1791: Telescopes: Refractor vs Reflector

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Telescopes: Refractor vs Reflector
On the other hand, the refractor's limited light-gathering means it's unable to make out shadow people or the dark god Chernabog.
Title text: On the other hand, the refractor's limited light-gathering means it's unable to make out shadow people or the dark god Chernabog.


This comic compares two major types of optical telescope: The refracting telescope and the reflecting telescope. A refracting telescope produces an image with a series of lenses. A reflecting telescope uses mirrors. (A third type, the catadioptric system telescope, uses both mirrors and lenses. It is not shown here.)

It first looks like the comic is simply trying to show that refracting has many flaws, such as expense, size and visibility (see more details below). However, the punchline invalidates these complaints with the (apparently major) flaw listed with the reflecting telescope: It can't see space vampires.

The unstated reason for this is that vampires, according to some cultures, cannot be seen in a mirror. As Space Vampires (like earth vampires) are widely believed to be made up and thus unlikely to interest most stargazers,[citation needed] this complaint is superfluous, and the reflecting telescope effectively has no flaws in comparison to the refracting telescope. There are other problems, though, with reflecting telescopes see details below. (Also there was a big problem in the original version of this comic).

Frequently, however, the right-angle transition at the base of the refractor telescope is done with a prism (an "image erector"). This uses the optical principle of total internal reflection. If mirror-non-appearance of vampires is due to the interaction of evil with silver, a refractor using a prism could still see vampires. On this theory, however, the reflector could too, since modern astronomical mirrors are coated with aluminum, not silver.

The title text expands on the seeing of supernatural beings, as another negative point is added to the refracting telescope; it apparently can't see Shadow People or the Slavic god Chernabog (sometimes spelled Chernobog), both of which are important although clearly not as important to the telescope's merit as seeing vampires since the fact is only mentioned in the title text. So of course the refracting telescope is still the best. Of course also neither the shadow people nor the god exists[citation needed] so this would likewise be a moot point.

In reality, "shadow people" are a psychological phenomenon wherein humans ascribe human shapes and movements to shadows in dark spaces. Chernobog is a 12th century Slavic deity, whose name translates to black god. His most famous appearance in modern media was in the 1940 Disney movie Fantasia (and Disney merchandise is also almost the only place that his name is spelled as Randall spelled it, with an "a" in the middle). Because shadows are dark and the god is also dark, they cannot be seen by the refracting telescope due to the reduced light-gathering which has already been mentioned as a drawback in the main comic.

Telescopes have been the subject of many comics on xkcd. Recently one about space telescope was released 1730: Starshade and before that a large "private" telescope was shown in 1522: Astronomy.

The real problems with refracting telescopes[edit]

The basic performance of a telescope is determined by its size: a wider telescope catches more light, making it easier to see faint objects, while a longer telescope is better for high magnification viewing. For looking at stars, the width is actually more important. No matter how much you zoom, a star is too far away to make bigger, but with a big aperture, you can see stars too faint for the naked eye. Planets benefit more from magnification, and distant galaxies need both.

In both respects, it's much easier to make a big reflector telescope than a big refractor one. Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. This means most refractor telescopes make do with narrow apertures only a couple of inches across. Reflector telescopes are sometimes called "light buckets" because they can have extremely big openings that can catch light from even very faint stars. In addition, because it has a mirror at one end, the reflector telescope is, in effect, twice as long as it appears - a refractor just cannot compete.

Refracting telescopes were only gradually overtaken by reflecting telescopes, however. In the age of great refractors, the largest telescopes in the world were refractors. Reflectors at the time had mirrors surfaced in speculum metal that began to tarnish only months after application, negatively affecting telescope performance. This problem was resolved when it became possible to surface a mirror in silver, but the problems with refractive lenses persist. Because of this, the largest optical telescopes ever built are reflectors, rather than refractors. In addition, a liquid mirror telescope uses a very cheap, but potentially very large mirror - with the drawback that the telescope can only look straight upwards.

Randall's points:

  • More expensive
    • Grinding a high quality lens is more expensive than producing an equivalent mirror - crown glass, which is needed for good quality telescope lenses, is expensive.
  • Less compact
    • In theory, a refractor could be made compact, but the image quality would be awful, because the lens would have to be extremely fat. The longer the telescope is, the less dramatic the focusing needs to be.
  • Chromatic aberration:
    • In optics, chromatic aberration is an effect resulting from dispersion in which there is a failure of a lens to focus all colors to the same convergence point, producing a rainbow effect around the image familiar to people who wear glasses, and with prisms. It occurs because lenses have different refractive indices for different wavelengths of light. Each colour is therefore focused slightly differently by the lens. Mirrors don't have chromatic aberration, since the light is reflected off the front of the mirror. The achromatic lens can reverse this effect, but it's expensive and its size is limited. Nevertheless, before telescope mirrors were perfected in the early 20th century, the best telescopes were achromatic refractors.
      • Note that this effect has also been mentioned in relation to photography by Black Hat in 1014: Car Problems, in a completely different context, but shows this is an issue Randall has considered before.
  • Reduced light-gathering
    • Apart from generally needing to be smaller than reflector telescopes a further problem comes from glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. All of this reduce the light gathered.

Other problems not mentioned by Randall:

  • Suspending a lens
    • Another important difference (and a big reason why large refracting telescopes don't exist) is that the lens of a refracting telescope has to be supported by the edges, so that light can pass through it. As a result there comes a point where it is no longer feasible to mount a large lens in a telescope due to its weight and the need to support it from the edges. In contrast the mirror of a reflecting telescope is supported from behind, and any support structures for the primary mirror are not in the path of the light. As a result, substantially larger mirrors can be easily mounted and supported. As an additional benefit this behind-the-mirror support has led to the creation of Adaptive Optics, a technique (which is impossible for refracting telescopes) that allows some of the atmosphere's distortions to be corrected for.
  • A mirror can be segmented to make a larger reflecting surface out of smaller (and hence easier to build/mount/support) mirrors. By using a segmented mirror it is possible to build an effective aperture much larger than what could be built even from a single mirror, which is itself much larger than the largest possible lens that might be built for a refracting telescope.

The real problems with reflecting telescope[edit]

It is worth noting that (apart from the vampire problem) a reflecting telescope also has disadvantages compared to a refracting telescope:

  • The main disadvantage is that in almost all reflecting telescope designs the focal point is directly in front of the mirror, i.e. in between the mirror and the target of interest.
    • As a result a secondary mirror is commonly used to direct the focal point somewhere outside of the field of view. However, this secondary mirror (and the struts that support it) will still block part of the field of view - although the focus of the telescope means that the secondary mirror is not visible when looking at distant objects, it will result in diffraction patterns that also hinder the image quality. In fact, this is the source of the diffraction spikes around stars which are commonly seen in astronomical images.
  • A reflecting telescope is also harder to maintain:
    • The mirrors need to be very precisely aligned (this is called collimation), and this can be a laborious process. They may also need re-polishing.
    • The telescope is open at one end, allowing dust and dirt to enter.
  • A reflecting telescope is not very portable. This is why bird-spotters use small refractor telescopes as an easy way to get a closer view of birds.

Despite this disadvantage, reflecting telescopes are used almost exclusively in modern astronomy because of practical limitations in making large refracting telescopes. Very few amateur astronomers use refracting telescopes - nowadays, they mostly exist to con people looking for Christmas presents in department stores (just because a telescope promises 100x zoom doesn't mean the image quality is any good!)


[A one panel comic showing two different telescope designs next to each other with labels above them and a bullet list of points below the them. The left drawing will be described first then the right.]
[A slim telescope design is shown. At the top the light enters shown in a light yellow shade between two thin parallel light gray lines that just fits inside the opening of the telescope which is slightly wider at the top than at the lens sitting a short way into the opening. The lens causes the light to focus just where the telescope again changes dimensions, and the light enters a small opening at the bottom of the long pipe of the telescope. Here the yellow light is a point as the two gray lines cross each other at that point. The light then broadens slightly again and the thin yellow light cone hits a mirror at the bottom of the telescope and is reflected to the left and out through the eyepiece. Below are the following points:]
  • More expensive
  • Less compact
  • Chromatic aberration
  • Reduced light-gathering
[A much broader (more than 150% of the first) but also much shorter (66%) telescope design is shown. At the top the light enters shown in a light yellow shade between two thin parallel light gray lines that still just fits inside the opening of the telescope. On it's way down to the bottom of the telescope the light passes by a small mirror turned down towards the bottom. When the hits the curved bottom mirror light is focus on it's way back back and a small light cone hits the small mirror mentioned before sitting almost at the top of the telescope. This mirror reflects the light to the left into an even thinner light cone that goes out through the eyepiece located near the top of the telescope. Below is the following point:]
  • Can't see space vampires


In an earlier version of this comic, the eyepiece of the refracting telescope included a mirror, often used with refractors to give an upright image and more comfortable access for the observer. This would of course invalidate the only advantage it has (vampire-visibility) over reflecting telescopes.

Randall later corrected this so the current/final version shows the light going straight out of the end.

An amici roof prism is sometimes used instead of a mirror with refractors, because it does not only deliver an upright image, but also one that is not a mirror image. In a prism, there is only total reflection, which, as opposed to a metal mirror, would probably work on vampires.

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Nitpick: The refracting telescope, drawn correctly, has a mirror in the optical path (image inverter), but it is made with a special vampire reflecting material Ichorium. (talk) (please sign your comments with ~~~~)

Doesn't the one in this image have a mirror too? at the bottom to make the image come out at the side instead of the end?

That's a good point: as drawn, the refracting telescope still has a mirror and also wouldn't be able to see space vampires. However, the refracting telescope doesn't have to have a secondary mirror, and there are plenty that don't, so it is more the drawing that is wrong rather than the text of the comic.Cmancone (talk) 14:31, 27 January 2017 (UTC)

Typically refractors use a prism rather than a mirror at the end, though it does the same thing. Can vampires be seen in a prism? 14:50, 27 January 2017 (UTC)

Only if they're pink. 14:57, 27 January 2017 (UTC)

Every time I press submit, it blocks me and makes me start over. Kynde, rather than making a small change every 30 seconds, perhaps you could do them all at once? -- Frankie (talk) 15:26, 27 January 2017 (UTC)

Well I'm sorry, I had the same problem as you. So I did not dare read it all through before I submitted, and thus so tried to fix the errors I found afterwards. That was also why I did not make the section for the real problems a subsection to the explanation so it (as the transcript) could be edited without conflicting with the other sections. --Kynde (talk) 15:30, 27 January 2017 (UTC)
Ah, it's network congestion. Sending small packets more quickly is indeed one way to get your message through, but it can lead to a tragedy of the commons. Everyone switching to larger packets is the optimal answer, but it's not a stable equilibrium. -- Frankie (talk) 15:39, 27 January 2017 (UTC)

Does anybody on the wiki HAVE a major in optics???? At least anybody who will see this page before MONDAY, when it will no longer be the latest??? That's right, Jacky720 just signed this (talk | contribs) 15:54, 27 January 2017 (UTC)

A telescope mirror typically would have no chromatic aberration, as it's a front-surface mirror. The light doesn't pass through the glass to get to the reflective material; the glass is on the back of the mirror for support. 16:15, 27 January 2017 (UTC)

I think where the commentary says "This means most reflector telescopes make do with narrow apertures" it should be "refractor telescopes". Rtanenbaum (talk) 16:18, 27 January 2017 (UTC)

FYI: Randall updated the comic, so that the refracting telescope doesn't have a mirror.-- 16:59, 27 January 2017 (UTC)

I would think that the main reason for Refractors over Reflectors is that it would be easier to make one with adjustable focus, so you would not need to wear glasses and – more importantly – you might be able to use the telescope as a binocular for things like birds. I don't know of any Reflectors with a significant adjustable focus, but I'm not an astronomer either. 17:31, 27 January 2017 (UTC)

Aren't there Space Vampires in Lovecraft somewhere? 22:48, 27 January 2017 (UTC) Not as such, but there are in the classic Queen of Blood [1] and in the unfortunate "Lifeforce" [2] . A refractor though should be able to see their interstellar umbrella [3] Anthony11 (talk) 07:05, 30 January 2017 (UTC)

An important advantage of refractors that keeps them popular is the very tiny amount of internal scattered light compared to reflectors. This really stands out when viewing planets and bright objects. Everybody loves that velvety black background field that refractors can provide. ExternalMonolog (talk) 09:59, 28 January 2017 (UTC)

Another issue with reflecting telescopes (though not pertinent to the joke) is that when used as a lens in photography, the bokeh, or unfocused highlights beyond the depth of field in an image, are rendered as circular 'doughnut' shapes, instead of the fuzzy points of light created by refracting lens systems. Run-on sentences, FTW. These Are Not The Comments You Are Looking For (talk) 01:20, 29 January 2017 (UTC)

Strictly speaking, bokeh is the "quality" of the OOF areas, not the areas themselves. Anthony11 (talk) 07:05, 30 January 2017 (UTC)

There can be no doubt that the original comic contains a *mirror*, not a prism. And a porro prism would not even work, because it reflects the projection by 180 degrees! Porro prisms are always used in pairs, to upright an image *without* changing the viewing direction! -- 12:26, 30 January 2017 (UTC)

The lens of a refractor flips the image, so having a single prism would render it upright. 01:48, 31 January 2017 (UTC)

These days, Chernobog may be better known as Czernobog, from Neil Gaiman's novel American Gods. 14:11, 1 February 2017 (UTC) Paul