Authors: Oleksandr Burlaka and Natalia Virnina
Individuals frequently purchase telescopes as a recreational activity for children and adolescents. However, in actuality, it is a device that is nearly as intricate as a smartphone. Therefore, why not select a telescope as if we were acquiring a technological device for personal use, appreciating its sophistication and utility?
What is a telescope?
Would you like to purchase a telescope for yourself? Yes, not for your child, but for your own use. For some reason, much of the advice available online is presented as if the primary purpose of this optical instrument is to entertain and occupy a child. Consequently, the guidance often centers on how to minimize expenses while still selecting a device of quality.
That said, when individuals purchase a high-technology device for personal use — something for their own enjoyment — they tend to approach it from a markedly different perspective. They examine the technical specifications of the device, aiming to comprehend the purpose of each feature, identify impressive capabilities, and discern less notable aspects. Therefore, let us adopt the same method when selecting a telescope.
Let us proceed on the understanding that our discussion pertains solely to the selection of a telescope for visual observations. Equipment dedicated to astrophotography constitutes a fundamentally different subject and is typically associated with higher costs. Nevertheless, it is entirely feasible to acquire a specialized smartphone mount and utilize it as an alternative solution.
The initial point to comprehend is that any telescope performs three main functions. Firstly, it gathers substantially more light than the human eye’s pupil, enabling the observation of fainter objects. Secondly, it permits the resolution of finer details. Thirdly, it enhances the angular resolution of the image. The first two capabilities are solely dependent on the diameter of the objective lens. The third capability relies on the ratio of the focal length of the objective lens to that of the eyepiece, which are invariably included in the technical specifications.
The optical system of a telescope can be broadly categorized into two principal components. The objective — comprising a lens or mirror — collects incident light and produces an image of the observed object within the focal plane. The eyepiece — a detachable optical element used for direct viewing — serves as a magnifying apparatus through which the observer examines the image generated by the objective.
Telescope specifications
When selecting a telescope, you may encounter various figures and unfamiliar terminologies within its specifications. Contrary to popular belief, the most critical aspect of the instrument is not the magnification, despite manufacturers often emphasizing this feature on the packaging of budget telescopes. Let us analyze each parameter comprehensively: identifying which ones are truly significant and which are merely decorative.
Aperture — the key figure
An essential characteristic of a telescope is its aperture, which refers to the diameter of the objective — the lens or mirror responsible for light collection. This measurement is expressed in millimeters and typically appears as the initial number in the model designation. For instance, a specification of “130/650” indicates an aperture of 130 millimeters and a focal length of 650 millimeters. The diameter is frequently also provided in inches (″). To convert inches to millimeters, multiply the inch value by 2.54; for example, 10 inches equals 254 millimeters.
Why is aperture so significant? Because a telescope primarily functions as a ″light collector.″ The human eye’s pupil dilates to approximately 6–9 mm in darkness, whereas the objective lens of a small telescope with a diameter of 100 mm captures hundreds of times more light. Furthermore, this capability increases not in direct proportion to the diameter, but relative to the surface area: a telescope with a 200-mm mirror gathers four times, rather than just twice, as much light as a 100-mm telescope. This underscores why each additional centimeter of aperture is both costly and highly sought after.
What can be observed depending on the aperture? A telescope with a diameter of 70–90 mm will reveal the craters on the Moon in considerable detail, Saturn’s rings, Jupiter’s four largest moons, and two or three dark bands on its disk, as well as the brightest star clusters and the Andromeda Galaxy. With an aperture of 130–150 mm, one will be able to observe nebulae and many more galaxies from the Messier catalog — although they will appear as diffuse, faint patches of light. In the brightest globular clusters, individual stars can be easily distinguished. An aperture of 200–250 mm signifies a substantial telescope that will serve the user for an extended period. It will permit the observation of structures within nebulae: the arms of luminous galaxies, filaments within nebulae, and other celestial objects that are much fainter than those visible with smaller apertures.
However, there is also a “downside” that is seldom emphasized in advertisements. Aperture correlates with weight and size. While it is relatively easy to pack a small telescope into a backpack and transport a 200-millimeter model to the yard independently, a telescope with an aperture of 300 mm or greater almost invariably necessitates the assistance of two individuals. Moving such a telescope can be quite challenging, as it may not even fit into every vehicle. It is important to remember that the most crucial factor is not the telescope with the largest aperture, but rather the one that is actually used with regularity.
Focal length — the second number
The second numeral in the model designation indicates the focal length of the lens, which is also expressed in millimeters. When a telescope is directed towards an object at a great distance, the light rays will converge at a single point — the focal point — and the span from the mirror or lens to this point is referred to as the focal length. Additionally, the eyepiece possesses its own focal length, which is typically inscribed directly on the eyepiece itself, also measured in millimeters.
There are two pragmatic justifications for this numerical value. Firstly, it approximately corresponds to the length of the tube, particularly in relation to the reflectors and refractors that are commonly accessible in the market. In essence, it addresses the issue of whether the telescope can be accommodated within the trunk. Secondly, in conjunction with the eyepiece, it defines the magnification.
How much will the increase be?
Magnification, or the magnification factor, is a value not inherent to the telescope itself. This is a fundamental aspect to comprehend prior to selecting an instrument. For this particular telescope, the magnification is determined exclusively in conjunction with the eyepiece and is expressed as the ratio of the focal length of the objective lens to that of the eyepiece. In essence, it is computed as follows:
magnification = objective lens focal length/eyepiece focal length.
Using a 130/650 telescope with a 25-millimeter eyepiece, the magnification is calculated as 650 / 25, which equals 26x. By replacing the eyepiece with a 10-millimeter one, the magnification increases to 65x. In essence, magnification is determined not solely by the telescope but by the combination of the telescope and the eyepiece. There is no such thing as “telescope magnification” independently of the eyepiece, regardless of claims made by manufacturers or sellers. When the packaging states “675× magnification” in prominent lettering, it signifies one thing: the product is of poor quality.
The fact is that the useful magnification is limited by the aperture, and the limit is simple — approximately twice the diameter of the lens in millimeters. For a 130-millimeter telescope, this is about 260x. However, keep in mind that not only does the apparent size of the object increase, but so does the distortion caused by atmospheric turbulence — the image will noticeably “shake.” In practice, it rarely makes sense to push the magnification beyond 120–150.
What are the consequences of employing a short-focus eyepiece to surpass the theoretical limit of twice the aperture diameter? The image will appear larger; however, it will also become dimmer and increasingly blurred. No additional detail will be visible — only the same Jupiter, extended into a pale, amorphous form. This phenomenon is referred to by astronomers as ’empty magnification.”
Resolution — the level of detail and clarity in visualization
It is noteworthy that all measurements of objects in the sky are expressed in angular units. A complete circle encompasses 360°, with each degree subdivided into 60 arcminutes (denoted by ‘), and each arcminute further divided into 60 arcseconds (″). To provide a comparative perspective: the angular diameter of the Sun’s disk or a full Moon is approximately 0.5°, equivalent to 30 arcminutes; Jupiter’s disk measures about 40 arcseconds; and when extending your little finger at arm’s length, it roughly covers about 1°.
Resolving power refers to the minimum distance between two points in the sky at which they remain distinguishable as separate entities and do not merge into a single object. It is solely dependent on the aperture and unaffected by other factors. A preliminary estimate can be obtained as follows:
resolving power in arcseconds is approximately ≈ 115 divided by the aperture in millimeters.
For a 100 mm aperture, the angular resolution is approximately 1.15 arcseconds; for a 200 mm aperture, the resolution doubles. Practically, this implies that with a 100-mm telescope, observers will be able to see the bands on Jupiter’s disk, though the fine swirls within them may not be distinguishable. Conversely, a 200 mm telescope allows for the observation of irregularities and dense patches within the bands. However, it is important to recognize that the Earth’s atmosphere is never perfectly stable. This instability causes image blurring and reduces resolution. Therefore, observers must exercise patience, observing the object for an extended period to capture moments of atmospheric stability.
The penetrating power — the capacity to observe faint stars.
The luminosity of stars is quantified using magnitudes, a logarithmic and inversely scaled measurement: the greater the magnitude value, the dimmer the celestial object. Under dark rural conditions, the naked eye can typically perceive stars up to approximately magnitude 6; within the center of a medium-sized city, visibility diminishes to about magnitude 4; and in major urban areas, it is limited to approximately magnitude 2. The brightness differential between two stars that vary by one magnitude corresponds to a factor of 2.512; specifically, a star of magnitude 1 is 2.512 times brighter than a star of magnitude 2.
The limiting magnitude of a telescope refers to the faintest star observable under clear, dark sky conditions. This metric is also influenced by the aperture size, approximating 12.5 for a 100-mm telescope and approximately 14 for a 200-mm telescope. It is important to highlight that light pollution from urban areas diminishes the limiting magnitude by two to three magnitudes. Therefore, relocating to a less polluted area will yield a more significant improvement in observational capability than investing in a higher-cost telescope. Consequently, the most economical enhancement in astronomy is to observe under dark skies outside urban environments.
Source: www.telescopeguide.org
Relative aperture: “fast” or “long-focus”?
Dividing the focal length by the aperture yields the concept known as the relative aperture. This is expressed as a fraction, for example: 650 / 130 = 5, or f/5. Telescopes characterized by small denominators (f/4–f/6) are referred to as fast, whereas those with larger denominators (f/10–f/15) are termed long-focus.
Initially, a word of caution, as the designation may be misleading: for visual observations, a fast telescope does not generate a brighter image. Brightness is determined by the aperture and magnification, rather than this numerical value. This measurement is particularly relevant in astrophotography; in that domain, a fast telescope indeed facilitates capturing images with shorter exposure durations.
What is the practical significance of this parameter? A telescope with a short focal length is characterized by its compact and lightweight design, facilitating low magnification and a broad viewing field. Such telescopes are ideal for observing nebulae, star clusters, and conducting general sky exploration. Nonetheless, they require higher quality eyepieces to prevent image blurring at the edges, as cheaper options tend to degrade image clarity. Conversely, a telescope with a long focal length is larger and more cumbersome but provides higher magnification capabilities and yields sharp, high-contrast images. This type is particularly suitable for detailed observations of the Moon, planets, and double stars.
The field of view refers to the portion of the sky that is visible
The field of view of a telescope is dependent upon the eyepiece and the magnification, and it is also straightforward to compute:
the field of view is calculated as the actual field of view of the eyepiece divided by the magnification.
The field of view of a standard eyepiece is approximately 50°, whereas that of a wide-field eyepiece ranges from 70° to 80° or more. Consequently, at 50x magnification with a standard eyepiece, an observer will perceive a section of the sky measuring one degree — equivalent to approximately two lunar diameters. At 200x magnification, the apparent field of view diminishes to a quarter of a degree, and the Moon will no longer be entirely visible within it.
Therefore, it is advisable to utilize an eyepiece with a long focal length and a wide field of view primarily for the purpose of locating celestial objects. It is nearly impossible to identify objects in the sky using a narrow-field eyepiece; instead, one should first locate the object at low magnification and then switch to a short-focus eyepiece if necessary. It is also important to note that extensive nebulae and galaxies are best observed at low magnifications.
An additional constraint pertains to the minimum magnification. The light beam emanating from the eyepiece should not surpass the diameter of the observer’s pupil; otherwise, a portion of the gathered light will bypass the eye. The beam’s width — which equals the aperture divided by the magnification — must not exceed approximately 7 mm. For a telescope with a 200-millimeter aperture, this implies that setting the magnification below 30× is impractical.
Key takeaways from this discussion
Consider two key metrics — the aperture and the relative aperture. The aperture defines the faintness and size of the objects observable; the relative aperture signifies the optimal use cases for the telescope and its portability. Magnification is not an inherent feature of the telescope itself; if it is specified on the packaging, it should be regarded with skepticism. All other factors — such as resolution, depth of field, and field of view — are derived from these two measurements and the characteristics of the eyepiece.
Types of telescopes: pros and cons
There are three principal categories of telescopes, each involving specific trade-offs. It is important to recognize that perfect optics do not exist; gaining an advantage in one aspect invariably results in a compromise in another. The optical imperfections, known as aberrations, have been a challenge for designers for over four centuries, and it is precisely these aberrations that cause telescopes with identical apertures to generate images of varying quality.
You need not elaborate extensively on this matter. It is sufficient to comprehend the trade-offs associated with each type — and to determine which option aligns most effectively with your personal preferences.
Refractors — lens telescopes
This description aligns with the common perception of a “telescope”: an elongated, enclosed tube featuring a lens at the front and an eyepiece at the rear. Historically, such instruments were among the first telescopes to be developed — Galileo likely observed through a device of similar design.
The issue with refracting telescopes is that the lens does not focus rays of all colors at a singular point. Depending on their wavelength, these rays are refracted at marginally differing angles and converge at various points along the optical axis. In practical applications, this causes colored halos around luminous objects such as the Moon, Jupiter, Venus, and the brightest stars. This phenomenon is termed chromatic aberration.
People learned how to combat this as early as the 18th century. The lens is made of two elements with different curvatures and from different types of glass, which partially compensate for each other. These lenses are cemented together — and this is how a so-called achromatic lens, or achromat, is produced. It is this type of lens that is found in the vast majority of modern, inexpensive refracting telescopes. While the halo does not disappear completely, it is significantly reduced, and the longer the tube, the less noticeable it becomes. In more complex lenses — apochromats — at least three lenses are typically used, and chromatic aberration is virtually absent. The price difference between an achromat and an apochromat of the same aperture is usually several times greater.
The reasons why individuals prefer refractors include their enclosed tube design, which prevents the ingress of dust and warm air; the optics’ requirement for minimal adjustment; and the delivery of sharp, high-contrast images. Per centimeter of aperture, this represents the most effective instrument for observing the Moon and the planets.
The reasons for the general disfavor towards certain optical instruments are multifaceted: as the aperture enlarges, the associated costs escalate exponentially. Manufacturing a large lens composed of multiple precisely matched elements poses significantly greater challenges compared to fabricating a mirror of identical diameter. Consequently, amateur refractors exceeding 150 mm in aperture are seldom encountered, and those with an aperture above 100–120 mm typically entail costs comparable to a high-quality reflector with twice the aperture. Additionally, the physical length of these instruments presents a disadvantage; for instance, a 90/1000 refractor necessitates a tube approximately one meter in length. It is also well understood that increased focal length correlates with a reduction in chromatic aberration.
Reflectors — mirror telescopes
In this configuration, light is gathered by a concave mirror positioned at the rear of the tube, and a small flat mirror redirects the rays towards the eyepiece. The most prevalent design is known as the Newtonian system. These models are among the most widely available in the current market.
A mirror reflects all colors uniformly, thereby eliminating any chromatic aberration in reflectors. This method offers the most economical approach to attain a large aperture: with the budget equivalent to that of a 100-mm apochromat, one can acquire a 250-mm reflector. For enthusiasts interested in observing faint objects such as nebulae, galaxies, and clusters, this option is evidently advantageous. Furthermore, this optical design ensures satisfactory performance when observing planets.
“There is a price to pay,” however, and it is prudent to be informed of it beforehand.
Initially, it is necessary to periodically adjust the mirrors to maintain their relative alignment. This process, known as collimation, typically requires approximately five minutes. It is generally advised to perform collimation regularly, particularly if the telescope is not kept in a fixed position but is transported via the trunk. In practical application, however, collimation is not frequently conducted. Nonetheless, it remains a routine procedure that is unnecessary for a refractor.
Secondly, in fast reflectors (f/5 and faster), stars located near the periphery of the field of view do not manifest as point sources; instead, they appear as slightly elongated objects reminiscent of comets, with tails extending outward. This form of aberration is referred to as coma. Nonetheless, the image remains sharp throughout most of the field of view, rendering this primarily a cosmetic defect that seldom impairs visual observation.
Thirdly, in the most economical models, the mirror is spherical rather than parabolic. Unlike a parabolic shape, a sphere does not focus all rays to a single point. Consequently, even with optimal focusing, stars may appear blurred. This phenomenon is known as spherical aberration. In telescopes with long focal lengths, small apertures, and a focal ratio of f/8–f/10, this effect is almost imperceptible. However, if you encounter a bright reflector telescope at an unusually low price, it is prudent to verify the type of mirror it possesses. It is advisable to avoid parabolic mirrors.
Furthermore, an open tube results in dust accumulation on the mirror and the presence of air currents within. Periodically — typically once every few years — the primary mirror must be removed and cleaned. Additionally, when a reflector stored indoors is taken outdoors, it requires approximately 20 to 60 minutes — depending on its size and the season — to equilibrate to the outdoor temperature. Failure to do so will cause the image to be blurred by turbulent air currents inside the tube.
Catadioptric lenses — comprising both a lens and a mirror
The third type integrates both methodologies: a corrector plate or meniscus lens is situated at the front, accompanied by internal mirrors. The most prevalent designs are the Schmidt-Cassegrain and the Maksutov-Cassegrain.
Their primary advantage is purely practical. Light entering the tube undergoes two additional reflections, first off the primary mirror and subsequently off the secondary mirror, which is situated at the center of the corrector plate or meniscus. Specifically, in the Schmidt-Cassegrain configuration, both mirrors are spherical. The primary mirror possesses a very short focal length; however, the convex secondary mirror corrects the image and prolongs the focal length.
Catadioptric telescopes are generally characterized by their compactness and relatively light weight. A 200-mm Schmidt-Cassegrain telescope can be accommodated within a large backpack, whereas a reflector of equivalent aperture may only just fit into a trunk — and not in every case. For residents of apartments planning to transport their telescope outside urban areas, this consideration is of considerable importance.
Catadioptric telescopes demonstrate minimal chromatic aberration, and their coma is considerably less pronounced than that of Newtonian telescopes. Maksutov telescopes are distinguished for producing highly detailed images of planets, whereas Schmidt-Cassegrain telescopes are recognized for their adaptability and multi-purpose utility.
Cons: The price per centimeter of aperture exceeds that of a reflector; it may take up to an hour for this tube to cool down; the field of view is limited due to the extended focal length; and the front plate has a tendency to accumulate dew, which necessitates the use of a lens hood — an insubstantial tube extension functioning as a visor to prevent air from cooling directly in front of the lens.
Regarding central shielding
In reflector and catadioptric telescopes, a secondary mirror is positioned at the center of the tube, which obstructs a portion of the light flux. The light loss incurred is minimal — merely a few percent. The more nuanced issue pertains to the redistribution of light caused by this obstruction: it slightly diverts light from the star’s central point and redistributes it into the diffraction rings surrounding the star. Consequently, this results in a diminution of contrast in fine details.
The practical conclusion is as follows: if the obstruction constitutes approximately 20% of the diameter, it can be disregarded. Should the obstruction range between 30–35%, as observed in a typical Schmidt-Cassegrain, the resulting impact relative to a refractor of identical aperture will become perceptible to the unaided eye during planetary observation. Concerning nebulae and star clusters, however, this consideration is irrelevant; in such cases, the effective aperture remains the determining factor.
Which optical circuit should you select?
If your primary objective is to observe faint celestial objects and to secure the largest feasible aperture within your budget constraints, it is highly advisable to opt for a reflector telescope — particularly as it is also quite comfortable for planetary observations. Should your focus be on the Moon, planets, double stars, and minimizing maintenance requirements, a refractor telescope would be suitable; however, it is important to consider that high-quality apochromatic models are considerably expensive. If portability and a compact design are your priorities, a catadioptric telescope is recommended, bearing in mind that this choice may necessitate a larger budget.
Eyeglasses and various accessories
The tube constitutes only one component of a telescope. The other critical component — without which observation is unfeasible — is the eyepiece, which dictates what you will observe tonight. While you may replace them at any moment, the mirror or lens cannot be altered. Therefore, it is imperative to regard eyepieces not as mere accessories, but as interchangeable and vital elements of the optical system.
The initial information you should be aware of regarding glasses
A telescope package typically includes one or two eyepieces, which are almost invariably the most economical options available. This is particularly evident for the eyepiece that provides the highest magnification. A 4–5 mm eyepiece from a budget set is notably difficult to use: the field of view is narrow, it requires pressing the eye directly against the lens, and the image quality is dim.
Therefore, the most sensible initial investment following the purchase of a telescope is a high-quality eyepiece. The difference will be immediately apparent, and it will incur considerably lower costs than upgrading to a larger aperture.
How many pairs of glasses do you need?
You will require three pieces; that is all necessary to commence. The initial — a long-focus eyepiece — is designed for low magnification, approximately 20–30x. This is a search eyepiece: it features a wide field of view and encompasses a substantial section of the sky; it is employed to locate an object and position it at the center of the field of view prior to a detailed examination. Additionally, it can be utilized to observe open clusters, large nebulae, and the Milky Way in general.
The second eyepiece provides medium magnification, approximately 50–70x. This serves as your “workhorse”: it is ideally suited for the comfortable observation of the Moon, planets, comets, globular clusters, and galaxies.
The third setting is near the maximum limit of your telescope — that is, approximately 1.5 to 2 times greater than the aperture measured in millimeters. It should be employed infrequently, exclusively during the clearest nights, but it is with this configuration that Saturn’s Cassini Division (the gap within its rings) becomes observable, Jupiter’s moons appear as minute disks, and close binary stars can be distinguished as two separate stars.
To convert this to eyepiece millimeters, divide the telescope’s focal length by the desired magnification. For a 130/650 telescope, the set would be approximately as follows: 25 mm, 10 mm, and 3 mm. Keep in mind that a high-quality eyepiece is never cheap. This is especially true for short-focus eyepieces.
Considerations when purchasing eyeglasses
Eyepiece diameter. The diameter of an eyepiece is measured in inches. On very inexpensive models, you may encounter eyepieces with a diameter of 0.965″, which indicates a low-quality product not worth consideration. In most reputable telescopes, the eyepiece port is designed for either 1.25 inches (the standard, suitable for most models) or 2 inches (which offers a wider field of view but is more costly). Frequently, a 1.25-inch adapter is provided with a 2-inch focuser, offering flexibility in your choice.
The field of view differs between eyepieces: a standard eyepiece typically offers approximately 50 degrees of vision, whereas a wide-angle eyepiece provides a range of 70 to 82 degrees. The distinction is significant; viewing through a wide-field eyepiece is akin to observing through a window, while the standard eyepiece resembles looking through a tube. Although a wide-field eyepiece generally costs three to four times more, it is ideally suited for the observation of open star clusters and large nebulae.
Eye relief refers to the distance at which the entire field of view is visible. When observing while wearing glasses — and in cases of astigmatism when removing them is not feasible — a minimum of 15 mm of eye relief is necessary; otherwise, only the central portion of the field of view will be visible through the glasses. In lower-quality, short-focus eyepieces, this distance often measures approximately 5 mm, requiring the eye to be positioned almost directly against the eyepiece. This situation can be particularly uncomfortable during winter observations, as moisture from the face and exhalation rapidly condenses on the eyepiece.
Diagrams are numerous, including types such as Plessl, orthoscopic, Erfle, and Nagler. A novice observer does not necessarily need comprehensive knowledge of all these types; generally, the Plessl eyepiece offers an optimal balance between cost and quality, fulfilling most requirements. Recently, the advent of zoom eyepieces — characterized by adjustable focal lengths typically ranging from 8 to 24 mm — has been noted. Although it may appear that a single zoom eyepiece could address all observational needs, in practice, such devices tend to be bulky and heavy, posing handling challenges for some focusers. Furthermore, high-quality zoom eyepieces are often quite costly.
Barlow lens
This component is not an eyepiece; rather, it is a small “spacer” that extends the telescope’s focal length and is physically inserted directly in front of the eyepiece. Barlow lenses generally have the same diameter as a standard eyepiece (1.25″) and are typically 2x or 3x; consequently, they double or triple the magnification while diminishing the field of view. In other words, with two eyepieces and one Barlow lens, four distinct magnification levels are accessible — which proves to be more economical than purchasing four separate eyepieces.
The tool in question proves to be advantageous; however, there are three important considerations to bear in mind. Firstly, an inexpensive Barlow lens tends to degrade image quality more significantly than it enhances it; therefore, it is advisable to acquire a reputable one, which typically costs as much as a quality eyepiece. Secondly, this accessory does not overcome the inherent limit on effective magnification: if the maximum magnification of your telescope is 200x, a Barlow lens will not increase it to 400x. Thirdly, it introduces an additional optical element that alters the focal point, potentially resulting in insufficient focuser range.
Diagonal prism: applicable for refractors and catadioptric telescopes
Unlike reflectors, in which the eyepiece assembly is located on the side, in refractors and catadioptric telescopes, the eyepiece is positioned on the opposite side of the objective lens. Consequently, when the telescope is aimed at a high position in the sky, it is necessary to exert some flexibility and strength to observe through the eyepiece. To facilitate more comfortable observations, these designs are frequently equipped with a diagonal prism. This optical component redirects the image to the side, thereby enhancing observational comfort. The diagonal prism is typically integrated into the focuser — directly in front of the eyepiece.
Sun filter
In contrast to the conventional perspective of the telescope as a device solely employed for nocturnal observations, it can nearly always be modified for solar observation. Achieving this necessitates the use of a specialized solar filter.
IMPORTANT! It is hazardous to observe the Sun directly without appropriate filtering. As previously stated, a telescope gathers significantly more radiation than the human eye, and viewing the Sun through such an instrument can cause permanent vision impairment, even if only for a moment or a minimal amount of light enters the lens.
A solar filter generally appears as a cover featuring a specialized film, such as AstroSolar, stretched across the central area. It should be positioned directly before the lens, entirely obstructing the passage of solar radiation. Reflecting approximately 99.999% of the light and heat, this filter ensures that viewing the Sun and its sunspots is entirely safe for your health.
When preparing to observe the Sun, it is imperative to securely attach the filter in place of the lens cap, insert a long-focus eyepiece into the focuser, and then carefully aim the telescope at the Sun. This can be achieved by using the telescope’s shadow as an indicator—ideally, it should be minimized. Upon completion of the observation, one must reverse the procedure: first, disengage the telescope from the Sun’s direction, then remove the eyepiece, and finally, remove the aperture filter.
The Finder — your helpful assistant within the expansive universe
Almost every reputable telescope includes a finder scope. This accessory functions as a supplementary miniature telescope or sight mounted onto the main telescope tube, aiding in accurately identifying the specific point in the sky that the telescope is directed towards. Typically, there are two types of finder scopes. In both instances, a specialized “dovetail” mount on the telescope tube is designed to accommodate the finder scope. Prior to commencing observations, it is essential to ensure that both the finder scope and the main telescope are aligned to observe the same celestial point. To achieve this, direct the telescope towards the Moon or a bright star and use the adjustment screws to align the finder scope accordingly. This process is efficient and straightforward.
For smaller apertures, manufacturers frequently incorporate what is referred to as a “Red Dot.” The image of its red LED — a small dot — is projected onto the sky, facilitating precise identification of the telescope’s orientation.
For the larger apertures within the set, an optical finder is provided — a miniature refracting telescope with a diameter measuring a few centimeters. It allows the observation of more stars than can be seen with the naked eye and possesses a considerably wider field of view than the main telescope. This assists in aligning oneself more precisely in the night sky compared to a Red Dot finder.
A critical notice for solar observations. If your telescope is fitted with an optical finder, it is advised not to mount it during solar observations, or to remove it if it is already attached. Should the finder be essential for balancing the instrument, its covers must be kept closed at all times. An uncovered finder scope poses a significant risk of causing burns if sunlight is inadvertently focused onto your skin.
Mount
Thus far, our discussion has primarily focused on the characteristics of the telescope tube. However, it is important to note that the telescope cannot be operated independently. The tube is affixed to a specialized support structure that stabilizes the instrument and facilitates its orientation towards specified regions of the sky. Generally, this supporting structure can be categorized into a mount (occasionally referred to as the “head”) and a tripod or column. The mount enables the telescope to pivot along two axes, while the tripod or column elevates the device to a position that is convenient for observation. In some instances, the mount and tripod constitute a single, integrated assembly supplied as a complete set with the telescope; alternatively, they may be purchased separately. It is also noteworthy that manufacturers sometimes use the term “mount” to denote a configuration comprising both the “head” and the tripod.
Mounting is generally regarded as the final consideration; however, it should, in fact, be prioritized. Even with the most superior optics, a wobbly or improperly positioned mount can compromise the observational experience and diminish interest in astronomy. An unstable mount, susceptible to the slightest contact or mild breezes, will induce image shake analogous to falling autumn leaves, thereby obstructing focus and obscuring details of celestial objects. Additionally, a mount that is cumbersome to operate can transform televised observation into a strenuous ordeal with the equipment. In either scenario, the evening is likely to conclude with pronounced disappointment.
The primary consideration when making a purchase is the load capacity. Ideally, this should be one and a half to twice the weight of your telescope. If the description indicates “supports 10 kg” and your telescope weighs 8.5 kg, then it is not an appropriate choice for your needs. Additionally, a slender aluminum tripod of the camera-style type is unsuitable for a telescope, regardless of the specifications on the packaging. Please verify the diameter of the legs and confirm whether they are constructed from steel or aluminum.
Altazimuth mount
An alt-azimuth, or simply azimuth, mount features the simplest design: one axis is oriented vertically, while the other is horizontal. The telescope is capable of movement in the left-right and up-down directions — precisely as intuition would indicate. This type of mount represents the most suitable starting point for comfortable visual observations.
There is also a disadvantage. Throughout the night, the sky appears to rotate relative to the observer not around a vertical axis but around an axis directed toward the celestial pole (proximate to the North Star). Consequently, objects frequently traverse diagonally across the field of view, necessitating simultaneous adjustments of the instrument’s positioning along both axes. At lower magnifications, this issue is minimal; however, at higher magnifications, adjustments must be performed every 30–40 seconds.
Most azimuth mounts available on the market are fork mounts (or, colloquially, “forks”). A fork mount consists of a U-shaped bracket with a tube positioned between its “arms” on the lateral axes. A basic version of this design is often included with an inexpensive refractor. More robust and reliable forks are well-suited for short catadioptric tubes. A variation of the fork mount is the J-shaped half-fork mount, where the telescope is attached on only one side. It is lighter and cheaper, but less rigid. Thanks to its compact size, a catadioptric telescope on a fork or half-fork mount can even fit on a balcony.
The Dobsonian mount, commonly known as the “Dobson,” represents one of the most prevalent types of fork mounts. Constructed from materials such as wood, plywood, or particleboard, it features a tube resting like a cannon on a carriage, capable of rotating on broad sliding supports. Its design appears straightforward and is cost-effective. Simultaneously, it provides unwavering stability for large apertures. The setup process is rapid, requiring no specialized expertise, and it offers reliable performance over an extended period. The majority of amateur astronomers engaged in visual observations favor Dobsonian mounts. For those seeking the maximum aperture with minimal financial and effort expenditure, this option is almost certainly the most appropriate.
Equatorial mount
Similar to the azimuth mount, the equatorial mount permits the telescope to rotate about two axes. However, in this instance, one axis is aligned with the Earth’s North Pole, inclined at an angle corresponding to the latitude of the observation site. The second axis is perpendicular to this. The primary benefit is that the Earth’s daily rotation is offset by rotating only a single axis. Once the telescope is aligned with the target object, it suffices to turn a single knob, ensuring the object remains within the field of view.
Equatorial mounts also present certain disadvantages. Firstly, they are considerably less intuitive, and the underlying logic of their movements can be confusing at first: rotating the handles results in the tube moving in a direction that may seem counterintuitive. Nonetheless, users typically become accustomed to this within a few evenings. Additionally, these mounts are heavier and more costly than an azimuth mount of comparable rigidity. Furthermore, they require counterweights, which are themselves quite heavy. Prior to each observation session, the axis must be aligned approximately toward the North Celestial Pole. Another important consideration is that when an object crosses the meridian, the telescope tube strikes the tripod leg, necessitating the flipping of the telescope to the opposite side of the axis.
“I want to press three buttons, and for it to just start moving on its own”
This leads us to an aspect for which certain individuals are prepared to incur additional costs — automation.
The most cost-effective method to eliminate the necessity for manual adjustments when observing a single celestial object is to employ a single-axis drive on an equatorial mount. This device comprises a small motor affixed to the polar axis, which rotates at the same velocity as the apparent movement of the sky. While the initial pointing of the telescope must be conducted manually, activating the drive enables the object to remain within the field of view throughout the night. Some mounts are equipped with this feature as standard, whereas others permit the purchase of the motor separately. From a cost-to-comfort perspective, this represents arguably the most advantageous investment.
The subsequent phase involves employing an equatorial mount equipped with a GoTo system. Both axes of this mount are motorized; the set includes a remote control featuring a comprehensive database of tens of thousands of celestial objects and its own electronic components. Your responsibility is to approximately align the telescope toward the celestial pole at the commencement of observations and to aim it at a few prominent stars. Once this is accomplished, simply select the desired object on the remote control, and the mount will automatically orient the telescope in the appropriate direction. Some contemporary models offer the capability to be operated directly via a smartphone.
GoTo systems are also available on azimuth mounts; this option is simpler and lighter: the electronics handle tracking along both axes, so there’s no need to align anything at all — you just need to point the telescope at a few bright stars at the start. The equatorial version is more complicated to set up, but it moves more smoothly along a single axis. However, in both cases, a motorized mount is significantly more expensive than a mechanical one. In addition, the electronics require a power source — for example, from a car.
If you are inclined towards a large aperture telescope but have budget constraints, and you wish to acquire skills in navigating the night sky (which is less challenging than many perceive), then a Dobsonian telescope is considered an appropriate selection. Should your current intentions involve observing the Moon and planets at high magnifications, it is advisable to opt for an equatorial mount, preferably equipped with a motor on the polar axis. If you are pressed for time and prefer observing rather than searching, investing in a mount with a GoTo system is recommended. However, it is important to note that this option is typically not cost-effective, and some time will be required for setup.
The answer to the Big Question about telescopes, mounts, and everything else
42.
Okay, just kidding! I was just wondering if anyone had read this far
Financial wealth is not the sole determinant of value
Individuals frequently inquire, “What do you suggest for this amount?” The reality is that there is no definitive response. Ultimately, it depends not only on the market offerings but also on various personal circumstances.
Regarding general considerations not previously addressed in this publication, it is advisable to pay attention to the secondary market. Amateur astronomers frequently upgrade their equipment and sell fully operational instruments. If feasible, establish contact with local amateur astronomers and inquire about the possibility of participating in an observation session. Determine which telescope you find most suitable, and seek similar specifications.
When purchasing a telescope, it is advisable not to allocate your entire budget solely for its procurement. Instead, it is prudent to reserve approximately 20–25% of your funds for accessories such as eyepieces, filters, and related items.
However, prior to making a purchase, you must resolve several issues.
Your observational objectives. The Moon and planets necessitate high magnification and contrast; therefore, optimal options include either a long-focal-length refractor or a Maksutov-Cassegrain telescope with an aperture ranging from 100 to 150 mm. For observing nebulae, galaxies, star clusters, and comets, a substantial aperture is essential; hence, the most suitable choice is the largest feasible and transportable Dobsonian reflector within your financial means. If a versatile instrument is desired, a reflector with an aperture of 150 to 200 mm and a focal ratio of approximately f/6 is recommended for long-term, reliable performance.
A designated space for your telescope is essential. Consider in advance where you will store your telescope when it is not in use. The storage area must be protected from moisture and dust. Suitable locations may include a covered balcony, a garage, or your personal room. Please be aware that the telescope may occupy a considerable amount of space within a bedroom.
Transportation. For residents of urban areas, it is highly advisable to utilize your telescope under genuinely dark skies. A compact catadioptric telescope can conveniently be stored in the trunk of any vehicle, accompanied by its mount and tripod. Conversely, a long refractor or reflector equipped with a Dobsonian mount may surpass the permissible size constraints.
Time is a crucial consideration when selecting a telescope, as observational activities can be time-consuming, particularly when conducted outside urban areas. It is essential to allocate sufficient time for travel, as well as for the setup and thermal stabilization of the telescope. The thermal stabilization process, which varies based on the design and size of the instrument, generally requires between 20 and 60 minutes.
What should you expect from your first night with a telescope?
It is highly advisable to test your recently acquired equipment under the actual night sky at your earliest convenience, ideally on the very first clear evening.
The Moon serves as an ideal initial celestial object for observation. It is a spectacle that invariably leaves a lasting impression. Gradually enhance the magnification to examine the seas, craters and their central peaks, mountain ranges, and subtle depressions in greater detail. If Saturn, Jupiter, or Mars are visible in the sky, ensure to observe them as well.
However, do not anticipate a spectacular display from nebulae and galaxies, even the brightest among them. Unlike the images produced by space telescopes or eminent astrophotographers, all diffuse objects will present as indistinct, grayish patches. Nevertheless, with continued practice, you will develop the ability to perceive more details as you acquire the subtle skill of visual observation. Nebulae will disclose a fibrous structure, and the spiral arms of galaxies will become discernible.
There exists a well-known humorous anecdote among amateur astronomers: upon acquiring new astronomical equipment, the sky frequently becomes overcast on that very night — the unfavorable weather conditions are reputed to be proportional to the aperture of the newly obtained instrument.
Actually, it is merely a joke (or is it? That warrants further investigation!). We wish you the best of luck in making your decision — one that will provide genuine enjoyment during many starry nights.
