by Zane Landers
Basically all telescopes ever made fall into three types. The largest optical telescopes in the world actually have remarkably similar workings, both optically and mechanically, to a humble backyard instrument. Even space telescopes work under the same principles for the most part. All telescopes focus light by converging rays, and this can be done either by refraction – bending light with lenses to focus an image – reflection, using a concave mirror to condense the image to a point, or a combination of mirrors and lenses to achieve the desired goals of working with different standards and sizes of eyepieces, cameras, and scientific instruments.
Pretty much all telescopes, even space telescopes, consist of three parts: the optical tube assembly, the mount, and whatever goes at the focal plane, usually either an eyepiece or camera. For space telescopes, the “mount” is a satellite housing with thrusters and gyroscopes to orient the telescope. For regular telescopes here on Earth, telescopes can use either alt-azimuth or equatorial mounts to aim around the sky. Large telescopes have to use alt-azimuth mounts as an equatorial mount would be too large, expensive, or unstable to make it practical; generally an alt-azimuth mount is easier to use for visual astronomy and better value for the money, but much harder to use for astrophotography, which is why only very large telescopes are typically seen on alt-azimuth mounts for astrophotography. We’ll delve more into the subject of telescope mounts in another article, as with eyepieces.
Regular telescopes can use either alt-azimuth or equatorial mounts to aim around the sky.
The optical tube assembly is technically the only part of the telescope which is actually a “telescope”. The OTA contains the optics needed to focus light into an image, almost always has some sort of focusing mechanism attached or built in, and usually has standardized rails and fittings for accessories, and/or attaching to a mount easily without the use of tools. Generally, we refer to the OTA as the OTA and only call it a “telescope” when it is paired with a mount.
Aperture refers to the diameter of the main lens or mirror in the telescope (usually called the primary mirror or objective), and is what dictates your telescope’s overall light gathering ability and resolving power. Aperture is usually expressed in inches or millimeters.
Note: This is a representation of focal length for refractors and reflectors, not catadioptrics; which have different optical designs that increase focal length, as shown below.
Focal length refers to the distance from the objective to the focus point of the telescope. In the case of some telescopes , this can be longer than the physical length of the instrument due to the way the optics work. Focal length dictates the image scale with cameras and the magnification you’ll get with a given eyepiece. A 25mm eyepiece provides 10x in a 250mm focal length, 40x with 1000mm and 80x with 2000mm, and so on. Contrary to popular belief you don’t always want to use high magnifications when looking through a telescope; low power is actually best on some objects and makes finding them easier. Your telescope itself and the turbulence of the Earth’s atmosphere also limit how much magnification you can use before getting blurry, dim, and unusable images.
Focal ratio or f/ratio is merely the focal length divided by the aperture of your telescope. For photography this works like an f/stop in a camera; for visual use it tends to correlate with eyepiece performance. Scopes of f/12 or above will work well with even the cheapest and worst eyepieces and provide sharp images out to the edge. Scopes between f/7-f/12 will generally perform well with inexpensive eyepieces. Scopes from f/5 to f/7 start to show issues with cheaper eyepieces towards the edge of the field of view; scopes below f/5 need fancy and expensive eyepieces to perform well and generally should have a coma corrector if they’re reflectors.
Refractors, which exclusively use lenses.
Catadioptrics, which use a mix of both lenses and mirrors, though the lenses are almost always not the main optical part producing the image; lenses in a catadioptric usually correct for aberrations caused by the mirror(s).
Reflectors, which exclusively use mirrors.
However, in the amateur world, these categories are a bit subdivided, and you’re likely to see these types specifically:
Achromatic refractors, which use a pair of crown and flint glass lenses. Most cheap refractors are achromats, and they generally suffer from chromatic aberration, which affects faster and larger refractors the most. Chromatic aberration in refractors limits their ability to produce sharp images, and causes lots of problems for long-exposure astrophotography.
Apochromats have become a bit of a meaningless buzzword in the world of astronomical equipment, and can be used to describe a number of the following categories.
ED doublets are technically achromats, but have one or both lenses made out of exotic glass, either fluorite or numerous proprietary glasses such as FPL-51 and FPL-53. They are better than achromats, and are mainly marketed for beginner astrophotography or visual astronomy where some optical aberrations are still permissible and can be ignored. ED doublets can be almost completely color-free if made at longer f/ratios, and faster ED doublets usually don’t present enough chromatic aberration for it to be visible with many cameras.
ED triplets use a three-lens arrangement at the front of the tube to produce sharp images. This makes them not only more expensive than ED doublets or achromats by far, but also quite a bit heavier. However, the third lens does not mean that ED triplets are free of chromatic aberration, and depending on the optical design and types of glass some ED triplets can actually be inferior to ED doublets. You should always do your research to decide which is better for your uses.
Petzval or “quadruplet” refractors use an ED doublet at the front (or occasionally triplet, making them technically “quintuplets” and then a set of corrector/reducer lenses somewhere around the back of the tube to make the telescope more compact, reduce chromatic aberration, and give the telescope a faster f/stop for astrophotography. Petzvals are very hard to scale up and are rarely optimized for use with eyepieces, so you typically see them as small astrophotography instruments. Many telephoto lenses are basically some type of Petzval, and the Petzval refractors you’re likely to come across tend to be similar in specs and performance, with optimizations made for astronomical versus terrestrial use.
If you’re a beginner looking for a first telescope just to look through and it has to be a refractor, 90% of your options are probably achromats and the rest are ED doublets. A triplet is typically too expensive and heavy, while Petzvals are often substandard for viewing at high magnifications and usually not very big at affordable price ranges.
For astrophotography, small refractors are great for beginners as they’re unlikely to have issues with flexure or focus shift during long exposures, and the typically low weight and short focal lengths mean they are not very demanding on your mount’s tracking accuracy.
Refractors are not commonly seen at large apertures, and have not been used for serious research since reflectors with glass mirrors became popular. They don’t scale up well and at large sizes are poorly suited for research or photography.
There’s a pretty finite number of ways you can focus light with just concave and convex mirrors, so there aren’t that many varieties of reflectors, especially ones you’re likely to see.
Newtonian reflectors use a concave, parabolic primary mirror to focus light, and a very flat secondary mirror to deflect that light to the side so that your head or cameras don’t obstruct the whole thing. Newtonians are extremely easy to construct, as well as fabricate the optics for, compared to pretty much any other type of telescope, so almost all large and/or homemade amateur instruments are Newtonians, typically those mounted on a simple alt-azimuth Dobsonian mount and thus referred to as Dobsonians (usually referred to as a “Dob” for short). Newtonians do suffer from edge-of-field aberrations - mainly coma - at faster f/ratios, but a coma corrector can mostly remedy this issue and allow for jaw-dropping images. Compared to the cost of a very large refractor or catadioptric, a coma corrector and nice eyepieces are a small price to pay for big, bright, and beautiful views.
Cassegrain reflectors use a parabolic primary mirror to focus light onto a convex, hyperbolic secondary mirror with extreme curvature, which then “magnifies” the image and sends it through a hole in the primary mirror, and the focused image is then located at the back of the telescope like a refractor. Cassegrains in their purest form, referred to as the Classical Cassegrain, aren’t the best for astrophotography and are very difficult to manufacture due to the nature of the secondary mirror, and as such you don’t tend to see them very often. The Cassegrain design performs best with a focal ratio of f/12 or longer, making it poor for wide fields of view for deep-sky viewing or astrophotography. However, variants on the Cassegrain are very common.
Dall-Kirkham reflectors use an ellipsoidal (basically halfway between spherical and parabolic) primary mirror with a convex spherical secondary mirror, and are very similar to Cassegrains with the advantage of being much easier to manufacture, and thus cheaper. However, the Dall-Kirkham design cannot provide a wide field of view without severe aberrations, and thus regular Dall-Kirkhams are only useful for viewing or photographing very small targets such as the planets. Thus, they usually have slower primary mirrors making them longer than a regular Cassegrain, and usually very long f/ratios; the fastest regular DKs are usually f/12 or above and many can be f/20(!)
Ritchey–Chrétien telescopes (usually referred to as RCs or RCTs), basically another variation on the Cassegrain, use an extremely hyperbolic primary mirror and an extremely hyperbolic convex secondary mirror. They achieve sharp images across a wider field of view than a classical Cassegrain which makes them ideal for photographic purposes, but usually can’t be made much faster than f/8.
Most of the early large research telescopes were Newtonian reflectors, most research instruments until recently tended to be Cassegrains, RCTs, or variations on them, and today the largest telescopes in the world are all reflectors. The Hubble Space Telescope is actually an RCT.
While other telescopes are fine, for the vast majority of cases a Newtonian reflector is usually the best telescope for visual astronomy, whether it’s for beginners or experienced hobbyists. Their simplicity leads to them being easily the most affordable, simplest to construct, and arguably easiest to use instruments possible. Newtonians are also great for long-exposure deep-sky astrophotography if you use a coma corrector, though the side-mounted nature of their focal plane can be irritating when it comes to keeping your mount and camera properly balanced and free of flexure.
A Newtonian reflector is usually the best telescope for visual astronomy.
The Classical Cass and Dall-Kirkham are little more than curiosities, with only a handful of commercial manufacturers and very limited use-cases. However, the RC is a great instrument for astrophotography, particularly of smaller deep-sky targets like globular star clusters and distant galaxies. This comes with a caveat: in addition to their cost, RCs are extremely difficult to collimate (align the optics) due to the extreme curvature of their optics, and these tight tolerances present issues with maintaining focus during long exposures as well; their typically high weights and long focal ratios also make them difficult to work with.
You can do a lot of weird and bizarre things with catadioptric optical designs, but the ones you’re likely to see are all just variations on reflectors.
Schmidt-Cassegrains (usually referred to as SCTs) use a concave primary mirror and convex secondary mirror, but both are spherical, which makes manufacturing extremely easy. The front of the telescope has a Schmidt corrector plate - a thin piece of glass with a series of strange curves created via vacuum pressing in conjunction with normal grinding and polishing - which corrects for the aberrations of using a pair of spherical mirrors. These telescopes are extremely common amongst amateurs, and can be used for a variety of purposes, though they primarily came into being due to convenience of manufacture and are not the most well-optimized.
Maksutov-Cassegrains (usually referred to as Maks) use spherical primary and secondary mirrors, with a massive, thick dish-shaped corrector lens on the front. The curvature on the back of this disk actually is very similar to that of the secondary mirror, so the “secondary mirror” on most commercially-made Maksutovs is nothing more than a portion of the corrector which has had a reflective “spot” deposited on top of the glass. Maks are very easy to manufacture to ridiculously high standards of optical quality, but large ones suffer from heat being trapped by the thick corrector lens and quickly become very heavy and expensive.
Schmidt-Newtonians and Maksutov-Newtonians are Newtonians with the aforementioned correctors and spherical primary mirrors; they have less coma than a regular Newtonian, but have few other advantages, and with today’s relatively affordable Newtonian coma correctors it makes little sense to buy, nor manufacture one of these catadioptric oddities. Many Schmidt-Newtonians made in the past also suffer from less-than-good optics, while Mak-Newtonians can take ages to cool down.
The dreaded Bird-Jones uses a spherical primary mirror with a small corrector mounted just before the focal plane to correct for the resulting optical aberrations. In theory, this is a workable and easy-to-make design. However, manufacturers have elected to make poor-quality Bird-Jones instruments with shoddy optics and low quality control, and usually poor mechanics, accessories, and mountings to match, while lying about their true nature and labeling them “Newtonians”. If you see a Newtonian reflector with a short and stubby tube that does not remotely correlate with a long focal ratio, beware - it’s likely one of these monstrosities. The most common of these are usually specified as 114mm x 1000mm, 127mm x 1000mm, and 150 x 1400mm. These telescopes cannot produce sharp images, are next to impossible to collimate, and should be avoided at any cost, as they are “hobby killers” with the tendency to make people quit amateur astronomy entirely. At least most of the cheap, poorly-mounted refractors have good optics.
Schmidt cameras are astrophotography-only instruments which use a spherical primary mirror and a Schmidt corrector, with the focal point usually being directly in front of the primary mirror. The Schmidt camera can achieve ridiculously fast focal ratios of f/2 or even faster, but regular Schmidt cameras have a curved focal plane and thus could only be used with film negatives bent correspondingly. Newer variations on the Schmidt camera - namely Celestron’s RASA and the HyperStar conversion kit for their SCTs - solve this problem, but come with various caveats.
Corrected Dall-Kirkhams are becoming increasingly popular, particularly among wealthy amateurs and smaller institutions, as they can achieve moderately fast focal ratios, large apertures, and can work with huge camera chips or those mounted to the side of the mounting via mirrors and prisms to re-direct light away from the tube. CDKs use a corrector lens much smaller than either mirror to fix the issues inherent in a regular Dall-Kirkham. CDKs are also much easier to manufacture and work with than an SCT or RCT, which are both beginning to be replaced in the world of small research instruments by CDKs.
Various sub-aperture corrected Cassegrains similar to the CDK exist which use small corrector lenses mounted inside various parts of the telescope; these can be marketed under a variety of names and can use various weird optical designs. Generally these telescopes are very hard to service or repair, and don’t always have the best performance, so buyer beware.
If you’re looking for a very compact and portable instrument primarily for viewing bright and small targets, a Maksutov-Cassegrain is a great telescope, though you should probably purchase it to accompany a larger instrument.
The best use for an SCT is in a backyard or club observatory where space is limited and you might want to do both viewing and astrophotography.
SCTs can be great for visual astronomy, but have narrower fields of view and usually slightly inferior image quality to similarly-sized reflectors, and cost a lot more – especially once a mount is factored in. The best use for an SCT is probably in a backyard or club observatory where space is limited and you might want to do both viewing and astrophotography; on the photography front an SCT can tackle planets, smaller deep-sky targets with its native focal ratio or an f/6.3 reducer, or wide-field vistas of deep-sky objects, though getting it up and running for long-exposure astrophotography can be a bit of a pain. SCTs are seldom made above 16” of aperture and the lone 16” contender from Meade tends to have quality control issues – as such, anything larger than 14” is probably going to need to be a CDK instead if you are building an observatory.
There are few cases where you’re likely to want – or be able to afford – the other catadioptric types we’ve mentioned. CDKs are enormously expensive, Schmidt cameras are not very versatile, and the other types we’ve mentioned are either inconsistent or simply bad the majority of the time.
Maximum aperture refers to the largest you’re likely to see and the largest that a typical person could operate, afford, and house on their own.
Efficiency of aperture refers to the light-collecting ability compared to most good refractors, which typically lose only 1% of the light going through the lenses thanks to efficient coatings and no obstructions. Big secondary mirrors and the inefficiency of cheaper or older mirror coatings can absorb light in any reflector or catadioptric.
Different standards of optical quality also lead to different performance. For instance, a good 5” or 6” refractor or Maksutov can out-perform an 8” Dobsonian on the Moon, planets, and double stars, while many Schmidt-Cassegrains (namely, large ones or those made in the 1980s during a period of high demand and correspondingly lax quality control during the hype around Halley’s Comet) can give outright mushy views. We’ve created a rough “sharpness per inch” metric as a result; e.g. a 4” scope with a sharpness per inch of 0.75 will give similarly good planetary and lunar views to a perfect 3” refractor with a score of 1.
We’ve also scored the average cost and weight per square inch (light collection scales squarely with aperture, so doubling aperture gives you 4 times the light) in USD, based on typical good instruments. Remember, however, that most telescope types have a ceiling of how much aperture they can reach before costs or their cumbersome nature become too much to handle, or at least mass-manufacture economically.
|Maximum aperture||16” common, mass mfg up to 25”, practical limit 40-50”||No more than 14” or 16”, 20-22” rare, practical limit ~36” if tried||Usually no more than 7-8”, practical limit ~10-12”||Usually no more than 6”, 8-12” possible but rare/bulky||Usually no more than 6”, few above 9” due to cost|
|Avg efficiency of aperture||75-90%, usually ~80%||68-80%||~80%||92-98%||95-99%|
|Sharpness per inch||0.8-0.9||0.5-0.8||0.9||0.25-1 due to f/ratio||0.7-1|
|Cost per in^2||~$8-$12||~$40||$36-$45||~$28||>=$75-$100|
|Weight per in^2||0.7-1.2 lbs||0.9-1.4 lbs||1.2-1.5 lbs||>1.5 lbs||>1.8 lbs|
It basically comes down to budget and portability considerations.
For deep-sky astrophotography, the capabilities of your mount (ultimately dictated by budget and portability requirements) and what you are interested in photographing are the main deciding factors.
For visual astronomy, your budget and portability requirements are what matter most. The biggest telescopes give the best views, but only if you are willing to set up and use them frequently.