Comparison OPTICON Sky Navigator 70F700EQ vs OPTICON ProWatcher 70F900EQ

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 finder provided in the design of the telescope.

A seeker is a device designed to point the device at a specific celestial object. The need for such a device is due to the fact that telescopes, due to the high magnification, have very small viewing angles, which greatly complicates visual guidance: such a small area of \u200b\u200bthe sky is visible in the eyepiece that it is possible to determine from these data exactly where the telescope is pointed and where it needs to be turning around is almost impossible. Pointing "on the tube" is very inaccurate, especially in the case of mirror models that have a large thickness and relatively short length. The seeker, on the other hand, has a low magnification (or works without magnification at all) and, accordingly, wide viewing angles, thus playing the role of a kind of “sight” for the main optical system of the telescope.

The following types of finders can be used in modern telescopes:

— Optical. Most often, such finders look like a small monocular directed parallel to the optical axis of the telescope. In the field of view of the monocular, markings are usually applied, showing which point in the visible space corresponds to the field of view of the telescope itself. In most cases, optical finders also provide a certain magnification — usually on the order of 5 – 8x, so when working with such systems, usually, the initial pointing of the telescope "...on the tube" is still required. The advantages of optics, as compared to LED finders, are the simplicity of design, low cost, and good suitability for observations in the city, suburbs, and other conditions with fairly bright skies. In addition, such devices do not depend on power sources. Against the background of a dark sky, the markings may be poorly visible, but for such cases there is a specific kind of finders — with an illuminated crosshair. However the backlight requires batteries, but even in the absence of them, the markings remain visible — as in a conventional, non-illuminated finder. Devices of this type are indicated by an index traditional for optics of two numbers, the first of which corresponds to the multiplicity, the second to the diameter of the lens — for example, 5x24.

— With point guidance (LED). This type of seekers is similar in principle to collimator sights: an obligatory design element is a viewing window (in the form of a characteristic glass in a frame), onto which a mark is projected from a light source. This mark can look like a dot or another shape — crosshairs, rings with a dot, etc. The device of such a finder is such that the position of the mark in the window depends on the position of the observer's eye, but this mark always points to the point at which the telescope is pointed. LED finders are more convenient than optical ones in the sense that the user does not have to bring the eye close to the eyepiece — the mark is well visible at a distance of 20 – 30 cm, which makes it easier to point in some situations (for example, if the observed object is located close to the zenith). In addition, such devices are great for working with dark skies. They usually do not have magnification, but this cannot be called a clear disadvantage — for a seeker, a wide field of view is often more important than zoom. But from the unambiguous practical shortcomings, it is worth noting the need for a power source (usually batteries) — without them, the system turns into a useless piece of glass. In addition, collimators as a whole are noticeably more expensive than classical optics, and the mark may be lost against the background of an illuminated sky.

Note that there are telescopes that do not have seekers at all — these are models with a small objective diameter, in which the minimum magnification (see above) is small and provides a fairly wide field of view.

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 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.