Comparison Arsenal 130/900 EQ2 vs Arsenal 130/650 EQ2

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 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 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.

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 type of mirror installed in a reflector or combined model (see “Design”).

Let us recall that the mirror in such models performs the same function as the objective lens in classical refracting telescopes - that is, it is directly responsible for magnifying the image. The type of mirror is indicated by its general shape:

- Spherical. The most common option, which is primarily due to ease of production and, as a consequence, low cost. On the other hand, a spherical mirror, purely technically, is not capable of concentrating a beam of light as effectively as a parabolic one does. This causes distortions known as spherical aberrations; they can lead to a noticeable deterioration in sharpness, and this effect becomes most noticeable at high magnifications. True, there are telescopes that are practically not susceptible to this phenomenon - namely, long-focus models in which the focal length is 8 to 10 times the size of the mirror; however, such devices are bulky and heavy. In light of this, it is worth specifically looking for models with this type of mirrors mainly in two cases: either if the telescope is planned to be used at a relatively small magnification (for example, for observing the Moon, planets, constellations), or if you are not bothered by the dimensions and weight.

— Parabolic. Mirrors in the shape of a paraboloid of rotation almost perfectly concentrate the rays entering the telescope at the desi...red point in the optical system. Thanks to this, reflectors with such equipment provide a very clear image even at high magnification levels and regardless of the focal length. The main disadvantage of this type of mirror is the rather high cost associated with the complexity of production. So it makes sense to pay attention to parabolic reflectors primarily when the described advantages clearly outweigh; A typical example is the search for a relatively compact telescope for observing deep space objects.

The total weight of the telescope assembly includes the mount and tripod.

Light weight is convenient primarily for "marching" use and frequent movements from place to place. However, the downside of this is modest performance, high cost, and sometimes both. In addition, a lighter stand smooths out shocks and vibrations worse, which may be important in some situations (for example, if the device is installed near a railway where freight trains often pass).