Comparison Arsenal 114/900 EQ1 vs National Geographic 114/500 Compact

The type of mount the telescope is equipped with.

A mount is a mechanical unit with which the telescope is attached to a tripod or (in some cases) installed directly on the ground. In addition to mounting, this unit is also responsible for pointing the optics at a certain point in the sky. The most popular nowadays are azimuth devices in different variations - AZ1, AZ2, AZ3, as well as in the form of the so-called Dobson mount. Equatorial mechanisms of different models ( EQ1, EQ2, EQ3, EQ4, EQ5) are noticeably more complex and more expensive, but they provide more possibilities. There are systems that combine both of these types of mounts at once - the so-called azimuth-equatorial ones. And finally, some telescopes are supplied without a mount at all. Here's a more detailed description of these options:

- Azimuthal. The full name is “alt-azimuth”. Traditionally, it has two axes of rotation of the telescope - one for pointing in altitude, the second in azimuth. Different models of such mounts differ in additional control capabilities:

• AZ1. They d...o not have a precision movement system.
• AZ2. Equipped with a system of precise vertical movement (around the horizontal axis).
• AZ3. Equipped with precision movement systems on both axes.

In any case, the second axis (azimuthal) in such systems is always located vertically, regardless of the geographical location of the telescope; This is the key difference from the equatorial mounts described below. In general, azimuth mechanisms are quite simple and inexpensive in themselves, while being quite convenient and practical, which is why this option is the most popular in our time. In addition, they are ideal for observing ground objects. The key disadvantage of this option is its poor suitability for continuous “accompaniment” of celestial bodies (moving across the sky due to the rotation of the Earth). If in a correctly configured equatorial mechanism you need to rotate the telescope along only one axis, then in the azimuthal mechanism you need to use both axes, and unevenly. The situation can be solved using an auto-tracking system, but this function significantly affects the price of the entire device. And even its presence does not guarantee that the telescope is suitable for astrophotography at long exposures - after all, with such use it is necessary to ensure not only accurate movement along each individual axis, but also correction for image rotation in the frame (which is not provided in every auto-tracking system, and also increases the price more).

- Dobson. A specific variation of the alt-azimuth mounts described above, used almost exclusively in reflectors. It also provides two axes of rotation - horizontal and vertical. The key feature of the Dobsonian mount is that it is not designed for a tripod and is mounted directly on the ground or other flat surface; For this purpose, the design provides a wide, massive base. Such systems are excellent for Newtonian telescopes, in which the eyepiece is located in the front part: thanks to the low position of the tube on the mount, the eyepiece itself is at a fairly convenient height. Also, the advantages of “Dobsons” include simplicity, low cost and at the same time good reliability, making them suitable even for large and heavy telescopes. Among the disadvantages, we should note the poor compatibility with uneven surfaces, especially hard ones, like solid rock (while tripods used with other types of mounts do not have this disadvantage).

- Equatorial. Mounts of this type make it possible to synchronize the movement of the telescope with the movement of celestial bodies across the sky, resulting from the rotation of the Earth. The conventional vertical axis, responsible for rotating the telescope from side to side, in such mechanisms is called the right ascension axis (RA), and the horizontal (for pointing along the conventional vertical) is called the declination axis (Dec.). Before use, the equatorial mount is adjusted so that the right ascension axis is directed to the “celestial pole”, parallel to the axis of rotation of the Earth (“the celestial axis”); the specific inclination relative to the vertical depends on the geographic latitude of the observation site. This format of work significantly complicates both the design of the mount itself and the installation procedure. On the other hand, equatorial systems are ideal for long-term “accompaniment” of astronomical objects: in order to compensate for the movement of a celestial body due to the rotation of the Earth and keep the target in the field of view, it is enough to rotate the telescope around the RA axis to the right (clockwise), and with a clearly defined speed - 15° per hour, regardless of the vertical position of the object. This makes such designs ideal for astrophotography, including deep space objects that require long exposures. In fact, this does not even require a full-fledged auto-tracking system - a relatively simple clock mechanism that rotates the telescope around the right ascension axis is enough. The downside of these advantages, in addition to the mentioned complexity and high cost, is their poor suitability for large, heavy telescopes - as the weight of the instrument increases, the weight of a suitable equatorial system increases even faster.
As for the different models of such mounts, they are marked with an alphanumeric index, from EQ1 to EQ5. In general, the higher the number in the designation, the larger and heavier the structure itself (including the tripod, if supplied), the less suitable it is for moving from place to place, but the better it dampens vibrations and shocks. But the restrictions on the weight of the telescope are not directly related to the equatorial mount model.

— Azimuthally-equatorial. Mechanisms that combine two types of mounts. It looks like this: an azimuthal system is installed on a tripod, and an equatorial system is installed on it, in which the telescope is already mounted. This design allows you to use the capabilities of both types of mounts. Thus, the azimuthal mechanism is quite suitable for observing large celestial bodies in near space (the Moon, planets) and large areas of the sky (such as constellations), and it does not require complex preliminary settings. And for astrophotography or for viewing deep space objects at high magnifications, it is more convenient to use the equatorial system. However, in practice, such versatility is extremely rarely required, despite the fact that the combination of two types of mounts complicates the design, increases its cost and reduces reliability. So this option can be found in single models of telescopes.

- Without a mount. The complete absence of a mounting system in the kit does not allow using the telescope out of the box. However, it can be the best option in some cases. The first is if the customer wants to choose the mount at his own discretion, without relying on the manufacturer's decision, or even assemble it himself (for example, quite a lot of astronomers make their own Dobsonian systems). The second typical case is if the household already has a mount (for example, from an old telescope that has fallen into disrepair), and there is simply no need to overpay for a second one. In any case, when choosing such a model, you should pay special attention to the type of fastening for which the pipe is designed - compatibility with a specific mount directly depends on it.

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 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.

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.

Availability of a lunar filter as standard equipment of the telescope.

This useful accessory reduces the brightness and contrast of the light coming from the Moon, allowing the observer to get a clearer image of the surface of the Earth's satellite. Moon filters usually have different degrees of darkness and are available in various designs (neutral gray, green, polarizing, etc.). The choice of a specific option depends on the observation conditions and user preferences.

Using a lunar filter allows you to more comfortably and in detail study various features of the moon's surface, such as craters, mountains and valleys, by preventing excessive illumination and softening contrast. Most modern telescopes are equipped with filters that are placed on the lens; eyepiece filters are also available - they are more compact and cheaper.

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).