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Telescope Types

|< < Prev #3182 (December 17, 2025)

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 comic shows diagrams of a number of different types of telescope, some real and others 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 telescope	Neither	Looking through a tube helps you focus by removing distractions, but doesn't magnify the object being viewed.
Kaleido	Yes, but not as a telescope	Reflector?	A kaleidoscope isn't really a telescope, because the non-viewing end is closed. You view many reflections of tiny objects 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 likely not end well for the drinker.
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, greatly magnified. This is inspired by the myth of Narcissus, who fell in love with his reflection in a pool of water. House of mirrors might feature such a "telescope", because it is basically a concave mirror, which would be a typical attraction at a funfair.
Gravitational	Yes	Refractor	These can't be constructed on Earth[citation needed], they're formed naturally by large stars (particularly black holes) and galaxies. 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, i.e. 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". As a telescope it would not be useful, not least because it cannot be pointed at anything in the sky.

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