave main mirror with an additional flat one, which reflects the image into the eyepiece. There are other variations of reflectors, but they are much more complicated and expensive, and therefore they have not received distribution in amateur astronomy. Anyway, telescopes of this type, being simpler, cheaper and more compact than refractors, have larger lenses and are less prone to distortion, which makes it possible to obtain high-quality images of fairly distant objects. Their main disadvantage is delicacy and difficulty in handling. Thus, mirrors are sensitive to shocks and shocks, the optics need to be adjusted from time to time, and before starting observation, it is necessary to wait for temperature equilibrium — otherwise the difference in air temperatures in the tube and outside will lead to a loss of image clarity (the same "haze" effect that can be seen above hot asphalt on a summer day). Also note that most reflectors give distortions at the edges of the image (the so-called "coma"), which narrows the actual field of view and makes it difficult to use them for astrophotography. However, in many models this drawback has been corrected, in others it is possible to use corrective lenses and other similar accessories, due to which reflectors are still the most popular option among astrophotographers.

— Mirror lens. Such telescopes, in fact, are mirror models (see above), designed according to specific schemes and supplemented with corrective lenses to eliminate various distortions. Thanks to this, it becomes possible to further improve the quality of the "picture" compared to classical refractors, while at the same time retaining their main advantages — primarily compactness and relatively low cost. Among the mirror-lens models, there are also several different systems. Thus, Schmidt-Cassegrain systems are compact, inexpensive, and not as sensitive to small shocks as classical Newtonian reflectors; and the Maksutov systems (Maksutov-Cassegrain for close objects and Maksutov-Newton for distant ones) are somewhat more expensive, but are considered more advanced.

Telescope objective diameter; this parameter is also called "aperture". In refractor models (see "Design"), it corresponds to the diameter of the entrance lens, in models with a mirror (see ibid.), it corresponds to the diameter of the main mirror. Anyway, the larger the aperture, the more light enters the lens, the higher (ceteris paribus) the aperture ratio of the telescope and its magnification indicators (see below), and the better it is suitable for working with small, dim or distant astronomical objects (primarily photographing them). On the other hand, with the same type of construction, a larger lens is more expensive. Therefore, when choosing for this parameter, it is worth proceeding from the real needs and features of the application. For example, if you do not plan to observe and shoot remote (“deep-sky”) objects, there is no need to chase high aperture. In addition, do not forget that the actual image quality depends on many other indicators.

Designing and manufacturing large lenses is not an easy and expensive task, but mirrors can be made quite large without a significant increase in cost. Therefore, consumer-grade refracting telescopes are practically not equipped with lenses with a diameter of more than 150 mm, but among reflector-type instruments, indicators of 100-150 mm correspond to the average level, while in the most advanced models this figure can exceed 400 mm.

The focal length of the telescope lens.

Focal length — this is the distance from the optical centre of the lens to the plane on which the image is projected (screen, film, matrix), at which the telescope lens will produce the clearest possible image. The longer the focal length, the greater the magnification the telescope can provide; however, keep in mind that magnification figures are also related to the focal length of the eyepiece used and the diameter of the lens (see below for more on this). But what this parameter directly affects is the dimensions of the device, more precisely, the length of the tube. In the case of refractors and most reflectors (see "Design"), the length of the telescope approximately corresponds to its focal length, but in mirror-lens models they can be 3-4 times shorter than the focal length.

Also note that the focal length is taken into account in some formulas that characterize the quality of the telescope. For example, it is believed that for good visibility through the simplest type of refracting telescope — the so-called achromat — it is necessary that its focal length is not less than D ^ 2/10 (the square of the lens diameter divided by 10), and preferably not less than D ^ 2/9.

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 highest resolution magnification that a telescope can provide. In fact, this is the magnification at which the telescope provides maximum detail of the image and allows you to see all the small details that, in principle, it is possible to see in it. When the magnification is reduced below this value, the size of visible details decreases, which impairs their visibility, when magnified, diffraction phenomena become noticeable, due to which the details begin to blur.

The maximum resolving magnification is less than the maximum useful one (see above) — it is somewhere around 1.4 ... 1.5 of the lens diameter in millimetres (different formulas give different values, it is impossible to determine this value unambiguously, since much depends on the subjective sensations of the observer and features of his vision). However, it is worth working with this magnification if you want to consider the maximum amount of detail — for example, irregularities on the surface of the Moon or binary stars. It makes sense to take a larger magnification (within the maximum useful one) only for viewing bright contrasting objects, and also if the observer has vision problems.

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 luminosity of a telescope characterizes the total amount of light "captured" by the system and transmitted to the observer's eye. In terms of numbers, aperture is the ratio between the diameter of the lens and the focal length (see above): for example, for a system with an aperture of 100 mm and a focal length of 1000 mm, the aperture will be 100/1000 = 1/10. This indicator is also called "relative aperture".

When choosing according to aperture ratio, it is necessary first of all to take into account for what purposes the telescope is planned to be used. A large relative aperture is very convenient for astrophotography, because allows a large amount of light to pass through and allows you to work with faster shutter speeds. But for visual observations, high aperture is not required — on the contrary, longer-focus (and, accordingly, less aperture) telescopes have a lower level of aberrations and allow the use of more convenient eyepieces for observation. Also note that a large aperture requires the use of large lenses, which accordingly affects the dimensions, weight and price of the telescope.

The penetrating power of a telescope is the magnitude of the faintest stars that can be seen through it under perfect viewing conditions (at the zenith, in clear air). This indicator describes the ability of the telescope to see small and faintly luminous astronomical objects.

When evaluating the capabilities of a telescope in terms of this indicator, it should be taken into account that the brighter the object, the smaller its magnitude: for example, for Sirius, the brightest star in the night sky, this indicator is -1, and for the much dimmer Polar Star — about 2. The largest magnitude visible to the naked eye is about 6.5.

Thus, the larger the number in this characteristic, the better the telescope is suitable for working with dim objects. The humblest modern models can see stars around magnitude 10, and the most advanced consumer-level systems are capable of viewing at magnitudes greater than 15—nearly 4,000 times fainter than the minimum for the naked eye.

Note that the actual penetrating power is directly related to the magnification factor. It is believed that telescopes reach their maximum in this indicator when using eyepieces that provide a magnification of the order of 0.7D (where D is the objective diameter in millimetres).

The resolution of the telescope, determined according to the Dawes criterion. This indicator is also called the Dawes limit. (There is also a reading of "Daves", but it is not correct).

Resolution in this case is an indicator that characterizes the ability of a telescope to distinguish individual light sources located at a close distance, in other words, the ability to see them as separate objects. This indicator is measured in arc seconds (1 '' is 1/3600 of a degree). At distances smaller than the resolution, these sources (for example, double stars) will merge into a continuous spot. Thus, the lower the numbers in this paragraph, the higher the resolution, the better the telescope is suitable for looking at closely spaced objects. However, note that in this case we are not talking about the ability to see objects completely separate from each other, but only about the ability to identify two light sources in an elongated light spot that have merged (for the observer) into one. In order for an observer to see two separate sources, the distance between them must be approximately twice the claimed resolution.

According to the Dawes criterion, the resolution directly depends on the diameter of the telescope lens (see above): the larger the aperture, the smaller the angle between separately visible objects can be and the higher the resolution. In general, this indicator is similar to the Rayleigh criterion (see "Resolution (Rayleigh)"), however, i...t was derived experimentally, and not theoretically. Therefore, on the one hand, the Dawes limit more accurately describes the practical capabilities of the telescope, on the other hand, the correspondence to these capabilities largely depends on the subjective characteristics of the observer. Simply put, a person without experience in observing double objects, or having vision problems, may simply “not recognize” two light sources in an elongated spot if they are located at a distance comparable to the Dawes limit. For more on the difference between the criteria, see "Resolution (Rayleigh)".