Comparison OPTICON Phobos 60F700 vs OPTICON StarRanger 45F600AZ

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 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 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 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.