explain xkcd - User contributions [en]

1791: Telescopes: Refractor vs Reflector

2017-01-27T16:33:23Z

Cmancone: /* The real problems with refractor telescopes */

{{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==
{{incomplete|The section [[#The real problems with refractor telescopes|The real problems with refractor telescopes]] could need someone with a major in optics looking over it. And probably the rest could also be smartened up?}}
This comic compares two types of telescopes: {{w|Refracting Telescope}} vs {{w|reflecting telescope}}.

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 refractor 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 stargazers, this complaint is superfluous, and the reflecting telescope effectively has no flaws in comparison to the refracting telescope.

The eyepiece of the refracting telescope appears 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 would invalidate the advantage it has over reflecting telescopes, but as can be seen on the Wikipedia page for Refracting telescope {{w|Refracting_telescope#Refracting_telescope_designs|it does not need the mirror}} drawn by [[Randall]]. So in principle such a telescope could then see vampires that do not show up in mirrors. But Randall's version would not be able to do so because of the mirror at the base. So it is for sure an error if it should be explained by the mirror folklore. 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. Since mirror-non-appearance of vampires is presumably due to the interaction of evil with silver, the refractor 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}}, both of which are apparently equally important to the telescope's merit despite neither the {{w|Shadow_person#History_and_folklore|shadow people}} or {{w|Chernobog#Folklore|the god}} exists. In reality, "shadow people" are a psychological phenomenon wherein humans ascribe human shapes and movements to shadows in dark spaces. Chernabog is a 12th century Slavic diety, whose name translates to ''black god''. His most famous appearance in modern media was in the 1940 Disney movie {{w|Fantasia (1940 film)|''Fantasia''}}. 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 has 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 refractor 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 reflector 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.

For 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 glasses 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 {{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.
*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.
**Finally, 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.

It is worth noting that:
:'''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 {{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 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 birdspotters 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 are the following point:]
:*Can't see space vampires

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Telescopes]]
[[Category:Rankings]]
[[Category:Science]]
[[Category:Astronomy]]
[[Category:Space]]
[[Category:Religion]]

1791: Telescopes: Refractor vs Reflector

2017-01-27T16:31:43Z

Cmancone: /* The real problems with refractor telescopes */

{{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==
{{incomplete|The section [[#The real problems with refractor telescopes|The real problems with refractor telescopes]] could need someone with a major in optics looking over it. And probably the rest could also be smartened up?}}
This comic compares two types of telescopes: {{w|Refracting Telescope}} vs {{w|reflecting telescope}}.

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 refractor 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 stargazers, this complaint is superfluous, and the reflecting telescope effectively has no flaws in comparison to the refracting telescope.

The eyepiece of the refracting telescope appears 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 would invalidate the advantage it has over reflecting telescopes, but as can be seen on the Wikipedia page for Refracting telescope {{w|Refracting_telescope#Refracting_telescope_designs|it does not need the mirror}} drawn by [[Randall]]. So in principle such a telescope could then see vampires that do not show up in mirrors. But Randall's version would not be able to do so because of the mirror at the base. So it is for sure an error if it should be explained by the mirror folklore. 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. Since mirror-non-appearance of vampires is presumably due to the interaction of evil with silver, the refractor 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}}, both of which are apparently equally important to the telescope's merit despite neither the {{w|Shadow_person#History_and_folklore|shadow people}} or {{w|Chernobog#Folklore|the god}} exists. In reality, "shadow people" are a psychological phenomenon wherein humans ascribe human shapes and movements to shadows in dark spaces. Chernabog is a 12th century Slavic diety, whose name translates to ''black god''. His most famous appearance in modern media was in the 1940 Disney movie {{w|Fantasia (1940 film)|''Fantasia''}}. 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 has 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 refractor 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 reflector 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.

For 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 glasses 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 {{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.
*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.
**Finally, 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}} 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.

It is worth noting that:
:'''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 {{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 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 birdspotters 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 are the following point:]
:*Can't see space vampires

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Telescopes]]
[[Category:Rankings]]
[[Category:Science]]
[[Category:Astronomy]]
[[Category:Space]]
[[Category:Religion]]

1791: Telescopes: Refractor vs Reflector

2017-01-27T16:22:04Z

Cmancone: /* The real problems with refractor telescopes */

{{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==
{{incomplete|The section [[#The real problems with refractor telescopes|The real problems with refractor telescopes]] could need someone with a major in optics looking over it. And probably the rest could also be smartened up?}}
This comic compares two types of telescopes: {{w|Refracting Telescope}} vs {{w|reflecting telescope}}.

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 refractor 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 stargazers, this complaint is superfluous, and the reflecting telescope effectively has no flaws in comparison to the refracting telescope.

The eyepiece of the refracting telescope appears 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 would invalidate the advantage it has over reflecting telescopes, but as can be seen on the Wikipedia page for Refracting telescope {{w|Refracting_telescope#Refracting_telescope_designs|it does not need the mirror}} drawn by [[Randall]]. So in principle such a telescope could then see vampires that do not show up in mirrors. But Randall's version would not be able to do so because of the mirror at the base. So it is for sure an error if it should be explained by the mirror folklore. 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. Since mirror-non-appearance of vampires is presumably due to the interaction of evil with silver, the refractor 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}}, both of which are apparently equally important to the telescope's merit despite neither the {{w|Shadow_person#History_and_folklore|shadow people}} or {{w|Chernobog#Folklore|the god}} exists. In reality, "shadow people" are a psychological phenomenon wherein humans ascribe human shapes and movements to shadows in dark spaces. Chernabog is a 12th century Slavic diety, whose name translates to ''black god''. His most famous appearance in modern media was in the 1940 Disney movie {{w|Fantasia (1940 film)|''Fantasia''}}. 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 has 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 refractor 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 reflector 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.

For 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 glasses 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 {{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.
*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.

It is worth noting that:
:'''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 {{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 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 birdspotters 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 are the following point:]
:*Can't see space vampires

{{comic discussion}}

[[Category:Comics with color]]
[[Category:Telescopes]]
[[Category:Rankings]]
[[Category:Science]]
[[Category:Astronomy]]
[[Category:Space]]
[[Category:Religion]]

1791: Telescopes: Refractor vs Reflector

2017-01-27T14:36:49Z

Cmancone: /* Explanation */

Talk:1791: Telescopes: Refractor vs Reflector

2017-01-27T14:31:42Z

Cmancone:

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.
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? [[Special:Contributions/162.158.89.187|162.158.89.187]]
Something went wrong on my comment - only the part after "doesn't" is from me, the part before was what i was commenting on, idk why it is not seperated [[Special:Contributions/162.158.89.187|162.158.89.187]]

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.[[User:Cmancone|Cmancone]] ([[User talk:Cmancone|talk]]) 14:31, 27 January 2017 (UTC)

1791: Telescopes: Refractor vs Reflector

2017-01-27T14:29:12Z

Cmancone: /* Explanation */

1791: Telescopes: Refractor vs Reflector

2017-01-27T14:27:53Z

Cmancone: /* Explanation */

1791: Telescopes: Refractor vs Reflector

2017-01-27T14:26:03Z

Cmancone: /* Explanation */

1032: Networking

2017-01-26T14:59:41Z

Cmancone: Added a quick note on Eusociality for "Eusocial Media Ventures"

{{comic
| number = 1032
| date = March 21, 2012
| title = Networking
| image = networking.png
| titletext = Our company is agile and lean with a focus on the long tail. Ok, our company is actually a polecat I found in my backyard.
}}

==Explanation==
Obviously, [[Beret Guy]]'s [[1021|business plan]] worked.

Networking, in business, is the act of expanding your group of contacts in order to help your career down the line. Here, in this comic, [[Beret Guy]] meets Chief Technology Officer (CTO, an executive level position overseeing development of new technologies) Connr Clark (perhaps a typo for "Connor" or perhaps a reference to common "Web 2.0" names like the businesses {{w|Flickr}}, {{w|Tumblr}}, etc.) and Beret Guy is as strange as he usually is: Although he is a business professional he has just photocopied a burrito. He also has a business card; usually this would contain contact information, but his only says "This is my business card". He calls his briefcase, or suitcase, a "handlebox", and it is full of a quarter of a million dollars in cash. Then Beret Guy proceeds to eat Connr's business card. All of these things are not common behavior.{{Citation needed}}

"Networking" is often an over-hyped, empty affair. There are zillions of networking meetings of every description going on every day everywhere, and mostly people trade cards and continue to not make money. So that's the joke – Beret Guy does the networking {{w|schtick}}, badly, and yet is somehow making huge amounts of money at it.

The comic is also likely a joke on the idea that many people are excited about becoming a "business professional" who carries a briefcase, hands out business cards, and makes tons of money, without having an adequate plan for how to make those things happen, or possibly even knowing what their actual job would be. Beret Guy never says what he does, simply introducing himself as a "business professional," and explains his piles of cash with "I am a business grown-up who makes business profits!" In this world —and in people's dreams— when you "grow up" and start a business, money magically appears. Obviously, that's not how it works.{{Citation needed}}

The "Eusocial" in "Eusocial Media Ventures" is a reference to {{w|eusociality}}, the highest level of social cooperation found in the animal kingdom. Eusocial animals (termites being a common example) cooperate together to raise their young, have different generations living in the same colony, and have specialized individuals for reproductive and non-reproductive tasks.

The title text is a pun on three common business buzzwords: agile, lean and long-tail. An agile business is one that can change course quickly based on customer demands and the business environment. A lean business is one with minimal inventory or assets; nothing is idle or warehoused, so everything in active use or on the move. Long-tail describes the strategy of offering a large number of unique items with relatively small quantities sold of each – usually in addition to selling fewer popular items in large quantities. Netflix is a popular example of long-tail because they have (almost) every movie imaginable, including rare titles that only a few people would be interested in.

And of course, the pun here is one animal that is agile and lean with a long ''tail'' is a {{w|polecat}}.

Furthermore, although "agile" and "lean" do mean a quick, nimble, and efficient business, they also refer to specific practices, as in {{w|agile software development}}, {{w|lean manufacturing}} and {{w|lean Six Sigma}}. Many people think these terms have devolved to overused jargon. While agile development is supposed to be a highly structured method to get programmers to produce more working code quickly, when someone from the marketing department says "''agile''" it often means "''We don't know what we're supposed to be producing, so we'll just chuck some stuff together, and keep those bits that the customer says he likes. We'll then do it all over again until we've got something that he'll pay for.''" "''Lean''" is supposed to mean that a business keeps its costs as low as possible, employing one person to do marketing and PR, not really having a Human Resources department, etc. But, in practice it often becomes "''Keep as little stock as possible so that we don't have a lot of money tied up in it, and don't need a big warehouse; make stuff just before it is supposed to ship so that we don't have to store it either; make frequent prayers and virgin sacrifices to whatever gods we can find to ensure that nothing slips up anywhere along the line that our lawyers can't get us out of.''"

See also [[1117: My Sky]].

==Transcript==
:[A man approaches Beret Guy at a party and they extend arms to shake hands. Beret Guy is holding a metal briefcase. Ponytail is a waitress in the background, carrying a tray with a wine glass on it.]
:Man: I'm Connr Clark, CTO at Eusocial Media Ventures.
:Beret Guy: I'm a business professional! Earlier I photocopied a burrito!

:[The man hands Beret Guy a business card. Beret Guy takes it and hands the man another business card. Beret Guy has put his suitcase on the floor.]
:Man: You should check us out! Here's my card.
:Beret Guy: Here's mine! Networking!

:[The man takes a closer look at the card, and Beret Guy holds up his case.]
:Man: ...this just says "This is my business card!"
:Beret Guy: Do you like it? I have more in my handlebox.

:[Beret Guy puts his case on a table and opens it to reveal it is full of cash. The man looks on in shock.]
:Man: Uh, that's ok, I think I'll—
:Beret Guy: Here, have ten of them!
:Man: —holy shit that thing is full of ''cash!''

:[The man raises his arms in excitement. Beret Guy turns to face him and chews on the man's business card.]
:Man: Where did you ''get'' that?
:Beret Guy: I am a business grown-up who makes business profits!
:Man: That's like a quarter of a million dollars!
:Beret Guy: Yay! Business is fun! Do you have more of your cards? They're ''delicious!''

{{comic discussion}}
[[Category:Comics with color]]
[[Category:Comics featuring Beret Guy]]
[[Category:Comics featuring Hairy]]
[[Category:Beret Guy's Business]]
[[Category:Comics featuring Ponytail]]

Talk:836: Sickness

2016-09-27T18:06:45Z

Cmancone: Retracing previous comment

Someone evidently didn't understand Hamlet too well. In "To be or not to be" he's contemplating suicide. "Take arms against..." means 'kill yourself so you won't have to put up with life's crappy bits. I would rewrite the Hamlet reference myself, but I'm too lazy. Could someone with a good understanding of the play do it? Please?[[Special:Contributions/173.245.48.85|173.245.48.85]] 01:42, 12 March 2014 (UTC)
:Agreed. Change made. [[User:Orazor|Orazor]] ([[User talk:Orazor|talk]]) 08:03, 21 July 2014 (UTC)
:"Take arms against a sea of troubles..." does not mean to commit suicide. It means to fight against the struggle referred to the in the previous line "the slings and arrows of outrageous fortune". The contemplation of suicide is expressed in the phrase "When he himself might his quietus make / With a bare bodkin" when one could end one's life with a dagger. [[Special:Contributions/141.101.70.151|141.101.70.151]] 08:35, 22 August 2014 (UTC)
::Yes, "to take arms against a sea of troubles" in this context does in fact does mean to commit suicide. The struggle refered to in the previous line is whether to put up with the unbearable situation he (Hamlet) has been placed in, or, in the next line(s), to exit the situation via suicide. Elsewise, why would he suddenly transition from "overcoming obstacles" to considering death? Doesn't make sense. For your reference, check out http://www.nosweatshakespeare.com/quotes/hamlet-to-be-or-not-to-be/.[[User:Orazor|Orazor]] ([[User talk:Orazor|talk]]) 10:53, 19 September 2014 (UTC)
: Why commit suicide. His mother tries to get him to accept the inevitable that his father's killer is now in power and get on with life instead of pretending to be daft.
:A bare bodkin is an arrow tipped with steel, a war arrow. As opposed to a neutered one for harmless purposes.
:[[User:Weatherlawyer| I used Google News BEFORE it was clickbait]] ([[User talk:Weatherlawyer|talk]]) 20:34, 24 January 2015 (UTC)

I'm going to add some more discussion to the explanation, as I have always read this comic completely differently. I saw white hat's question as asking if Cueball considered seeking cures for his illness outside of science, i.e. in pseudoscience. Given the wide variety of "cures" for all manner of illnesses (especially cancer) in alternative medicine, this seems like the most straight-forward meaning of his question "are you looking for answers outside of science". Randall has obviously made plenty of comics about pseudoscience, so it is a relevant theme for him. It also brings the comic together nicely, as Cueball is basically answering "Science has given us plenty of great tools: no need to rely on that crap!". Frankly, I'm a bit confused as to how the explanation went to discussions of suicide. I understand that "Slings and arrows of fortune" is a Hamlet reference that involves a discussion of suicide, but the comic makes way more sense if you understand it as a contrast between the tools of modern medicine and the "tools" of ancient/alternative medicine. I don't really think suicide was what Randall was aiming for. However, I'll leave that all in as an alternative explanation for someone else to remove, if desired. [[User:Cmancone|Cmancone]] ([[User talk:Cmancone|talk]]) 18:01, 27 September 2016 (UTC)
:Having read everything over again, I think the current explanation is fine. I realized that the explanation talks less about suicide than the discussion, and that the explanations understanding of "Beyond science" is a better fit than him asking about alternative medicine. So ignore me [[User:Cmancone|Cmancone]] ([[User talk:Cmancone|talk]]) 18:06, 27 September 2016 (UTC)

2016-06-27T12:42:09Z

Cmancone:

Come to think of it, I haven't accidentally hit a porn site in years. Is Randall even referring to a real problem? Anyone remember whitehouse dot com? And for the record, kids, [http://purl.net/net/tbc/writing/xxx.htm don't do porn]. ''— [[User:Tbc|tbc]] ([[User talk:Tbc|talk]]) 12:27, 24 June 2016 (UTC)''

I think the sentences "It is hard to steal nuclear launch codes. And a good thing too since they could be used to start a nuclear war." are weird... to me on the first read it sounded like it is a good thing to steal them... [[Special:Contributions/162.158.85.63|162.158.85.63]]

What is it with Randall and stealing wienermobiles? [http://www.xkcd.com/935/ xkcd 935] [[Special:Contributions/173.245.52.62|173.245.52.62]] 15:12, 24 June 2016 (UTC)
:I added it to the explanation, thanks! [[User:Elipongo|Elipongo]] ([[User talk:Elipongo|talk]]) 16:16, 24 June 2016 (UTC)
:There's also a wienermobile in xkcd 1110 parked to the right of the Burj. [[Special:Contributions/198.41.239.33|198.41.239.33]] 11:03, 27 June 2016 (UTC)

A somewhat similar thing really happened in one of the URL shortening services in Taiwan. This case is not that the domain is stolen; the problem is that its database storing shortened URL mappings, because of some mis-operation in converting database data, is rolled back and some shortened URLs are "double-booked." According to the announcement of the service, this affects over 234 thousand entries in the database. This leads to PTT, the largest terminal-based bulletin board system in Taiwan, bans shortened URLs from this service. --[[Special:Contributions/108.162.222.40|108.162.222.40]] 20:21, 24 June 2016 (UTC)

''sites can be particularly vulnerable if they do not maintain their web site'' - what? You can have domain name without ANY web site at all. "lapse" likely refers to owners stopping paying. -- [[User:Hkmaly|Hkmaly]] ([[User talk:Hkmaly|talk]]) 11:09, 25 June 2016 (UTC)

(Trying again... the CAPTCHA is glitching out on me.) "It is also hard to steal the {{w|Crown Jewels of the United Kingdom|Crown Jewels}}, since they are protected by a [http://yeomenoftheguard.com/Windsor%20Castle.jpg complex security system]." - The items that are the first linked items are not at the location the second link points to... [[Special:Contributions/141.101.98.131|141.101.98.131]] 16:20, 25 June 2016 (UTC)

In line with the above comments: the whole section on the crown jewels and the wienermobile seem to miss the point and get hung up on very minor details. Stealing the crown jewels would make a few people fabulously rich, a few people significantly poorer (or jailed, or court-martialled, depending), but would hardly affect anyone else in real terms other than making millions of people - all around the world - very upset. Saying that ''Randall erroneously assumes'' that there would be little consequence to stealing the wienermobile is just silly: there is nothing erroneous about it since it could never have a material effect on more than a few individuals, and the possibility of someone being injured or killed during the robbery is irrelevant since it applies equally well to the nuclear or crown jewels options. [[Special:Contributions/108.162.229.44|108.162.229.44]] 16:12, 26 June 2016 (UTC)

In regards to stealing tinyurl.com, I don't think it would actually be that easy. In the title text Randall suggests picking up the domain name when it expires. Because some domains were stolen that way in the past, ICANN has changed the rules for the major top-level-domains, including .com. Now, after a domain name expires, the original register has a 45 day auto-renew grace period where they can re-register it without penalty. If they miss that period, they have an additional 30 day grace period where it can be re-registered with a penalty. The domain name stops working when it initially expires so it would be nearly impossible for a company like tinyurl to get to the end of both grace periods without noticing and fixing the problem. These new rules make it effectively impossible for an organization to lose its domain name by failing to renew on a timely basis. [https://www.icann.org/resources/pages/expired-2013-05-03-en Reference]

Since Randall only mentioned domain expiration as the way it might be stolen, it is unclear whether or not he was considering a more direct domain name hijacking. I'm less familiar with how easy domain hijacking might be but considering that their entire business depends on their domain name, I can't imagine it would actually be that easy.

Regarding the current explanation (and has been pointed out already), saying that "sites can be particularly vulnerable if they do not maintain their web site" is very wrong. This has nothing to do with maintaining a website, and only has to do with maintaining thei domain name. The website and domain name are two very different things, so this isn't just a matter of nitpicking. However, as I have explained above, the entire concept is no longer correct. There is now a grace period up to 75 days long for .com domains during which registrars are not allowed to sell the domain name to another third party.

Talk:1638: Backslashes

2016-02-03T14:08:14Z

Cmancone:

It should be noted that this also occurs in almost every programming language where "\" is the escape character. i.e.
print("Hello")
> Hello
print("\"Hello\"")
> "Hello"
print("\\Hello\\")
> \Hello\
Oh, and by the way, isn't this the third comic to mention "Ba'al, the Soul Eater"? Maybe we should start a category. (Others are [http://www.explainxkcd.com/wiki/index.php/1246:_Pale_Blue_Dot 1246] (title text) and [http://www.explainxkcd.com/wiki/index.php/1419:_On_the_Phone 1419].)
[[Special:Contributions/173.245.54.29|173.245.54.29]] 06:14, 3 February 2016 (UTC)
:[[:Category:Ba'al|Did that]] before seeing you comment, so yes I agree. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:47, 3 February 2016 (UTC)

;I don't think the regex is invalid

According to <tt>man grep</tt> you need to specify the <tt>-E</tt> option to use extended regex; without it unescaped parentheses are not interpreted, so they don't need to match.

My - very wild - guess is that it was the command he used to find the line with the most special characters, but I am not confident enough to edit the article (if someone can confirm?). {{unsigned ip|141.101.66.83}}

If it was supposed to do that, it doesn't work. Running it on my bash history matches no lines, and I have lots of special characters in there [[Special:Contributions/197.234.242.243|197.234.242.243]] 07:12, 3 February 2016 (UTC)

Explain it to me like I'm dumb. What is this comic going on about? I think the explanation needs more examples like that hello, above, because that's almost understandable. --[[Special:Contributions/198.41.238.231|198.41.238.231]] 07:47, 3 February 2016 (UTC)
:I agree. But I cannot help either.--[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:51, 3 February 2016 (UTC)

This is the third time Randall has mentioned Ba'al the Soul Eater xD [[User:International Space Station|International Space Station]] ([[User talk:International Space Station|talk]]) 08:26, 3 February 2016 (UTC)
:Yes, that was already mentioned a few hours before you comment, see the first comment. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:51, 3 February 2016 (UTC)

After passing the regex through bash, you get <nowiki>\\[[(].*\\[\])][^)\]]*$</nowiki> That is, the literal character \, followed by [ or (, followed by any number of any characters, followed by \, followed by ] or ), followed by any number of characters that aren't ) or ], until the end of the line. [[Special:Contributions/108.162.216.44|108.162.216.44]] 08:33, 3 February 2016 (UTC)
:It sounds like you know what you are talking about. Anyone who can explain it good enough for the explanation, and correct the explanation of the title text if it is wrong to say that it would not work. I have added this as the reason for incomplete. But maybe also examples are needed for people with not programming skills/knowledge. We also enjoy xkcd ;-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:51, 3 February 2016 (UTC)
For fun:
cat ~/.bash_history | xargs -d "\n" -n 1 -I {} bash -c 'chars="$(echo "$1" | grep -o "[a-zA-Z0-9 ]" | wc -l)"; echo "$(( 100 - $(( $chars * 100 / ${#1} )) )) $1"' _ {} | sort -nrk 1 | less

Outputs your bash_history, ordered by relative gibberishness. This was copied by hand from desktop to mobile, might well have a few typos.--[[Special:Contributions/162.158.90.208|162.158.90.208]] 10:04, 3 February 2016 (UTC)

The problem in the comic is not with regexes per se but with situations when the entered text or expression passes through several interpreters, like bash -> grep/sed/awk, or program text -> external shell command. In such cases, you have to escape backslashes for each program in the sequence, and it gets worse if you have 'real' backslashes in the final text that you're processing with the utilities (Windows' file paths, for example). See https://en.wikipedia.org/wiki/Leaning_toothpick_syndrome.
Feel free to lift this to the explanation page, since I'm not good at longer and more careful explanations than this one.
Also, gotta notice that Feedly stripped paired backslashes in the title text (probably passed it through some 'interpreter' embedded in its scripts). [[User:Aasasd|Aasasd]] ([[User talk:Aasasd|talk]]) 10:13, 3 February 2016 (UTC)

:A funny comment about the MediaWiki software, which is even worse than this comic: <code><nowiki><Nikerabbit> I looked the code for rlike and didn't find where it does this. Can you point me to it? <vvv> $pattern = preg_replace( '!(\\\\\\\\\\\\\\\\)*(\\\\\\\\)?/!', '$1\\/', $pattern ); <Nikerabbit> I thought that was ascii art :)</nowiki></code> ([https://phabricator.wikimedia.org/P110$275 source]) --[[Special:Contributions/162.158.91.215|162.158.91.215]] 10:18, 3 February 2016 (UTC)

Interestingly, I first looked at this on my phone (using <strike>Chrome</strike> Feedly for Android), but the title text did not display correctly in that the backslashes didn't appear (which was a little confusing!). In Chrome on my Windows desktop, the title text appeared correctly. [[User:Jdluk|Jdluk]] ([[User talk:Jdluk|talk]]) 11:36, 3 February 2016 (UTC)

enough with the harry potter fancruft. "elder" is a [[Wiktionary:elder|perfectly good word]]. just because you came across it for the first time in harry potter means you are *typing carefully* the kind of person that likes harry potter. unless this is a ''harry potter reference'' wiki, of course. in which case i'll prepare a complete list of every word that appears both here and there and put a list on every page. oh, right, no i won't. --[[Special:Contributions/141.101.106.161|141.101.106.161]] 12:41, 3 February 2016 (UTC)

Remember that "Elder" is used in a lot of RPGs to denote high level enemies or items. I feel like that's what Randall's referring to here, more than Harry Potter or the general sense of the term "Elder."

Attempting to add to the discussion: This regex is not necessarily invalid or incomprehensible. It looks like he was looking for a line with a regular expression or definitely some code. You just have to work your way through the backslashes. Although it might be invalid depending on the precise rules. He has some unescaped closing brackets and closing parenthesis. If these have to always be escaped then the regex is invalid. If however you don't have to escape a closing bracket with no opening bracket, then things are fine. I'm not familiar enough with grep's regex parser to know how it handles that edge case. Presuming those unescaped paren and brackets are fine, his regex searches for:

1. A backslash

2. An opening bracket

3. An opening parenthesis (this is a character set but the only character in it is an opening paren)

4. Any number of any characters

5. A backslash

6. An opening bracket

7. A closing bracket

8. A closing paren (presuming it doesn't have to be escaped when there is no opening paren)

9. A closing bracket (presuming it doesn't have to be escaped when there is no opening bracket)

10. Any number of character that are not a closing paren or closing bracket

11. The end of the line

Basically he is looking for a string that looks like:

\[(AAAAA\[])]AAAAA

Looks like a regex to me, and it looks like this regex also doesn't escape closing paren/brackets that don't have an opening paren/bracket, so I'm guessing that he knows what he is doing and his regex is fine. Maybe he was playing regex golf?
[[User:Cmancone|Cmancone]] ([[User talk:Cmancone|talk]])cmancone

Talk:1638: Backslashes

2016-02-03T14:06:25Z

Cmancone:

It should be noted that this also occurs in almost every programming language where "\" is the escape character. i.e.
print("Hello")
> Hello
print("\"Hello\"")
> "Hello"
print("\\Hello\\")
> \Hello\
Oh, and by the way, isn't this the third comic to mention "Ba'al, the Soul Eater"? Maybe we should start a category. (Others are [http://www.explainxkcd.com/wiki/index.php/1246:_Pale_Blue_Dot 1246] (title text) and [http://www.explainxkcd.com/wiki/index.php/1419:_On_the_Phone 1419].)
[[Special:Contributions/173.245.54.29|173.245.54.29]] 06:14, 3 February 2016 (UTC)
:[[:Category:Ba'al|Did that]] before seeing you comment, so yes I agree. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:47, 3 February 2016 (UTC)

;I don't think the regex is invalid

According to <tt>man grep</tt> you need to specify the <tt>-E</tt> option to use extended regex; without it unescaped parentheses are not interpreted, so they don't need to match.

My - very wild - guess is that it was the command he used to find the line with the most special characters, but I am not confident enough to edit the article (if someone can confirm?). {{unsigned ip|141.101.66.83}}

If it was supposed to do that, it doesn't work. Running it on my bash history matches no lines, and I have lots of special characters in there [[Special:Contributions/197.234.242.243|197.234.242.243]] 07:12, 3 February 2016 (UTC)

Explain it to me like I'm dumb. What is this comic going on about? I think the explanation needs more examples like that hello, above, because that's almost understandable. --[[Special:Contributions/198.41.238.231|198.41.238.231]] 07:47, 3 February 2016 (UTC)
:I agree. But I cannot help either.--[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:51, 3 February 2016 (UTC)

This is the third time Randall has mentioned Ba'al the Soul Eater xD [[User:International Space Station|International Space Station]] ([[User talk:International Space Station|talk]]) 08:26, 3 February 2016 (UTC)
:Yes, that was already mentioned a few hours before you comment, see the first comment. --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:51, 3 February 2016 (UTC)

After passing the regex through bash, you get <nowiki>\\[[(].*\\[\])][^)\]]*$</nowiki> That is, the literal character \, followed by [ or (, followed by any number of any characters, followed by \, followed by ] or ), followed by any number of characters that aren't ) or ], until the end of the line. [[Special:Contributions/108.162.216.44|108.162.216.44]] 08:33, 3 February 2016 (UTC)
:It sounds like you know what you are talking about. Anyone who can explain it good enough for the explanation, and correct the explanation of the title text if it is wrong to say that it would not work. I have added this as the reason for incomplete. But maybe also examples are needed for people with not programming skills/knowledge. We also enjoy xkcd ;-) --[[User:Kynde|Kynde]] ([[User talk:Kynde|talk]]) 09:51, 3 February 2016 (UTC)
For fun:
cat ~/.bash_history | xargs -d "\n" -n 1 -I {} bash -c 'chars="$(echo "$1" | grep -o "[a-zA-Z0-9 ]" | wc -l)"; echo "$(( 100 - $(( $chars * 100 / ${#1} )) )) $1"' _ {} | sort -nrk 1 | less

Outputs your bash_history, ordered by relative gibberishness. This was copied by hand from desktop to mobile, might well have a few typos.--[[Special:Contributions/162.158.90.208|162.158.90.208]] 10:04, 3 February 2016 (UTC)

The problem in the comic is not with regexes per se but with situations when the entered text or expression passes through several interpreters, like bash -> grep/sed/awk, or program text -> external shell command. In such cases, you have to escape backslashes for each program in the sequence, and it gets worse if you have 'real' backslashes in the final text that you're processing with the utilities (Windows' file paths, for example). See https://en.wikipedia.org/wiki/Leaning_toothpick_syndrome.
Feel free to lift this to the explanation page, since I'm not good at longer and more careful explanations than this one.
Also, gotta notice that Feedly stripped paired backslashes in the title text (probably passed it through some 'interpreter' embedded in its scripts). [[User:Aasasd|Aasasd]] ([[User talk:Aasasd|talk]]) 10:13, 3 February 2016 (UTC)

:A funny comment about the MediaWiki software, which is even worse than this comic: <code><nowiki><Nikerabbit> I looked the code for rlike and didn't find where it does this. Can you point me to it? <vvv> $pattern = preg_replace( '!(\\\\\\\\\\\\\\\\)*(\\\\\\\\)?/!', '$1\\/', $pattern ); <Nikerabbit> I thought that was ascii art :)</nowiki></code> ([https://phabricator.wikimedia.org/P110$275 source]) --[[Special:Contributions/162.158.91.215|162.158.91.215]] 10:18, 3 February 2016 (UTC)

Interestingly, I first looked at this on my phone (using <strike>Chrome</strike> Feedly for Android), but the title text did not display correctly in that the backslashes didn't appear (which was a little confusing!). In Chrome on my Windows desktop, the title text appeared correctly. [[User:Jdluk|Jdluk]] ([[User talk:Jdluk|talk]]) 11:36, 3 February 2016 (UTC)

enough with the harry potter fancruft. "elder" is a [[Wiktionary:elder|perfectly good word]]. just because you came across it for the first time in harry potter means you are *typing carefully* the kind of person that likes harry potter. unless this is a ''harry potter reference'' wiki, of course. in which case i'll prepare a complete list of every word that appears both here and there and put a list on every page. oh, right, no i won't. --[[Special:Contributions/141.101.106.161|141.101.106.161]] 12:41, 3 February 2016 (UTC)

Remember that "Elder" is used in a lot of RPGs to denote high level enemies or items. I feel like that's what Randall's referring to here, more than Harry Potter or the general sense of the term "Elder."

Attempting to add to the discussion: This regex is not necessarily invalid or incomprehensible. It looks like he was looking for a line with a regular expression or definitely some code. You just have to work your way through the backslashes. Although it might be invalid depending on the precise rules. He has some unescaped closing brackets and closing parenthesis. If these have to always be escaped then the regex is invalid. If however you don't have to escape a closing bracket with no opening bracket, then things are fine. I'm not familiar enough with grep's regex parser to know how it handles that edge case. Presuming those unescaped paren and brackets are fine, his regex searches for:

1. A backslash
2. An opening bracket
3. An opening parenthesis (this is a character set but the only character in it is an opening paren)
4. Any number of any characters
5. A backslash
6. An opening bracket
7. A closing bracket
8. A closing paren (presuming it doesn't have to be escaped when there is no opening paren)
9. A closing bracket (presuming it doesn't have to be escaped when there is no opening bracket)
10. Any number of character that are not a closing paren or closing bracket
11. The end of the line

Basically he is looking for a string that looks like:

\[(AAAAA\[])]AAAAA

Looks like a regex to me, and it looks like this regex also doesn't escape closing paren/brackets that don't have an opening paren/bracket, so I'm guessing that he knows what he is doing and his regex is fine. Maybe he was playing regex golf?
[[User:Cmancone|Cmancone]] ([[User talk:Cmancone|talk]])cmancone