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Telescopes Telescopes refracting focal plane focal length, f optical axis image lens (diameter, D) focal plane focal length, f optical axis reflecting prime focus Primary mirror (paraboloid + hyperboloid) flat mirror Newtonian focus Secondary mirror (hyperboloid) Diffraction and resolution I n=2 Airy disk n=1 n=1 n=2 distance I Airy disk 1.22 /D distance I 1.22 /D distance Rayleigh Criterion: A point source is said to be resolved if the closest peak of any other Airy disk falls at least as far away as its own first minimum. aberrations chromatic aberration (refracting) spherical aberration (spherical mirror) centre focus edge focus coma (lense and parabolic mirror) instrument detector Typical instruments: • Camera • Spectrograph • Polarimeter • Photometer Detectors: • Charge-coupled devices (CCDs) • Photographic plates ground HST HST repaired HST repaired + ground without with adaptive optics spectrograph slit in telescope focal plane beam from telescope collimator mirror CCD diffraction grating For normal incidence: imaging lens n = order number = wavelength d = groove spacing infrared - ionized H visible (emission nebula illuminated by stars) globular cluster M79 visible star light UV - massive stars Radio interferometry a L signals combined The effective baseline, L = a cos ZD (where a is the distance between the telescopes, and ZD is the zenith distance of the source.) 350 m radio telescope in Arecibo Very Large Array in Sorocco - 36 km Very Long Baseline Interferometry galaxy with radio lobes X- ray grazing telescope CCD Charge Coupled Device Chandra XMM - Newton