3182: Telescope Types

2026-04-25T00:11:28Z

2600:4040:2D29:6800:B8C2:10C7:93B8:B85C:

{{comic
| number = 3182
| date = December 17, 2025
| title = Telescope Types
| image = telescope_types_2x.png
| imagesize = 517x680px
| noexpand = true
| titletext = I'm trying to buy a gravitational lens for my camera, but I can't tell if the manufacturers are listing comoving focal length or proper focal length.
}}

==Explanation==
This comic shows diagrams of a number of different types of {{w|telescope}} — some real, while others are other objects, or made up by [[Randall]]. It includes both refracting and reflecting designs; see [[1791: Telescopes: Refractor vs Reflector]] for the important (according to Randall) differences between them.

{| class="wikitable"
! Type !! Real telescope? !! Optic type !! Description
|-
| {{w|Reflecting telescope#Prime_focus|Prime focus}} || Yes || Reflector || A telescope design where the observer/receiver is situated at the focal point of a single mirror. Rare in optics, but a common design in radio telescopes.
|-
| {{w|Herschelian telescope|Herschelian}} || Yes || Reflector || A telescope design much akin to Prime Focus but with the mirror tilted so that the observer does not block incoming light. Named after astronomer William Herschel.
|-
| {{w|Newtonian telescope|Newtonian}} || Yes || Reflector || Newtonian telescopes employ a second, flat mirror along with the primary parabolic mirror.
|-
| {{w|Galilean telescope|Galilean}} || Yes || Refractor || What might usually come to mind when picturing a telescope. A long tube that uses lenses rather than mirrors (making it a refracting telescope) to magnify images.
|-
| {{w|Keplerian telescope|Keplerian}} || Yes || Refractor || An improvement on Galilean telescopes, using a convex lens rather than a concave one at the eyepiece (as shown in the diagram). It does however invert images.
|-
| {{w|Gregorian telescope|Gregorian}} || Yes || Reflector || Uses two concave mirrors, the secondary being placed beyond the primary's focal point. The image is reflected back through a hole in the primary mirror. Unique among reflectors in that the image is not inverted.
|-
| {{w|Cassegrain telescope|Cassegrain}} || Yes || Reflector || Similar to prime focus, but uses a secondary mirror to reflect light through a hole in the primary mirror to the observer (situated at the rear)
|-
| {{w|Cardboard}} tube || No, does not magnify || N/A || Children may sometimes use tubes, particularly the cardboard middles from paper rolls, as a play 'telescope'. Looking through a tube can give an illusion of magnification by removing distractions and focusing your attention on the object in view, but it doesn't actually magnify the object being viewed. It will still cause a minor optical effect due to {{w|diffraction}} on the edges of the tube.
|-
| {{w|Kaleidoscope|Kaleido''(scope)''}} || No, just used to create decorative patterns || Reflector || A {{w|kaleidoscope}} is similar in form to the stereotypical 'ship's telescope', being a tubular object that you look in to one end of. However, it isn't really a telescope, because you can't use it to magnify arbitrary objects of interest. The non-viewing end is closed, and you view patterns created by many fragmented reflections of tiny objects contained at the end, rather than remote objects. The mirrors (set lengthways and angled to each other) are also usually flat, so provide no magnification.
|-
| {{w|Liquid mirror telescope|Liquid mirror}} || Yes || Reflector || A telescope with the same design as Prime Focus, using a rotating pool of reflective liquid (most commonly mercury) as a mirror, the steady rotation creating a parabolic lens. The diagram adds a straw so that someone can drink the liquid. This would not improve telescope performance or end well for the drinker (if the liquid was to be anything but water).
|-
| Narcissian || No, observers view themselves || Reflector || This is like a prime focus telescope, but the focus is outside the end of the telescope where the viewer is located, so they can only see themselves, magnified by the concave mirror. This is inspired by the myth of {{w|Narcissus}}, who fell in love with his reflection in a pool of water. A {{w|house of mirrors}} (a typical attraction at a funfair) might feature such a 'telescope', because it is basically a concave mirror.

Furthermore, a narcissist, someone who is inordinately self-centered and arrogant (named for Narcissus), would likely appreciate this kind of mirror, as a narcissist considers self-viewing more worthwhile than viewing the outside world.
|-
| {{w|Gravitational lens|Gravitational}} || Not as shown || Refractor || Using the gravitational effect of very large objects on the light passing around them to gain a magnified (if distorted) view of objects beyond them. These are formed naturally by large stars (particularly {{w|black holes}}) and galaxies, but can't be constructed on Earth{{cn}}. There are proposals to launch missions to the very far reaches of the Solar System to "construct" a {{w|Solar gravitational lens}} telescope, but the masses and distances involved are not compatible with consumer camera hardware.

In the title text, Randall is concerned about whether the listed focal length of a gravitational lens is measured in the {{w|comoving and proper distances|comoving or proper}} reference frame — that is, whether the expansion of the universe (between the place and time of the lens's creation or construction and Randall's decision to purchase) has been factored out or not. At the cosmological scales between stars and galaxies, where gravitational lensing is most relevant, this is a useful distinction to make, but [https://iauarchive.eso.org/public/themes/buying_star_names/ stars are not for sale] (by any legitimate commercial entity) and so nobody would be advertising any focal length in either reference frame for any purchaser.
|-
| Geological || No || Reflector || This 'telescope' employs a single mirror to show the observer the 2003 movie {{w|The Core}}, which was broadly derided by science-minded people. As a telescope it would not be useful, not least because it cannot be pointed at an arbitrary object, although movie projectors do have a similar structure. Its relevance to real geology is also dubious.{{Citation needed}}
|}

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}

[The comic contains cross-sections/diagrams of 13 different types of telescopes, each with a small eye representing the observer and shaded areas representing the path of light, with darker areas indicating where light overlaps itself. The light originates from the left in all but the Liquid Mirror and Geological diagrams.]

[Prime Focus: A telescope that contains a concave mirror at one end, which reflects and focuses a column of light directly to an observer within the telescope.]

[Herschelian: Similar to Prime Focus, except the mirror is angled such that its focal point is outside the telescope, along with the observer.]

[Newtonian: Similar to Prime Focus, although there is a smaller angled mirror that changes the direction of the light by 90° after being focused so that the focal point is outside of the telescope.]

[Galilean: Rather than being reflected, the light is refracted by a convex lens and focused until it hits a concave lens at the other end, where the observer is.]

[Keplerian: Similar to Galilean, although the light narrows to a point and effectively "flips" before being straightened out by a second convex lens.]

[Gregorian: Similar to a Prime Focus telescope, though the light is refracted a second time by a small concave mirror slightly after the focal point and exits through a gap in the first.]

[Cassegrain: Similar to Gregorian, though the second mirror is convex and slightly in front of the focal point.]

[Cardboard Tube: A simple tube that allows only a small potion of light in, which is then seen by the viewer.]

[Kaleido: A tube with several small pieces of some material at one end, which light passes through and is then repeatedly bounced around in a reflective inner tube.]

[Liquid Mirror: Similar to Prime Focus, except the mirror is vertical and made of liquid being rotated at a constant speed by a motor underneath. A drinking straw leads out of the liquid and toward a small closed mouth.]

[Narcissian: Similar to Prime Focus, except the light is emanating from Cueball, who is standing ~4m away from a Cueball-sized mirror. The light is being refracted directly into Cueball's face.]

[Gravitational: A small black hole is in the middle of the telescope, positioned just right to warp two streams of light coming around it directly to the observer.]

[Geological: Similar to a Newtonian telescope, except the primary mirror is replaced by a small television labeled "TV playing ''The Core'' (2003)."]

{{comic discussion}}<noinclude>

[[Category:Science]]
[[Category:Astronomy]]
[[Category:Geology]]
[[Category:Telescopes]]
[[Category:Movies]]
[[Category:The Core]]
[[Category:Comics featuring Cueball]]

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3182: Telescope Types