Comparison Levenhuk Strike 50 NG vs Celestron PowerSeeker 50AZ

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 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.

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 magnification of the Barlow lens supplied with the telescope.

Such a device (usually, it is made removable) is a diverging lens or lens system installed in front of the eyepiece. In fact, the Barlow lens increases the focal length of the telescope, providing a greater degree of magnification (and a smaller angle of view) with the same eyepiece. In this case, the magnification factor with a lens can be calculated by multiplying the “native” magnification with a given eyepiece by the magnification of the lens itself: for example, if a telescope with a 10 mm eyepiece provided a magnification of 100x, then when installing a 3x Barlow lens, this figure will be 100x3=300x. Of course, the same effect can be achieved with an eyepiece with a reduced focal length. However, firstly, such an eyepiece may not always be available for purchase; secondly, one Barlow lens can be used with all eyepieces suitable for the telescope, expanding the arsenal of available magnifications. This possibility is especially convenient in those cases when the observer needs an extensive set of options for the degree of magnification. For example, a set of 4 eyepieces and one Barlow lens provides 8 magnification options, while working with such a set is more convenient than with 8 separate eyepieces.

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).

The method of attaching the tube to the mount provided in the telescope.

Nowadays, three main such methods are used: rings, screws, plate. Here is a more detailed description of each of them:

— Mounting rings. A pair of rings with screw terminals mounted on a mount. The inner diameter of the rings is approximately equal to the thickness of the pipe, and tightening the screws ensures a tight fit. In this case, the telescope tube, usually, does not have any special stops and is held in the rings solely due to friction. In fact, this allows, by loosening the screws, to move the pipe forward or backward, choosing the optimal position for a particular situation. However, one should be careful here: too much displacement of the mount from the middle, especially in refractors with a long tube length, can upset the balance of the entire structure.
Anyway, the rings are quite simple and at the same time convenient and practical, and compatibility with them is limited solely by the diameter of the tube. Thus, it is this type of fastening that is most popular nowadays. Its disadvantages include the need to independently select a fairly stable position of the telescope, as well as monitor the reliable tightening of the screws — loosening them can lead to the tube slipping and even falling out of the rings.

— Mounting plate. In fact, we are talking...about a dovetail mount. A special rail is provided for this on the telescope body, and a platform with a groove on the mount. When installing the pipe on the mount, the rail slides into the groove from the end and is fixed with a special device such as a latch or screw.
One of the key advantages of mounting plates is the ease and speed of mounting and dismounting the telescope. So, unscrewing and tightening a single retainer screw is easier than fiddling with screw fastening or puffs on rings — especially since in many models this screw can be turned by hand, without special tools. And there is no need to talk about latches. The disadvantage of this option can be called exactingness in the quality of materials and manufacturing accuracy — otherwise, a backlash may appear that can noticeably "spoil the life" of the astronomer. In addition, such a mount has very limited possibilities for moving the telescope back and forth on the mount, or even does not have them at all; and the bars and slots can vary in shape and size, which makes it somewhat difficult to select third-party mounts.

— Mounting screws. Mounts with such a mount have a seat in the form of the letter Y, between the “horns” of which the telescope is installed. At the same time, it is attached to the horns on both sides with screws that are screwed directly into the tube; there are at least two screws on each side so that the pipe cannot rotate around the attachment point on its own.
In general, this fixation option is highly reliable and convenient in the process of using the telescope. The screws are tight, without backlash, hold the tube; when they are weakened, the very backlash may appear, but that’s all; in addition, the telescope will stay on the mount and will not fall if at least one screw remains at least partially tightened. In addition, the fixation point is usually located near the centre of gravity, which by default provides optimal balance and eliminates the need for the user to independently look for an attachment point. On the other hand, the installation and removal of the pipe in such mounts requires more time and hassle than in the systems described above; and the location of screw holes and mounting threads are generally different between models, and designs of this type are usually not interchangeable.