Telescopes Refractor vs Reflector
Telescopes: Refractor vs Reflector
[A one panel comic showing two different telescope designs next to each other with labels above them and a bullet list of points below them. The left drawing will be described first then the right.]
[Left:] Refractor
[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
[Right:] Reflector
[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.

This comic compares two major types of optical telescopes : 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 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:
Other problems not mentioned by Randall:
It is worth noting that (apart from the vampire problem) a reflecting telescope also has disadvantages compared to a refracting telescope:
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!)
On the other hand, reflecting telescopes help astronomers gaze at Beige Gorgons (mentioned in comic 2360: Common Star Types ).
[A one panel comic showing two different telescope designs next to each other with labels above them and a bullet list of points below them. The left drawing will be described first then the right.]
[Left:] Refractor
[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
[Right:] Reflector
[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.
