Comparison Celestron CPC 1100 GPS XLT vs Celestron CPC 925 GPS XLT

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 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)".

The resolution of the telescope, determined according to the Rayleigh criterion.

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.

The Rayleigh criterion is a theoretical value and is calculated using rather complex formulas that take into account, in addition to the diameter of the telescope lens (see above), the wavelength of the observed light, the distance between objects and to the observer, etc. Separately visible, according to this method, are objects located at a greater distance from each other than for the Dawes limit described above; therefore, for the same tel...escope, the Rayleigh resolution will be lower than that of Dawes (and the numbers indicated in this paragraph are correspondingly larger). On the other hand, this indicator depends less on the personal characteristics of the user: even inexperienced observers can distinguish objects at a distance corresponding to the Rayleigh criterion.

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.