Have you ever wondered about how many different kinds of telescopes there are?
The largest telescope on Earth is the Gran Telescopio Canarias which measures 34 feet across, is located on the Canary Islands of Spain, and was built in 2009. The Hubble Space Telescope orbits our planet while staring deep into the darkness of the universe. These are only two examples of how much our telescopes today vary, but there are many more.
It’s time to find out just how many different types of telescopes there really are.
In telescope terms, the part of the telescope that gathers the light is called the optical tube assembly, the OTA. This will have a lens in the front, a mirror in the back, or both.
The device that holds the OTA and allows you to point and control it is called the mount. This may be in the form of a tripod or other arrangement. The OTA plus the mount would be called a telescope.
In this discussion, I will focus on the optical tube assembly. There are three types that are most common in the astronomy hobby market. These are:
- Refractors – based on an objective lens to gather light
- Newtonian reflectors – based on a mirror to gather light
- Catadioptric – based on a combination of lenses and mirrors
Let’s define three terms or specifications that are common to all optical tubes.
This one is easy. This is the diameter of the primary lens in the front or the primary mirror in the back of the tube. The larger the aperture the more light the OTA gathers. The more light gathered the dimmer the objects that can be seen, the more detail that can be seen and the greater the magnification that can be applied to the image. Typical apertures in the hobby market range from 50 mm to about 400 mm or about 2 inches to about 16 inches. There are optical tubes larger than 16 inches in the hobby market but they are more the exception than the rule.
- Focal Length
Whether the telescope uses a lens or a mirror as its primary light gathering tool, that lens or mirror is shaped to focus the light to a point known as the focus. Whatever that distance may be, it is called the focal length, shown as f’ in the diagram. It is worth noting that eyepieces, which are a collection of lenses, also have a specified focal length.
- Focal Ratio
Take the focal length and divide it by the aperture and you get the focal ratio. You will see numbers like F5, F8, F10, F12, F15, and other variations.
A key thing to keep in mind about focal ratio is that numerically high focal ratio optical tubes deliver the light stream to the eyepiece in a much more parallel way than OTAs with low focal ratios. Where this comes into play is how the eyepiece has to handle the light.
A numerically high focal ratio, say over F10, makes it fairly easy for the eyepiece to present a uniformly good image all the way across the field of view. A numerically low focal ratio OTA, say F6 and lower, presents the light rays at the edge of the field to the eyepiece at a steeper angle making it harder for the eyepiece to present a uniformly good image all the way across the field of view. This will come up later as we discuss each of the optical tube designs.
Developed in the early 1600s, this is the OTA that was popularised by Galileo Galilei just a few years after the invention of telescope. It has a curved lens in the front that gathers light and bends it to the focal point along the focal length of the optical tube. The light will usually hit a mirror or prism device called the diagonal which directs it to the eyepiece. A focuser moves some part of the optical path in order to bring the light stream to proper focus, creating a sharp image. The eyepiece then magnifies the image and presents it to the eye of the observer or the sensor of a camera.
A key characteristic to this OTA design is that there is nothing obstructing the optical path which means the full aperture, the span of the front lens, is available to gather light and present it to the eyepiece. This will be a key difference from the other designs we will discuss.
A negative character of this design is that light passing through the aperture lens is broken up into its respective colours in a similar fashion to a prism that casts rainbows on the wall. Unfortunately, a side effect of this is that not all of the colour streams arrive at the eyepiece at exactly the same time. As a result, the refractor exhibits chromatic aberration or false colour in the image. A later development involved the addition of one or more additional lenses in order to minimise the chromatic aberration.
The more basic modern refractor design, the achromatic refractor, uses two lenses which greatly reduces but does not completely eliminate this chromatic aberration. This is the design that we see in low-cost to midrange refractors. This colour aberration is most evident at low focal ratios and is most noticeable around bright objects. The Moon, for example, might have a blue/purple edge which is not truly there. It is a chromatic aberration resulting from the lens system. The main benefit of this design is that it is low cost and makes for a fairly light optical tube.
The more advanced refractor form, the apochromatic refractor, introduces specialised glass and perhaps a third lens to further reduce chromatic aberration, especially in low focal ratio designs. This results in truer colour rendition that is required for astrophotography and visual observers who demand the best image. However, this also results in a heavier optical tube in addition to being much more expensive.
In the hobby market, it is rare to see refractors with an aperture of more than 8 inches as it is difficult and very expensive to produce lenses greater than that. And the more common sizes are 5 inches, 127 mm, down to about 50 mm, about 2 inches.
The lens system, when used with a diagonal, produces a correct image up and down but reversed left and right. For astronomy, this is not an issue as there is no up or down, left or right in the heavens. But when the refractor design is used for terrestrial purposes such as spotting scopes and binoculars, a correcting prism is introduced to correct the image for left and right.
Developed by Sir Isaac Newton in the mid-1600s, this uses a mirror rather than a lens to focus the light. A major benefit is that there is no chromatic aberration introduced.
As we see in the diagram, light enters from the left, hits a mirror in the back which focuses it and sends it to a secondary flat mirror angled at 45 degrees to direct it to the eyepiece. The original design used polished metal but today we use glass as the base that has had an aluminized layer deposited on the glass to create a highly reflective surface, the mirror.
The primary mirror can be spherical in shape but larger mirrors, typically larger than 100 mm, are usually parabolic.
The image produced by a Newtonian reflector is inverted. This doesn’t matter much when viewing the sky but it means that the Newtonian is not suitable for daytime terrestrial use. Seeing the boats on the lake upside down would not be desirable.
As compared to the refractor, we can see that the Newtonian has a central obstruction in the form of the secondary mirror. This means that some of the light entering the optical tube is blocked by the secondary mirror.
As a result of this central obstruction, the refractor can gather more actual light in a smaller aperture than the Newtonian. Where the Newtonian gains advantage is that it is easier to make big mirrors than big lenses. So the Newtonian design scales up nicely. It is not uncommon to see mass market Newtonian reflectors up to 16 inches, about 400 mm aperture and larger. Of all the OTA design we will discuss, the Newtonian reflector offers the lowest cost per inch of aperture.
When you get beyond 16 inches aperture in the hobby market, especially for visual astronomy, the optical tubes are almost all Newtonian reflectors. In the larger designs, typically 14 inches or larger, they go from a solid tube to a truss design that allows the OTA to be taken apart for easier transport. As shown in the picture, often a soft cover is put over the framework to control stray light
In simple terms, a catadioptric optical tube assembly is a combination of lenses and mirrors. The two most common designs in the hobby market are the Schmidt-Cassegrain telescope, SCT, and the Maksutov-Cassegrain telescope, MCT. They both have a front corrector plate, which is a lens, a rear primary mirror with a hole in the centre, and a secondary reflector which is usually attached to the corrector plate.
The SCT is based on a spherical corrector plate up front, a spherical primary and spherical secondary mirror. The secondary directs the light back through a hole in the primary mirror, to the diagonal and then to the eyepiece.
SCTs are extremely popular in the hobby market. Sizes commonly run from 4”/102 mm to 16”/400 mm. Larger sizes are possible but very expensive.
The MCT, as shown in the diagram, appears similar to the SCT except that the corrector plate is based on a thicker convex corrector plate design and rather than a spherical mirror as the secondary, a silvered area on the back of the corrector plate handles the reflection back through the hole in the spherical primary mirror.
MCTs are common in the 4”/102 mm to 7”/180 mm range. The MCT uses a thicker, and therefore heavier corrector than the SCT making it harder to manufacture economically in larger sizes for the hobby market.
As you can see above, both the MCT and SCT designs have a central obstruction, like the Newtonian. As a result part of the light is blocked by that central obstruction.
Their main benefit of the MCT and SCT is that they fold the light path inside the optical tube. This allows for longer focal lengths in a more compact package.
Like the refractor, the MCT and SCT, when used with a diagonal, present a correct image up and down, but reversed left and right. You can add a correct image diagonal for daytime use.
How Do the MCT and SCT Compare to the Newtonian?
A popular sized 8” F6 Newtonian would have a focal length of about 1200 mm in a 48” optical tube. That tube would weigh about 20 pounds.
An 8” F10 SCT would have a focal length of about 2000 mm in an optical tube length of about 17 inches and weigh about 12 pounds.
A 7” F15 MCT would have a focal length of about 2700 mm, an optical tube length of about 22” and weight 16 pounds.
The downside of the SCT and MCT is a higher cost per inch of aperture than the Newtonian design. Also, their longer focal lengths limit their low power field of view as compared to the Newtonian. So we consider MCTs and SCTs to be narrow field of view optical tubes as compared to most Newtonians of similar aperture.
How Do the SCT, MCT, and Newtonian Compare to Refractors for Light Gathering?
Refractors have no central obstruction so a smaller aperture refractor can provide comparable light gathering to a larger Newtonian, SCT or MCT. I usually give the refractor about a 1” or 25 mm advantage but some would advise 2”. So a 4”/102 mm refractor will provide approximately the same light gathering as a 5”/ 127 mm Newtonian, SCT or MCT. Many would say it is really closer to a 6”/150 mm.
Newtonians required regular collimation, a process of aligning the mirrors. This only takes a few minutes, once you get the hang of it. Normally you don’t have to do it every time you use the scope, but it is a maintenance item. Some people do it every time because it only takes a couple of minutes.
MCTs and SCTs do require collimation, but only rarely. The time intervals can be measured in years in most cases.
Refractors generally do not need collimation. The rigid mounting of the lenses tends to hold up for the life of the scope. This is one reason they make good travel scopes.
Which Type is Right for You?
The primary job of the optical tube assembly is to gather light, and they all do that very well. Each optical tube can be used for planets or deep sky objects. Refractors, SCTs, and MCTs can be used during the day when used with a correct image diagonal.
The SCTs and MCTs, due to their long focal lengths, tend to be more optimised to planets and smaller DSOs. Newtonians and refractors typically occupy the more medium to short focal lengths so they often present wider fields of view. This can provide an advantage for wide DSOs, but they can be used on planets as well. This differences here are more of shading then black and white cutoff on what the optical tubes can be used for what targets.
The MCT and SCT tend to have focal ratios of F8 to F15. This means that they are less demanding on eyepieces so you can use less expensive, less corrected eyepieces and still get a good view that is fairly clean from edge to edge.
Newtonians are typically in the F4 to F8 range. These are a little more demanding on the correction ability of the eyepiece. Below F6 this becomes more of a factor. There is a device called a coma corrector that can help with this, or you can simply select better, more expensive eyepieces in order to have a clean image across the field. Many people feel a little edge distortion is tolerable.
Refractors can run the full range of focal ratios from F4 to F20. So, like the Newtonians, if you are below F6, give some extra consideration to the types of eyepieces you are buying or what your tolerance is for edge distortion.
For travel scopes, short tube refractors and smaller SCTs and MCTs work best. There are small Newtonians but they tend to be bulkier than the others and more likely to get out of alignment from bouncing around during travel.
Spotting scopes which are typically used for daytime outdoor use are based on the refractor design. Binoculars are also refractor based designs. These and astronomical refractors are very popular for travel as they are fairly rugged and don’t require any maintenance.
If you get a refractor with a shorter focal length it will show you wide vistas and many can even fit in a carryon bag for the airline. If you are focused on planets, you may prefer a longer focal length of the MCT or SCT. Planets don’t require as much aperture as fainter DSOs.
I have covered the types of scopes that are typically seen in the hobby market. There are others but these are the most common. And I have tried to bring out some of the strengths and challenges that each type presents.
If you become a long term astronomy hobbyist, you will likely end up with more than one telescope. And you are likely to have more than one type. My experience is that most people end up with:
- Binoculars – an implementation of the refractor design
- A grab and go scope – smaller and lighter, often in the 70 mm to 150 mm range
- A light bucket – typically 8”/203 mm or larger for those fainter targets
With what has been covered in this article you will be better prepared to pick which type of optical tube will fit your needs and interests. If you have more than one you can have more than one type. You can then pick the tool you want to use to optimise your experience for that observing session.