Editing 1791: Telescopes: Refractor vs Reflector
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==Explanation== | ==Explanation== | ||
− | This comic compares two | + | {{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 | + | 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| | + | 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. |
− | + | 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 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. | |
− | 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 | + | 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. |
− | + | ==The real problems with refractor telescopes== | |
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*More expensive | *More expensive | ||
− | ** | + | **It is very difficult to make a refractor telescope as large as a reflector telescope, and would thus be more expensive it it needed to be as good as the reflector telescopes used almost exclusively today. |
+ | **One of the problems in very large apertures, is the problem of lens sagging, a result of gravity deforming glass. 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. | ||
*Less compact | *Less compact | ||
− | ** | + | **If it needed to be as good as a reflector telescope it would have to be very long. |
*{{w|Chromatic aberration}}: | *{{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 | + | **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. It occurs because lenses have different refractive indices for different wavelengths of light. |
+ | **The problem manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image, because each color in the optical spectrum cannot be focused at a single common point. | ||
***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. | ***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 | *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. | + | **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 compared to the other type. |
− | + | It is worth noting that: | |
− | + | '''A reflecting telescope also has disadvantages compared to a refracting telescope'''. | |
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*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. | *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 | + | **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 and 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. |
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− | Despite this disadvantage, reflecting telescopes are used almost exclusively in modern astronomy because of practical limitations in making large refracting telescopes | + | Despite this disadvantage, reflecting telescopes are used almost exclusively in modern astronomy because of practical limitations in making large refracting telescopes. |
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==Transcript== | ==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 them. The left drawing will be described first then the right.] | + | :[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:] | :[Left:] | ||
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:*Reduced light-gathering | :*Reduced light-gathering | ||
− | :[Right | + | :[Right] |
:<big>Reflector</big> | :<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 | + | :[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 | :*Can't see space vampires | ||
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{{comic discussion}} | {{comic discussion}} | ||
[[Category:Comics with color]] | [[Category:Comics with color]] | ||
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[[Category:Rankings]] | [[Category:Rankings]] | ||
[[Category:Science]] | [[Category:Science]] |