1791: Telescopes: Refractor vs Reflector

Explain xkcd: It's 'cause you're dumb.
Revision as of 14:27, 27 January 2017 by Cmancone (talk | contribs) (Explanation)
Jump to: navigation, search
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 shows two types of telescopes: Reflecting and Refracting. It first looks like the comic is trying to show that refracting has many flaws, such as expense, size and visibility. 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 cannot be seen in a mirror. As space vampires do not exist [citation needed], this complaint is moot, and the reflecting telescope technically has no flaws in comparison to the refracting telescope. 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, both of which are apparently equally important to the telescope's merit despite also not existing. Although unclear, refracting telescopes presumably can't see shadow people or Chernabog because both are "dark", and with their smaller collecting areas, reflecting telescopes can see fainter objects than refracting telescopes.

An error in the comic would seem to be that both telescope illustrations contain a mirror (in the refracting, it's at the eyepiece). This would invalidate the advantage it has over reflecting.

The title text adds an additional drawback to a refracting telecsope: it cannot see Shadow People or Chernabog. 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 apperance in modern media was in the 1940 Disney movie Fantasia.

It is worth noting that in reality 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 and 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. Despite this disadvantage, reflecting telescopes are used almost exclusively in modern astronomy because of practical limitations in making large refracting telescopes.


Ambox notice.png This transcript is incomplete. Please help editing it! Thanks.

[The comic is one panel showing two different telescope designs.] REFRACTOR More expensive Less compact Chromatic aberration Reduced Light-gathering

REFLECTOR Can't see space vampires

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

comment.png add a comment! ⋅ comment.png add a topic (use sparingly)! ⋅ Icons-mini-action refresh blue.gif refresh comments!


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)

Colin Wilson also wrote a novel titled The Space Vampires. https://www.goodreads.com/book/show/869314.The_Space_Vampires Nitpicking (talk) 03:38, 6 March 2022 (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

It is odd that apochromatic refractors are not mentioned. These have much better color correction than achromatic telescopes. Further, contrary to the article's claims, refractors ARE widely used by amateur astronomers--especially for photography. There are several reasons for this, including that refractors are physically more compact, generally have give higher contrast images, and do not require collimation. Most important, though, is that they generally have shorter focal lengths than reflecting or catadioptric designs. This results in lower magnification and a wider field of view. This is helpful not only because many deep sky objects have a large angular size, but because highly accurate tracking is required to prevent star trails at higher magnifications. A google search will confirm that many experts in astrophotography recommend refractors (esp. for beginners). Cheers.