1. Intro to Stellar Astrophysics L21 The tools of astrophysics ä Virtually all information about the external Universe is received in the form of electromagnetic radiation. ä The EM spectrum covers a range >10 20 in wavelength. ä The Planck-Einstein relation implies higher energy = shorter wavelength

2. Intro to Stellar Astrophysics L22 The EM spectrum *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. ä Radio ä Millimetre ä Microwave ä Infrared* ä Visible ä Ultraviolet* ä X-rays*   -rays*

3. Intro to Stellar Astrophysics L23 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 ä Atmospheric transmission varies ä Telescopes and detector vary ä Different parts of the spectrum reveal different objects and different kinds of information…..

4. Intro to Stellar Astrophysics L24 For example … M104 Sombrero Galaxy. © NASA/HST and Spitzer HST visible Spitzer IR 3.6 (blue), 4.5 (green), 5.8 (orange), and 8.0 (red)  m Combined - HST visible ( blue-cyan), Spitzer 3.6-4.5  m (green) and 8.0  m ( red)

5. Intro to Stellar Astrophysics L25 Milky Way at many wavelengths © NASA ADF - http://adc.gsfc.nasa.gov/mw/mmw_sci.html

6. Intro to Stellar Astrophysics L26 Telescopes Telescopes at many wavelengths are basically similar. Important factors are: ä Configuration - lens/mirror, paraboloids, prime focus, cassegrain, grazing incidence…

7. Intro to Stellar Astrophysics L27 Telescopes - 2 ä Surface materials - glass, metal sheet, chicken wire,.. ä Surface accuracy - ‘diffraction limited’ is < /8 (p-p in the surface) or /4 in the wavefront ä Magnification - not very important ä Collecting area - light gathering power (sensitivity)  D 2 with possible ‘secondary obstruction’

8. Intro to Stellar Astrophysics L28 W.M. Keck Observatory - Hawai’i © NASA/JPL-Caltech

9. Intro to Stellar Astrophysics L29 Keck primary mirror © NASA/JPL-Caltech

10. Intro to Stellar Astrophysics L210 Parkes radio telescope © CSIRO/ATNF ?

11. Intro to Stellar Astrophysics L211 Sensitivity Factors affecting sensitivity: ä Atmospheric transmission ä Collecting area ä System throughput ä Detector quantum efficiency ä Observing time ä Background - e.g. scattered light. As well as natural sources, man-made pollution is a major problem for astronomy. At optical wavelengths for example….

12. Intro to Stellar Astrophysics L212 Light pollution © unknown © Pearson Education 2007

13. Intro to Stellar Astrophysics L213 Resolution The final important factor is resolution ä Theoretical resolution - Rayleigh’s criterion: ä In practice, this is limited (for optical, IR) by ‘seeing’ - practical limit is 0.3 ~ 1.0 arcsec. ä At radio wavelengths, telescope size is the limiting factor.

14. Intro to Stellar Astrophysics L214 Resolution - single telescopes Band /8Resolution Typicalmin.surfacefor D=10 m actual telescopes Typicalmin.surfacefor D=10 m actual telescopesaccuracy UV 100 nm13 nm0.0025” 0.010” (HST 2.4 m) Optical 500 nm63 nm0.013” (Keck 10 m) Near IR 2  m250 nm0.050” (Keck 10 m) mm 1 mm0.13 mm25” (JCMT 10 m) cm 21 cm26 mm1.5° 9’ (Greenbank 100m) Now, concentrating on the optical for a moment…….

15. Intro to Stellar Astrophysics L215 Adaptive optics ä Active Optics:  slow image correction (f < 1 Hz), to correct mirror and structural deflections ä Adaptive Optics: ä fast image correction (f ≥ 1 Hz), primarily to correct random phase fluctuations of wavefronts caused by atmospheric turbulence - resulting image motion and blurring

16. Intro to Stellar Astrophysics L216 Where does Seeing arise? Turbulence in the atmosphere leads to refractive index variations. Contributions are concentrated into layers at different altitudes. © John O’Byrne

17. Intro to Stellar Astrophysics L217 10 minutes of data refractive index structure constant (C n 2 ) v. altitude Scidar measurements at SSO © John O’Byrne

18. Intro to Stellar Astrophysics L218 Seeing parameters  Fried parameter r o (  z) = 0.185  6/5 cos 3/5 z(∫ C n 2 dh) -3/5  Seeing disk FWHM without AO ≈  /r o for large telescopes  So at ~500nm, r o ≈ 10 cm for 1 arcsec FWHM seeing At 2.5  m, this corresponds to r o ≈ 70 cm and 0.7 arcsec seeing

19. Intro to Stellar Astrophysics L219 Essentials of an AO system ä Wavefront sensor ä Computer ä Phase modulator © John O’Byrne

20. Intro to Stellar Astrophysics L220 AO example © University of Hawaii ?

21. Intro to Stellar Astrophysics L221 Keck - Io ä Upper Left: Keck AO; K-band, 2.2micron. Keck AO; K-band, 2.2micron. ä Upper Right: Galileo; visible light. Galileo; visible light. ä Lower Left: Keck AO; L-band, 3.5micron. Keck AO; L-band, 3.5micron. ä Lower Right: Keck without adaptive optics. Keck without adaptive optics. © NASA/JPL-Caltech

22. Intro to Stellar Astrophysics L222 Interferometry ä If EM waves from two or more apertures are coherently combined, the resolution is set by the “baseline” B between the apertures. ä 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  Orionis (Betelgeuse). ä 1950s: Discovered by radio astronomers! ä Now widely used in radio, difficult at optical/IR.

23. Intro to Stellar Astrophysics L223 Basic principle of an optical interferometer - the Sydney University Stellar Interferometer (SUSI) at Narrabri is a 2-dimensional example Basic principle of an optical interferometer - the Sydney University Stellar Interferometer (SUSI) at Narrabri is a 2-dimensional example © University of Sydney

24. Intro to Stellar Astrophysics L224 Resolution - interferometers BaselineResolution BaselineResolution Typicalmax. SUSI 400 nm640 m0.0002” ATCA 6 cm~20 km2.5” VLBI 6 cm~5000 km0.003” © CSIRO/ATNF