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, Fig.1-6, p. 14.

7. Radio Interferometry
Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.6 meters). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of approximately 100 ft. and stood 20 ft. tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of a received signal could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system.
After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially surmise that he was detecting radiation from the Sun.
After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also determined that the signal repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is a typical astronomical characteristic of any "fixed" object located far from our solar system (see sidereal day). By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius.
His discovery was widely publicized, appearing in the New York Times of May 5, He published his classic paper "Electrical disturbances apparently of extraterrestrial origin" in Proc. IRE in This paper was re-printed in Proc. IEEE in 1984 (for their centennial issue, where they note the research most likely would have won a Nobel prize, had not the author died young) and again in 1998, for the first centennial of radio. Jansky wanted to follow up on this discovery and investigate the radio waves from the Milky Way in further detail. He submitted a proposal to Bell Labs to build a 30 meter diameter dish antenna with greater sensitivity that would allow more careful measurements of the structure and strength of the radio emission. Bell Labs, however, rejected his request for funding on the grounds that the detected emission would not significantly affect their planned transatlantic communications system. Jansky was re-assigned to another project and did no further work in the field of astronomy.

8. Resolution of Single Dish and Interferometer
Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.6 meters). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of approximately 100 ft. and stood 20 ft. tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of a received signal could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system.
After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially surmise that he was detecting radiation from the Sun.
After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also determined that the signal repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is a typical astronomical characteristic of any "fixed" object located far from our solar system (see sidereal day). By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius.
His discovery was widely publicized, appearing in the New York Times of May 5, He published his classic paper "Electrical disturbances apparently of extraterrestrial origin" in Proc. IRE in This paper was re-printed in Proc. IEEE in 1984 (for their centennial issue, where they note the research most likely would have won a Nobel prize, had not the author died young) and again in 1998, for the first centennial of radio. Jansky wanted to follow up on this discovery and investigate the radio waves from the Milky Way in further detail. He submitted a proposal to Bell Labs to build a 30 meter diameter dish antenna with greater sensitivity that would allow more careful measurements of the structure and strength of the radio emission. Bell Labs, however, rejected his request for funding on the grounds that the detected emission would not significantly affect their planned transatlantic communications system. Jansky was re-assigned to another project and did no further work in the field of astronomy.

9. Resolution of VLBI
Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz (wavelength about 14.6 meters). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name "Jansky's merry-go-round". It had a diameter of approximately 100 ft. and stood 20 ft. tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of a received signal could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system.
After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. He spent over a year investigating the source of the third type of static. The location of maximum intensity rose and fell once a day, leading Jansky to initially surmise that he was detecting radiation from the Sun.
After a few months of following the signal, however, the brightest point moved away from the position of the Sun. Jansky also determined that the signal repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is a typical astronomical characteristic of any "fixed" object located far from our solar system (see sidereal day). By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center the galaxy, in the constellation of Sagittarius.
His discovery was widely publicized, appearing in the New York Times of May 5, He published his classic paper "Electrical disturbances apparently of extraterrestrial origin" in Proc. IRE in This paper was re-printed in Proc. IEEE in 1984 (for their centennial issue, where they note the research most likely would have won a Nobel prize, had not the author died young) and again in 1998, for the first centennial of radio. Jansky wanted to follow up on this discovery and investigate the radio waves from the Milky Way in further detail. He submitted a proposal to Bell Labs to build a 30 meter diameter dish antenna with greater sensitivity that would allow more careful measurements of the structure and strength of the radio emission. Bell Labs, however, rejected his request for funding on the grounds that the detected emission would not significantly affect their planned transatlantic communications system. Jansky was re-assigned to another project and did no further work in the field of astronomy.

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 = 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
Hydrogen 21-cm Emission Diagram of a ground-level hydrogen atom changing from a higher-energy state (electron and proton spins are parallel) to a lower-energy state (spins are antiparallel). The emitted photon carries away an energy equal to the energy difference between the two spin states.
21-cm Lines Typical 21.1-cm radio spectral lines observed from several different regions of interstellar space. The peaks do not all occur at a wavelength of exactly 21.1 cm, corresponding to a frequency of 1420 MHz, because the gas in the Galaxy is moving with respect to Earth.

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.
A scan of our Milky Way galaxy at 21 cm shows that the distribution of neutral hydrogen is concentrated in the spiral arms. The Sun is marked as the yellow arrow, Galactic center is a blue dot. Notice how there is a cone of avoidance behind the Galactic center due to confusion in the HI signal.
See for interesting history.
See for map from 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
The two peaks correspond to approaching and receding parts of the galaxy.
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) has ~104 dipole anttenae and no dishes. 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.)
Note that Johnson voltage noise is sqrt(4kTBR). The power is V^2/R, or 4kTB.

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

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

26. Atmospheric opacity Atmosphere
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. Don’t Let This Happen to Your Radio Telescope
300 foot radio telescope in Green Bank, WV
9:43pm, Nov. 15, 1988
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 Unblocked aperture
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 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 km;
C array km; and
D array -- 1 km.

45. VLA Receivers Receivers Available at the VLA 4 Band P Band L Band
4 Band
P Band
L Band
C Band
X Band
U Band
K Band
Q Band
Frequency (GHz)
22-24
40-50
Wavelength (cm)
400 90 20 6
3.6 2
1.3
0.7
Primary beam (arcmin)
600
150 30 9
5.4 3 1
Highest resolution (arcsec)
24.0
6.0
1.4
0.4
0.24
0.14
0.08
0.05
System Temp
,000.K K
37-75.K
44.K
34.K
110.K K K

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 Center RADIO NIR

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

50. Molecules