Editing 1791: Telescopes: Refractor vs Reflector

{{comic
| number    = 1791
| date      = January 27, 2017
| title     = Telescopes: Refractor vs Reflector
| image     = telescopes_refractor_vs_reflector.png
| titletext = On the other hand, the refractor's limited light-gathering means it's unable to make out shadow people or the dark god Chernabog.
}}

==Explanation==
This comic compares two major types of {{w|optical telescope}}: The {{w|refracting telescope}} and the {{w|reflecting telescope}}. A refracting telescope produces an image with a series of lenses. A reflecting telescope uses mirrors. (A third type, the {{w|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 [[#The real problems with refracting telescopes|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 {{w|vampires}}, {{w|Vampire#Apotropaics|according to some cultures}}, cannot be seen in a mirror. As {{w|space vampires}} (like earth vampires) are widely believed to be {{w|Vampire#Origins_of_vampire_beliefs|made up}} and thus unlikely to interest most [[1644: Stargazing|stargazers]], 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 [[#The real problems with reflecting telescope|details below]]. (Also there was a big problem in the [[#Trivia|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 {{w|Shadow person|Shadow People}} or the Slavic god {{w|Chernobog|Chernabog}} (normally 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 {{w|Shadow_person#History_and_folklore|shadow people}} nor {{w|Chernobog#Folklore|the god}} exists 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 {{w|Fantasia (1940 film)|''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 [[:Category:Telescopes|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===
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 {{W|great refractors}}, the largest telescopes in the world were refractors. Reflectors at the time had mirrors surfaced in {{W|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 {{w|List of largest optical telescopes historically|largest optical telescopes ever built}} are reflectors, rather than refractors. In addition, a {{w|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 - {{w|Crown glass (optics)|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. 
*{{w|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 color 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 {{w|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 {{w|Adaptive_Optics|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 {{w|Segmented_mirror|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====
It is worth noting that (apart from the vampire problem) 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 {{w|Secondary_mirror|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 {{w|Diffraction_spike|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 {{w|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 {{w|Birdwatching|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 most 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!)

==Transcript==
:[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.]

:[Left:]
:<big>Refractor</big>

:[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:]
:<big>Reflector</big>

:[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


==Trivia==
*In an '''[http://www.explainxkcd.com/wiki/images/archive/b/b2/20170127171253%21telescopes_refractor_vs_reflector.png earlier version]''' of this comic, the eyepiece of the refracting telescope appeared to include either a mirror or a prism (possibly {{w|porro prism}} or {{w|amici roof prism}}). 
**These make the image upright and allow the observer to look through the telescope from a more comfortable position. 
**A mirror, however, would invalidate the only advantage it has over reflecting telescopes.
**So even though it could be said to be a prism (without the vampire problem) [[Randall]] later choose to correct this "error" so the '''[http://www.explainxkcd.com/wiki/images/b/b2/telescopes_refractor_vs_reflector.png current/final version]''' shows the light going straight out of the end without the risk of anyone interpreting the offensive part for a mirror destroying the joke.
**Now it also looks just like one of the depictions of this type of telescopes on the Wikipedia page for the refracting telescope.
***See this {{w|Refracting_telescope#Refracting_telescope_designs|design}} that does not include a mirror/prism.

{{comic discussion}}

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