3182: Telescope Types

Telescope Types

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Title text: 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.

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Explanation

This is one of 57 incomplete explanations: This page was created recently ACCORDING TO A TELESCOPE POINTING BACK IN TIME. Don't remove this notice too soon. If you can fix this issue, edit the page!

This comic shows diagrams of a number of different types of 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.

Type	Real?	Refractor/Reflector	Description
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.
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.
Newtonian	Yes	Reflector	Newtonian telescopes employ a second, flat mirror along with the primary parabolic mirror.
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.
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.
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.
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)
Cardboard tube	Yes, but not as a (functional) telescope	Neither	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 diffraction on the edges of the tube.
Kaleido	Yes, but not as a telescope	Reflector	A 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 are also usually flat, so there's no magnification.
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 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 is anything but water).
Narcissian	Yes, but not as a telescope	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 Narcissus, who fell in love with his reflection in a pool of water. A 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 would consider self-viewing more worthwhile than viewing the outside world.
Gravitational	Only in theory	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 black holes) and galaxies, which can't be constructed on Earth[citation needed]. There are proposals to launch missions to the very far reaches of the Solar System to "construct" a Solar gravitational lens telescope, but the masses and distances involved are not compatible with consumer camera hardware. In the title text, Randall makes a pun on whether the listed focal length of a gravitational lens is measured in the 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 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 The Core, which was universally derided by science-minded people. As a telescope it would not be useful, not least because it cannot be pointed at an arbitrary object. Its relevance to real geology is also dubious.

Transcript

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Discussion

no vampire jokes 🥀 (1791) TheTrainsKid (talk) 00:08, 18 December 2025 (UTC)

Got down some preliminary descriptions of each telescope type used 185.132.133.218 01:44, 18 December 2025 (UTC)

insert that one mickey mouse meme with the caption "what a fucking narcissist" Yaokuan ITB (talk) 02:33, 18 December 2025 (UTC)

abnormally low joke-to-real ratio for this format of comic! 2601:241:8002:3E0:C0A2:9DA:ED39:D13F 03:21, 18 December 2025 (UTC)

I noticed that... I think this might've originally been 'look at all these cool telescope types', but then he realized he had to put some sort of joke somewhere. --DollarStoreBa'al^Converse 03:27, 18 December 2025 (UTC)

Can someone make a category for The Core (2003)? It's been mentioned often enough. 83.245.251.49 09:22, 18 December 2025 (UTC)

Can you list 4 more comics then I will make the category. I think that is about the limit for when to make a new category. I know there are a few more but is it only 2-3more? --Kynde (talk) 14:00, 18 December 2025 (UTC)

All I can think of is 673: The Sun. --DollarStoreBa'al^Converse 15:09, 18 December 2025 (UTC)

Also mentioned in the title text of 2858: Thanksgiving Arguments. --208.59.176.206 15:24, 18 December 2025 (UTC)

> This would not […] end well for the drinker.

Would it though? Drinking elemental mercury, while not great on nutritional value, should be mostly safe (and I'm using that word quite loosely). The most danger would be while drinking and expelling it, when there's a danger of inhaling mercury vapors, right? --Coconut Galaxy (talk) 10:29, 18 December 2025 (UTC)

It wouldn't take much work to make the "Real?" column all contain only "yes" 136.32.133.124 12:05, 18 December 2025 (UTC)

As all others are refractors or reflectors, can cardboard tube be considered a diffractor? As it is the only thing that it does.--Trimutius (talk) 15:43, 18 December 2025 (UTC)

> Children may sometimes use tubes [...]

It's not just children. I've seen "viewing tubes" in at least a couple of places, hard-mounted metal tubes that point at particular points of interest. I'm not finding any good references, but here's a photo showing some at the top of a nearby mountain: https://maps.app.goo.gl/wwnYJ1zEQEXzjyJS8 Jordan Brown (talk) 18:07, 18 December 2025 (UTC)
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