1791: Telescopes: Refractor vs Reflector
|Telescopes: Refractor vs Reflector|
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 explanation may be incomplete or incorrect: The section 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?|
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It first looks like the comic is simply trying to show that refracting has many flaws, such as expense, size and visibility (see more details below). However, the punchline invalidates these complaints with the (apparently major) flaw listed with the reflecting telescope: It can't see space vampires.
The unstated reason for this is that vampires, according to some cultures, cannot be seen in a mirror. As space vampires (like earth vampires) are widely believed to be made up and thus unlikely to interest most stargazers, 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 Porro prism or 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 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 Shadow People or the Slavic god Chernobog, both of which are apparently equally important to the telescope's merit despite neither the shadow people or 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 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
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 - 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.
- 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 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.
- 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 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 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 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 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!)
- [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.]
- [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
- [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
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