Oct. 18, Review: Telescopes – their primary purpose… Across the full EM spectrum (radio through very high energy gamma- rays) telescopes fundamentally similar: “light” collectors, to detect (and allow study) faint astronomical objects Most (but not all) telescopes form direct image or faithful reconstruction of position and flux of sources (e.g. stars) within the field of view – a telescope only images a “patch” of sky; you have now derived FoV in EL2 for Clay + CCD The image (e.g. point like star) can be detected (today’s class) on film (glass plates), electronic pixel detectors (CCDs) or other photon detectors, usually through filters Individual (or many) objects (e.g. stars) in the image can instead be detected through a spectrograph to reveal their spectra (e.g. absorption or emission lines) in great detail

Oct. 18, How sharp are optical images from ground vs. HST? Hubble Space Telescope (HST) is above the blurring effects of atmosphere, so even before its faulty mirror shape was fixed (Dec. 1993) it was better than anything from ground. But the fix gave diffraction limit: Θ ~λ/D=0.06” for λ=640nm (red) & D=2.4m = galaxy in Virgo

Oct. 18, Telescopes on ground vs. space Atmosphere is like looking through “blobby lenses”; light is distorted and jiggles from a point source (e.g. star) more than from an extended source (Moon or planet). Hence stars twinkle! Adaptive optics (Tf6-18) is powerful way to “anti-jiggle” the optics to take out atmospheric jiggles in “real time”. But works only over tiny field of view (FoV), though nearly reaches diffraction limit, Θ = λ/D (see last slide for scaling) Atmosphere prevents most of EM spectrum from passing through. Optical is only narrow slice; most of IR, UV, and all of x-ray and gamma-ray absorbed. Need telescopes in space! A radio telescope on/near back side of Moon would be much more sensitive at long wavelengths than on Earth since no radio pollution! Read article on Astronomy from the Moon, Oct. Sky & Tel, p. 24

Oct. 18, How do we detect light and make images? Photographic plates were the long-time standard (until c. 1982). Harvard has world’s largest collection (500,000 plates giving 100y of coverage full sky; our DASCH project will digitize them! Now we use Charge Coupled Devices (Tf6-19), like our CCD on the Clay Telescope (EL2): 1K x 1K pixels, 11microns each! (see blackboard explanation for how they work…) We also put spectrographs at focus of telescope (Tf6-20) to diffract the light and disperse into a spectrum to reveal both the continuum shape (and thus BB temperature) and also spectral lines