When looking to enter the world of amateur astronomy, the task of choosing the right telescope can be daunting. There are a myriad of different types, each with their own capabilities and function. Navigating the terminology and foreign descriptions, along with vast discrepancies in pricing, can easily frighten an intrigued beginner from taking their first steps toward what can amount to a very fulfilling and captivating hobby. This article aims to break down the three most popular types of telescopes and their ideal usage into an easily digestible synopsis, that works to explain the history, application, and pricing of the three main telescope types: the Refractor, the Reflector, and the Catadioptric.
Refractor Telescopes
Refractor Telescopes (also known as dioptric telescopes) are highly portable, basic telescopes that are ideal for viewing stars, the moon, or other nearby planetary objects. All modern telescopes can trace their parentage to the Refractor telescope, a design first seen in the early seventeenth century, and popularized by none other than the father of observational astronomy, Galileo Galilei. Commanded by Galileo’s watchful eye, the Refractor Telescope produced the first documentation of Jupiter’s four largest moons, Ganymede, Callisto, Io, and Europa, as well as the first observation of the Moon’s mountainous and cratered surface, sunspots, and the appendages of Saturn, later discovered to be the planet’s iconic rings.
Through the utilization of a curved objective lens, which bends—or refracts—parallel beams of light into a single focal image. Photons are carried through the objective lens, and then flow down a straight path to the eyepiece. The length of a Refractor is determined by the size of the telescope’s lens—meaning the larger the lens, the longer the telescope’s tube. This is the same basic principle seen in binoculars, opera glasses, and rifle scopes.
Because of this design, Refractor Telescopes are often longer than their counterparts, but are allowed a smaller diameter. However, this comes at the cost of limited aperture—the amount of light admitted based on the size of the optical opening—which typically ranges from three to five inches. This basic structure, along with the fact that the lenses are immobile, make Refractors easy to transport and maintain, and thus a great telescope for burgeoning and younger astronomers. It is also worth noting that because Refractor Telescopes inverse images only on a vertical axis, so that a viewer is able to see objects right-side up, allowing novice Astrophotographers greater simplicity in capturing a shot. Basic Refractor Telescopes can be found in most department stores, and as such are a very affordable and can be found for as low as $100, creating an easy means for one to dip their toes into astronomy as hobby.
Where Refractor Telescopes succeed in cost, portability, and ease of maintenance, they do suffer from an effect known as Chromatic Aberration, or Color Fringing, where the color wavelengths of incoming light split, disabling the wavelengths from coming together into a perfect single focal point, particularly in the high levels of magnification needed for astronomical viewing. Chromatic Aberration manifests in images as blue or purple fringing that outline the edge of a subject, creating an unwanted aura of color dispersion. Unlike Galileo’s original design, which utilized a single objective lens and whose imaging was prone to Chromatic Aberration, as well as blurred, out of focus sights, modern Refractors contain two or three lenses, respectively known as Doublet Refractors and Triplet Refractors, which—while more expensive—are designed to minimize this problem, if not practically eliminate it.
However, when going to purchase a telescope, the terms one will more often than not come across are Achromatic and Apochromatic Refractors, rather than Doublet or Triplet refractors, and this is where things tend to get a little, and somewhat needlessly, complicated. Explaining the relationship between the levels of chromatic adjustment, the number of lenses, and whether something is Achromatic or Apochromatic tends to unleash a semantical argument that can send even the most seasoned amateur astronomers running to the hills.
First developed by Chester Moore Hall in the mid-eighteenth century, Achromatic Refractor Telescopes utilize two lenses often placed against one another, once concave—which has a high level of wavelength dispersion— and one convex—which has a low level of wavelength dispersion—to effectively balance each other out by refracting the two paralleling waves of light to a single focal point, limiting the effects of Chromatic Aberration. The term Achromatic, simply put, means ‘without color.’ Still, Achromatic Refractors do not work to completely eliminate all levels of Chromatic Aberration.
For that, Peter Hall developed the Apochromatic Refractor Telescope some twenty years after Hall’s Achromatic Refractor, which worked to further cleanse the issue of Chromatic Aberration, as well as Spherical Aberrations caused by the inherent curvature of telescopic lenses in general. This allows for Red, Green, and Blue wavelengths to be brought together on a single focal point, creating a sharper image practically free of any Chromatic or Spherical Aberration at all.
Regardless, Achromatic and Apochromatic Refractors can considerably lighten ones wallet, ranging anywhere from $500 to $10,000 depending on aperture. This is where the design of Refractor Telescopes begins to counteract their efficiency, with price, size, and weight scaling at a much steeper rate than other available telescope options. Still, the low maintenance, and sharpness of image, make Achromatic and Apochromatic telescopes some of the best for Astrophotography, despite the elevated cost.
Reflector Telescopes
Although Moore and Hall developed means for overcoming the problem of Chromatic Aberration in Refractor Telescopes, Isaac Newton found a means of circumventing color fringing nearly a hundred years prior with the invention of his Reflector Telescope (also known as a catoptric telescope) in the seventeenth century. Newton’s Reflector telescope was an improvement on an initial reflector design invented by Niccolo Zucchi earlier in the century, which employs the use of mirrors to fold light into focus, rather than additional lenses. Light is accepted into an open-air optical tube and onto a primary mirror opposite the objective, which then reflects the light onto a secondary mirror and into the eyepiece, positioned on the side of the tube, rather than on the end. Because photons are allowed to enter the optical tube unimpeded by a lens, the light wavelengths are not bent, allowing the mirror to reflect light onto a single focal point. Unlike Refractors, however, Reflector Telescopes invert their views so that they appear upside down, which can be corrected through the use of prisms or diagonals in modern devices, although this is not needed.
While Newton’s design solved the issue of Chromatic Aberration, the limitations of mirrors in the seventeenth century created their own problems—these early mirrors were often made of metal, which created an image that lacked the crispness of focus seen in the Refractor Telescopes of the same period. The mirrors were also susceptible to tarnishing, and did not initially allow for the same level of aperture as Galileo’s design, despite the larger optical tube. Early Reflector Telescopes also suffered from cost and difficulty in their production, both of which were surmounted by the passage of time and evolving mirror technologies. Although Newton’s Reflector Telescope could view the Galilean moons of Jupiter as well, these early disadvantages barred his Reflective telescope from popularity within the scientific community. Still, Newton’s design, along with his understanding of physics, deepened our understanding of universal gravitation and its effects on planetary motion.
Modern Reflector Telescopes are best suited for deep sky observation, and so the larger diameter of the optical mirror,—thus granting a larger aperture with improvements—does not face the same field of view and magnification limitations as Refractor Telescopes, enabling stunning views of both distant nebulae and galaxy clusters. Due to the large aperture of Reflector Telescopes, as well as the employment of a mirror fed eyepiece, allowing light to fold rather than travelling a straight line, Reflector telescopes have a much shorter design as compared to Refractors. This, combined with the fact that it is now cheaper to produce mirrors over lenses, make Reflector Telescopes the most cost effective designs regarding price per aperture of any consumer telescope today.
The biggest downside to Reflectors is the maintenance involved, as the open-air design of the tube allows for dust contamination, and damage if the mirrors aren’t cleaned properly. The mirrored design also requires semi-frequent collimation, or the manual alignment of the focal eye piece with the secondary mirror and the primary mirror to maximize the amount of photons reaching the eye piece. The need for collimation in Reflector telescopes varies from weeks to months depending on use and treatment, and almost every time they travel. For astrophotography, a coma corrector is also recommended to avoid the astigmatism caused by the parabolic mirror for objects at the edge of the telescope’s focal point.
Overall, Reflector Telescopes are the most affordable telescopes for their capabilities, and the price point can range anywhere from $50 for a child’s entry telescope to $2,000. It wasn’t until amateur-astronomer John Dobson’s invention of the Dobsonion Telescope in 1965 that large aperture Reflector Telescopes became commercially viable to the average amateur-astronomer. Easily the most popular Reflector Telescope, The Dobsonian Telescope is a Newtonian Reflector attached to a built in alt-azimuth mount, allowing for a compact, easy to operate, easy to transport design, which allows for unparalleled access to amateur-astronomy and astrophotography at an affordable cost per aperture. A decent Dobsonian can range anywhere from $350 to $2,500—and it’s important to remember that the mount is built in.
Catadioptric Telescopes
Catadioptric Telescopes, as the name suggests, are also commonly called Compound Telescopes and merge the best features of Refractor and Reflector Telescopes, employing both the optical lens of Refractors and mirrored reflection system to an eyepiece just like Reflectors. The design of the Catadioptric Telescope, being the most recent of optic telescope designs, is rooted in Augustin-Jean Fresnel’s reflective lighthouse designs of the early nineteenth century. Just a few decades later, Léon Foucault also took advantage of Catadioptic systems to remove aberrations present in high powered microscopes. It wasn’t until 1931, however, that Bernhard Schmidt applied the compound design to telescopes, with the creation of the Schmidt Astrophotography Camera, a telescope that utilized an aspheric lens to correct the spherical aberrations presented by the mirrors in Reflector Telescopes, while also mitigating coma and chromatic aberration. This system was also utilized by the Kepler Space Telescope which launched in 2009 and was retired in 2018.
Catadioptric Telescopes are easily the most versatile of the consumer telescope types, allowing for both terrestrial and deep space viewing, while also remaining the most portable due to its compact design, enabled by taking advantage of both Refractor and Reflector designs. The optical tubes of Catadioptric Telescopes are shorter than even Reflector Telescopes. The compound design folds light which enters through its aspheric corrector lens, which is reflected by the primary mirror, and magnified by the secondary mirror before the image is received by the eyepiece. This allows the primary and secondary focal points to be changed with great variation, enabling superior near focus when compared to any other telescope, while still providing excellent deep space imaging. This versatility, however, makes perfect focus on the telescope hard to achieve, requiring a high level of skill and patience to operate at professional levels. However, while they still require some amount of collimation, this is needed far less frequent than with Reflector Telescopes. For astrophotographers, most Catadioptric Telescopes offer something special, as they are made with them in mind, with the eyepiece thread crafted for easy adaptability to most digital cameras.
The most popular type of Catadioptic Telescope is the Schmidt-Cassegrain, which had the most portable, compact, and lightweight design for a telescope that is so well-rounded. The main drawback to the Schmidt-Cassegrain, and Catadioptic Telescopes in general, is that they are far more expensive than Reflectors of comparable aperture, but for the serious amateur-astronomer and astrophotographer the price, ranging from $500 to $9,000, is well worth the expenditure.
So, what’s right for you?
When it comes down to it, that’s entirely up to you. Take into account what you wish to accomplish, and weigh what you’re willing to spend against the available aperture and portability. In any case, you can be sure that the stars will be at your fingertips. Check out our beginners guide to buying your first telescope!
