Comparison Celestron PowerSeeker 70AZ vs Celestron PowerSeeker 50AZ

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).

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 magnification of the inverting lens supplied with the telescope.

Without the use of such a lens, the telescope, usually, produces an inverted image of the object under consideration. In astronomical observations and astrophotography, this is in most cases not critical, but when considering terrestrial objects, such a position of the “image” causes serious inconvenience. The inverting lens provides a flip of the image, allowing the observer to see the true (not inverted, not mirrored) position of objects in the field of view. This function is found mainly in relatively simple telescopes with a low magnification factor and a small lens size — they are considered the most suitable for ground-based observations. Note that, in addition to "clean" lenses, there are also inverting systems based on prisms.

As for the magnification, it is very small and usually ranges from 1x to 1.5x — this minimizes the impact on image quality (and it is more convenient to increase the overall magnification in other ways — for example, using the Barlow lenses described above).