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I'm having trouble finding a definition of telescope that works for all telescopes (optical, radio, or otherwise). So far, all the definitions I've found only apply to optical telescopes.

ODO:

telescope
An optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified.

Dictionary.com:

telescope
an optical instrument for making distant objects appear larger and therefore nearer. One of the two principal forms (refracting telescope) consists essentially of an objective lens set into one end of a tube and an adjustable eyepiece or combination of lenses set into the other end of a tube that slides into the first and through which the enlarged object is viewed directly; the other form (reflecting telescope) has a concave mirror that gathers light from the object and focuses it into an adjustable eyepiece or combination of lenses through which the reflection of the object is enlarged and viewed.

These definitions work just fine for optical telescopes, but there are many different types of telescopes, and not all of them fit either definition. According to Wikipedia,

[In certain] types of high energy particle telescopes there is no image-forming optical system.

A telescope's optical components seem to be a defining feature, and I'm uncertain how some of these types of telescopes are considered telescopes under the current definition.

Is there another, more generic definition of "telescope" that I've missed? If not, is there a reason why the definition of "telescope" hasn't been updated to include a wider range of devices?

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  • 1
    Perhaps some are telescopes by analogy.
    – Jim Mack
    Jan 10 '18 at 3:13
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    OED Online adds this to the first definition: "Also: an instrument or apparatus of analogous design used to study distant sources of electromagnetic radiation lying outside the visible spectrum."
    – 0..
    Jan 10 '18 at 3:26
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    @ermanen Good point. However, even the assertion that all telescopes measure electromagnetic radiation is false as neutrino detectors are sometimes also considered to be telescopes. For example, ANTARES: "Astronomy with a Neutrino Telescope and Abyss environmental RESearch"
    – Vyren
    Jan 10 '18 at 3:55
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    Photon detection and neutrino detection work similarly, though of course the detectors are vastly different. Both can be considered telescopes. BTW, there is also a verb telescope, which means to condense like a collapsing telescope. Jan 10 '18 at 3:58
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    @Vyren: Technology is advancing rapidly, dictionaries are not that fast :) Most detect electromagnetic radiation so that's why it is included in the definition also.
    – 0..
    Jan 10 '18 at 4:06
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Etymonline says that the word telescope comes from the Greek words tele, meaning far, and skopos, meaning watcher.

And in fact, I can't think of anything called a telescope that isn't used for observing something far away. But to make it a more precise definition, you should add that it observes it by using some kind of radiation, since otherwise a Mars lander would qualify as a telescope, which it clearly shouldn't.

The Collins Dictionary has the definition

  1. any instrument, such as a radio telescope, for collecting, focusing, and detecting electromagnetic radiation from space.

So they have updated their definition.

However, we now have telescopes that use neutrinos and gravitational waves, and these don't focus the radiation in any meaningful sense, so removing the words "electromagnetic" and "focusing" from the definition would probably be a good idea.

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In astronomy a telescope isn't used so much to make things appear closer as to concentrate a weaker signal to detect fainter objects.

The point of a telescope is to have a larger aperture than your eye and so to collect more light and allow you to see fainter, more distant objects - so Peter Shor's answer is the best.

The Collins definition is a bit off - it doesn't have to be electromagnetic (we have neutrino telescopes) and it doesn't have to focus (interferometers e.g. radio telescopes don't).

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    The Collins definition might have been written when there weren't neutrino telescopes, and when radio telescopes were generally composed of parabolic antennas, which definitely focused things. But you're right, it's off now. Jan 10 '18 at 4:14
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It is possible that telescope is assumed to refer to a device that operates at visible frequencies, since that is what telescopes originally did. Telescopes that detect frequencies beyond visible spectrum are usually referred to by the frequencies that they detect, e.g. microwave telescopes, radio telescopes, infra-red telescopes, etc. However, they all detect electromagnetic radiation (i.e. photons), albeit at different frequencies (or wavelengths).

Note that dictionary definitions may be somewhat vague:

radio telescope noun [ C ]

a device for receiving, for scientific study, the electromagnetic waves sent out by objects in space such as stars

Cambridge Dictionary

This definition is a little misleading, since radio telescope normally refers to a device that operates at frequencies below the infra-red spectrum.

Like microscope, the term telescope is a reasonable catch-all, although its meaning may be misunderstood.


Note that there are such things as neutrino telescopes. Neutrinos are not a form of electromagnetic radiation, although they are fundamental particles.

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I think you need to refer to a specialist dictionary. I have a copy of The Penguin Dictionary of Physics copyright 1975 (abridged 1977).

Under telescope, definition 1 starts An optical device for producing a magnified or intensified image of distant objects and runs to 4 paragraphs, mentioning refracting and reflecting types.

Definition 2 is of interest here, and I quote it in its entirety:

An electronic device for detecting and amplifying electromagnetic radiation outside the visible region of the spectrum, including the *radio telescope and *electron telescope

(the asterisks indicate terms defined elsewhere in this dictionary). Note the lack of references to focussing in this definition, thus it would include even phased array telescopes like some of the square kilometre array.


For those puzzled by the electron telescope (as I was) it used a photocathode, high-voltage accelerating grids, and a phosphor screen to image UV and IR.

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