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The tools of astrophysics Virtually all information about the external Universe is received in the form of electromagnetic radiation. The EM spectrum covers a range >1020 in wavelength. The Planck-Einstein relation E hf hc implies higher energy = shorter wavelength Intro to Stellar Astrophysics L2 1 The EM spectrum Radio Millimetre Microwave Infrared* Visible Ultraviolet* X-rays* -rays* Intro to Stellar Astrophysics L2 *Note: The atmosphere is opaque (or partially so) for radiation in these bands. They can only be observed from high altitude observatories, balloons, rockets or satellites. 2 Different ‘astronomies’ Astronomy/Astrophysics today gathers its information from across the EM spectrum, but we still sometimes talk about different ‘astronomies’ (optical astronomy, radio astronomy, X-ray astronomy) because Atmospheric transmission varies Telescopes and detector vary Different parts of the spectrum reveal different objects and different kinds of information….. Intro to Stellar Astrophysics L2 3 HST visible For example … Combined - HST visible (blue-cyan), Spitzer 3.6-4.5 m (green) and 8.0 m ( red) Spitzer IR 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red) m M104 Sombrero Galaxy. Intro to Stellar Astrophysics L2 4 © NASA/HST and Spitzer © NASA ADF - http://adc.gsfc.nasa.gov/mw/mmw_sci.html Milky Way at many wavelengths Intro to Stellar Astrophysics L2 5 Telescopes Telescopes at many wavelengths are basically similar. Important factors are: Configuration lens/mirror, paraboloids, prime focus, cassegrain, grazing incidence… Intro to Stellar Astrophysics L2 6 Telescopes - 2 Surface materials - glass, metal sheet, chicken wire,.. Surface accuracy - ‘diffraction limited’ is < /8 (p-p in the surface) or /4 in the wavefront Magnification - not very important Collecting area - light gathering power (sensitivity) D2 with possible ‘secondary obstruction’ Intro to Stellar Astrophysics L2 7 W.M. Keck Observatory - Hawai’i © NASA/JPL-Caltech Intro to Stellar Astrophysics L2 8 Keck primary mirror © NASA/JPL-Caltech Intro to Stellar Astrophysics L2 9 Parkes radio telescope © CSIRO/ATNF ? Intro to Stellar Astrophysics L2 10 Sensitivity Factors affecting sensitivity: Atmospheric transmission Collecting area System throughput Detector quantum efficiency Observing time Background - e.g. scattered light. As well as natural sources, man-made pollution is a major problem for astronomy. At optical wavelengths for example…. Intro to Stellar Astrophysics L2 11 Light pollution QuickTime™ and a YUV420 codec decompressor are needed to see this picture. © Pearson Education 2007 Intro to Stellar Astrophysics L2 12 © unknown Resolution The final important factor is resolution Theoretical resolution - Rayleigh’s criterion: min 1.22 D In practice, this is limited (for optical, IR) by ‘seeing’ - practical limit is 0.3 ~ 1.0 arcsec. At radio wavelengths, telescope sizeis the limiting factor. Intro to Stellar Astrophysics L2 13 Resolution - single telescopes Band UV Optical Near IR mm cm /8 Typical min.surface accuracy for D=10 m actual telescopes 100 nm 500 nm 2 m 1 mm 21 cm 13 nm 63 nm 250 nm 0.13 mm 26 mm 0.0025” 0.010” (HST 2.4 m) 0.013” (Keck 10 m) 0.050” (Keck 10 m) 25” (JCMT 10 m) 1.5° 9’ (Greenbank 100m) Resolution Now, concentrating on the optical for a moment……. Intro to Stellar Astrophysics L2 14 Adaptive optics Active Optics: slow image correction (f < 1 Hz), to correct mirror and structural deflections Adaptive Optics: fast image correction (f ≥ 1 Hz), primarily to correct random phase fluctuations of wavefronts caused by atmospheric turbulence - resulting image motion and blurring Intro to Stellar Astrophysics L2 15 Where does Seeing arise? Turbulence in the atmosphere leads to refractive index variations. Contributions are concentrated into layers at different altitudes. © John O’Byrne Intro to Stellar Astrophysics L2 16 Scidar measurements at SSO 10 minutes of data refractive index structure constant (Cn2 ) v. altitude Intro to Stellar Astrophysics L2 QuickTime™ and a GIF decompressor are needed to see this picture. 17 © John O’Byrne Seeing parameters Fried parameter ro(,z) = 0.185 6/5cos3/5 z(∫ Cn2dh)-3/5 Seeing disk FWHM without AO ≈ /ro for large telescopes So at ~500nm, ro ≈ 10 cm for 1 arcsec FWHM seeing At 2.5 m, this corresponds to ro ≈ 70 cm and 0.7 arcsec seeing Intro to Stellar Astrophysics L2 18 Essentials of an AO system Wavefront sensor Computer Phase modulator © John O’Byrne Intro to Stellar Astrophysics L2 19 AO example © University of Hawaii ? Intro to Stellar Astrophysics L2 20 Keck - Io Upper Left: Keck AO; K-band, 2.2micron. Upper Right: Galileo; visible light. Lower Left: Keck AO; L-band, 3.5micron. Lower Right: Keck without adaptive optics. © NASA/JPL-Caltech Intro to Stellar Astrophysics L2 21 Interferometry If EM waves from two or more apertures are coherently combined, the resolution is set by the “baseline” B between the apertures. Interferometry first proposed by Fizeau but first successful astronomical interferometer was due to Michelson (1891 Galilean satellites). In 1921 Michelson & Pease measured angular diameter of a Orionis (Betelgeuse). 1950s: Discovered by radio astronomers! Now widely used in radio, difficult at optical/IR. Intro to Stellar Astrophysics L2 22 © University of Sydney Basic principle of an optical interferometer - the Sydney University Stellar Interferometer (SUSI) at Narrabri is a 2-dimensional example Intro to Stellar Astrophysics L2 23 Resolution - interferometers Baseline Typical max. SUSI 400 nm ATCA 6 cm VLBI 6 cm Intro to Stellar Astrophysics L2 640 m ~20 km ~5000 km Resolution 0.0002” 2.5” 0.003” 24 © CSIRO/ATNF