- instrument for observing distant objects by collecting electromagnetic radiation
- optical telescope: refracting (lens) or reflecting (mirror), invented 1608
- radio telescope: dish antenna collecting radio waves, reveals cold gas and pulsars
- space telescopes: above atmospheric distortion (Hubble, James Webb, Chandra)
- aperture determines resolving power; larger aperture captures more photons
- extends human vision from planetary scale to cosmic scale (13.8 billion light-years)
- interferometry links multiple telescopes for effective aperture spanning continents
- adaptive optics corrects atmospheric turbulence in real time using semiconductor sensors
- complementary to microscope: one reaches outward, the other inward
- energy source for deep-space instruments: solar panels and radioisotope generators