= Telescope
{wiki}
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 $D$, the <Rayleigh criterion> gives an angular scale $\theta\simeq1.22\lambda/D$. 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.<University Physics Volume 3 by Samuel J. Ling and others/2.8 Microscopes and Telescopes>[telescopes: objective, eyepiece and angular magnification]{ref}
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