Astronomers and physicists now observe the sky with instruments that are sensitive to signals that are not a form of Maxwell's electromagnetic waves or their equivalent, photons. These "telescopes" have been designed to detect (i) neutrinos from the sun, supernova explosions and nuclear interactions in the earth's atmosphere, (ii) cosmic ray protons and other heavier atomic nuclei that gain their high energies at acceleration sites in the Galaxy and possibly beyond, and (iii) gravitational waves which should be created by large rapidly accelerating masses, such as a binary system containing two neutron-stars that spiral into one another to form a black hole.
Neutrinos and cosmic ray fluxes are now being detected and studied. Gravitational waves have yet to be detected but there is much reason, both observational and theoretical, to believe they exist and will be found when experiments reach the required sensitivity.
These detection systems do not focus the particles from a distant point source to an image as do optical telescopes, and so are often called "experiments" or "observatories". Nevertheless they do have methods of determining arrival directions to within several degrees in some cases, and they do probe the secrets of the universe. Thus "telescope" in its broadest sense is certainly an appropriate term for these instruments.
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