Astronomical Observational Techniques and Instrumentation

1. Astronomical Observational Techniques and Instrumentation Professor Don Figer Radio Astronomy

2. Aims of Lecture • review radio imaging chain • describe radio sources • describe radio detection • describe some common radio telescopes • give examples of radio objects

3. Radio Imaging Chain

4. Introduction • Radio regime covers ~1 mm to 10 m, but best atmospheric transmission is over 1-20 cm. • Radio detection requires a receiver (plus dish in some cases). • The unit of intensity for radio measurements is the Jansky, named for early radio pioneer, Karl Jansky • A strong source has an intensity of a few Jy. • A very weak source has an intenstiy of a few mJy.

5. Pioneers of Radio Astronomy • Radio astronomy was developed relatively early (~75 years ago) • atmosphere is transparent at radio wavelengths • important commercial applications (communications) • WWII military applications (communications and radar) • Jansky made first measurements and identified source in Galactic center in 1933. • Grote Reber first noted sources with increasing flux for lower frequencies, i.e. synchrotron emission, in 1937. Grote Reber Karl Jansky

6. Basic Radio Telescope Kraus, 1966. Fig.1-6, p. 14.

7. Radio Interferometry

8. Resolution of Single Dish and Interferometer

9. Resolution of VLBI

10. Resolution Comparisons

11. Radio Sources

12. Spectral Index • The spectral energy distribution at radio wavelengths is often described by the “spectral index,” alpha. • Thermal emission (blackbody) is described by the Rayleigh-Jeans tail of the Planck function, so alpha~2. • For non-thermal radiation, alpha<0. • For a stellar wind from a hot star, alpha~0.6 (see Wright & Barlow, 1975, ApJ, 170, 41).

13. Blackbody Sources • Peak in cm-wave radio requires very low temperature: lmaxT = 0.2898 cm K • Cosmic Microwave Background is about the only relevant blackbody source • Ignored in most work – essentially constant source of static (same in all directions) and much weaker than static produced by instrumentation itself

14. Continuum Sources • Due to accelerating electrons: • Synchrotron radiation • Bremsstrahlung (free-free) • Quasars, Active Galactic Nuclei, Pulsars, Supernova Remnants, HII regions, etc.

15. Spectral Line Sources • Neutral hydrogen (H I) spin-flip transition • Recombination lines (between high-lying atomic states) • Molecular lines (CO, OH, etc.)

16. 21-cm Radiation • due to electron spin flip • seen in emission and absorption • useful for tracing spiral arms

17. 21-cm Radiation: Map of Galaxy • Galactic center is blue dot, and Sun is at yellow arrow. • Signal is attributed to distance along line of sight by comparing measured radial velocity to a model that assumes circular Galactic rotation curve. • Similar structure is seen in HII, OB stars, and star forming regions.

18. 21-cm Radiation: Rotation Curve • H I spectral line from a galaxy shifted by expansion of universe (“recession velocity”) and broadened by rotation Frequency

19. Radio Recombination Lines • These transitions are all “hydrogen-like” in that the upper-state electron “sees” a nucleus with almost one positive charge.

20. Radio Detection

21. Radio Telescope Components • Reflector(s) • Feed horn(s) • Low-noise amplifier • Filter • Downconverter • IF Amplifier • Spectrometer

22. Antenna Fundamentals • An antenna is a device for converting electromagnetic radiation into electrical currents or vice-versa, depending on whether it is being used for receiving or for transmitting. • In radio astronomy, antennas are used for receiving. • The antenna receiver usually receives radiation from a dish, but it doesn’t have to. • For instance, the Long Wavelength Array (LWA) that has ~104 dipoles. At a wavelength of 15m, the dipoles have ~106 m2 of effective collecting area, where collecting area goes as wavelength squared, divided by 4 pi.

23. Equivalent Antenna Temperature • One can equate the power due to a source to the equivalent power (Johnson noise) of a resistor having a certain temperature. • Power WA detected by an antenna due to a source of flux density S where antenna temperature is TA , and effective aperture is Ae. (Factor of one-half because detector is only sensitive to one polarization.)

24. System Temperature = total noise power detected, a result of many contributions Thermal noise T = minimum detectable signal For GBT spectroscopy

25. Smoothing by the beam Kraus, 1966. Fig. 3-6. p. 70; Fig. 3-5, p. 69.

26. Atmospheric opacity • The amount of absorbed radiation depends upon the number of absorbers along the line of sight t0 t=1.4*t0 Atmosphere where t is the optical depth and z is the angle from zenith.

27. Beam & sidelobes • Essentially diffraction pattern of telescope functioning as transmitter • Uniformly illuminated circular aperture: central beam & sidelobe rings

28. Polarization • Radio waves can be linearly or circularly polarized. • A radio receiver can only detect linearly polarized radiation along one axis. • Two receivers are needed to sense both polarizations or circular polarization.

29. Interferometry and Aperture Synthesis

30. VLA

31. Constructieve Interference

32. Interference Pattern

33. Aperture Synthesis

34. Baselines

35. Optical Interferometry?

36. Examples of Radio Telescopes

37. 9:43pm, Nov. 15, 1988 Don’t Let This Happen to Your Radio Telescope 300 foot radio telescope in Green Bank, WV 9:42pm, Nov. 15, 1988

38. Green Bank Telescope (GBT) • Out with the old, in with the new…. 100x110m Unobstructed aperture reduces reflections into telescope from terrestrial transmitters and reduces diffraction. GBT paint is white in the visible portion of the spectrum to reflect sunlight because differential solar heating would expand and deform the reflector. It is black in the mid-infrared so that the GBT can cool itself efficiently by reradiation. It is transparent at radio wavelengths so that it neither absorbs incoming radio waves nor emits thermal noise at radio wavelengths.

39. GBT Main Features • Fully steerable antenna • 5 deg - 95 deg elevation range; 85% coverage of the celestial sphere. • Unblocked aperture • Active surface • Allows for compensation for gravitational and thermal distortions. • Ultimate frequency coverage of 100 MHz to 100 + GHz • 3 orders of magnitude of frequency coverage for scientific flexibility. Current frequency coverage of 290 MHz to 49 GHz (0.6 to 100 cm) • Location in the National Radio Quiet Zone

40. Unblocked Aperture • 100 x 110 m section of a parent parabola 208 m in diameter • Cantilevered feed arm is at focus of the parent parabola

41. Subreflector and receiver room

42. GBT Receiver Turret

43. Very Large Array (VLA)

44. VLA Main Features • 27 radio antennas in a Y-shaped configuration • fifty miles west of Socorro, New Mexico • each antenna is 25 meters (82 feet) in diameter • data from the antennas are combined electronically to give the resolution of an antenna 36km (22 miles) across • sensitivity equal to that of a single dish 130 meters (422 feet) in diameter • four configurations: • A array, with a maximum antenna separation of 36 km; • B array -- 10 km; • C array -- 3.6 km; and • D array -- 1 km.

45. VLA Receivers

46. Very Long Baseline Array (VLBA) • ten radio telescope antennas • 25 meters (82 feet) in diameter and weighing 240 tons • Mauna Kea to St. Croix in the U.S. Virgin Islands • VLBA spans more than 5,000 miles, providing astronomers with the sharpest vision of any telescope on Earth or in space. • efforts to reduce funding • efforts to increase sensitivity (~6x)

47. Objects in Radio

48. Sagittarius A: Mystery Mass in Galaxy CenterRADIO NIR

49. Virgo A (M87): Hidden Massive Black Hole shooting out a JetRADIO OPTICAL

50. Molecules