A telescope collects electromagnetic radiation from distant objects and forms an image or supplies a signal to a detector. An optical telescope can use lenses or mirrors; instruments for other wavelengths can require different collecting structures. Collection increases the detectable signal, while the aperture and wavelength constrain the angular resolution.
For an ideal unobscured circular aperture of diameter , the Rayleigh criterion gives an angular scale . Increasing the magnification of an already blurred image cannot recover angular detail lost through diffraction, optical aberrations or atmospheric seeing. This makes a double star a useful observing test: a larger apparent image is different from two genuinely resolved intensity peaks.[ref]
An X-ray observatory used for high-resolution imaging and spectroscopy of astronomical sources. Its observations of the Perseus cluster reveal structure in the emitting intracluster gas around radio bubbles.[ref]
A space observatory that measured stellar light curves to discover exoplanets and constrain their exoplanet occurrence rate through exoplanet transit photometry.
