Comparison Arsenal 76/700 AZ2 vs Sigeta Virgo 76/700

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 smallest magnification that the telescope provides. As in the case of the maximum useful increase (see above), in this case we are not talking about an absolutely possible minimum, but about a limit beyond which it makes no sense from a practical point of view. In this case, this limit is related to the size of the exit pupil of the telescope — roughly speaking, a speck of light projected by the eyepiece onto the observer's eye. The lower the magnification, the larger the exit pupil; if it becomes larger than the pupil of the observer's eye, then part of the light, in fact, does not enter the eye, and the efficiency of the optical system decreases. The minimum magnification is the magnification at which the diameter of the exit pupil of the telescope is equal to the size of the pupil of the human eye at night (7 – 8 mm); this parameter is also called "equipupillary magnification". Using a telescope with eyepieces that provide lower magnification values is considered unjustified.

Usually, the formula D/7 is used to determine the equal-pupillary magnification, where D is the diameter of the lens in millimetres (see above): for example, for a model with an aperture of 140 mm, the minimum magnification will be 140/7 = 20x. However, this formula is valid only for night use; when viewed during the day, when the pupil in the eye decreases in size, the actual values of the minimum magnification will be larger — on the order of D / 2.

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 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 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 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.

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).