Transcript Radio Astronomy - Center for Detectors

Astronomical Observational Techniques
and Instrumentation
Professor Don Figer
Radio Astronomy
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Aims of Lecture
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review radio imaging chain
describe radio sources
describe radio detection
describe some common radio telescopes
give examples of radio objects
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Radio Imaging Chain
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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
1 Jy  10- 26 W m- 2 Hz -1  10- 23 ergs s 1 cm- 2 Hz -1.
• A strong source has an intensity of a few Jy.
• A very weak source has an intenstiy of a few mJy.
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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
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Basic Radio Telescope
Kraus, 1966. Fig.1-6, p. 14.
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Radio Interferometry
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Resolution of Single Dish and Interferometer
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Resolution of VLBI
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Resolution Comparisons
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Radio Sources
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Spectral Index
• The spectral energy distribution at radio wavelengths is often
described by the “spectral index,” alpha.
F    .
• Thermal emission (blackbody) is described by the RayleighJeans tail of the Planck function, so alpha~2.
F 
2kT 2 2kT
  2 .
2
c

• For non-thermal radiation, alpha<0.
• For a stellar wind from a hot star, alpha~0.6 (see Wright &
Barlow, 1975, ApJ, 170, 41).
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Blackbody Sources
• Peak in cm-wave radio requires very low temperature:
maxT = 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
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Continuum Sources
• Due to accelerating electrons:
– Synchrotron radiation
– Bremsstrahlung (free-free)
• Quasars, Active Galactic Nuclei, Pulsars, Supernova
Remnants, HII regions, etc.
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Spectral Line Sources
• Neutral hydrogen (H I) spin-flip transition
• Recombination lines (between high-lying atomic states)
• Molecular lines (CO, OH, etc.)
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21-cm Radiation
• due to electron spin flip
• seen in emission and absorption
• useful for tracing spiral arms
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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.
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21-cm Radiation: Rotation Curve
• H I spectral line from a galaxy shifted by expansion of
universe (“recession velocity”) and broadened by rotation
Frequency
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Radio Recombination Lines
• These transitions are all “hydrogen-like” in that the upper-state
electron “sees” a nucleus with almost one positive charge.
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Radio Detection
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Radio Telescope Components
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Reflector(s)
Feed horn(s)
Low-noise amplifier
Filter
Downconverter
IF Amplifier
Spectrometer
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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.
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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.
2
V
Power  IV 

4R

4kTBR
4R

2
 kTB.
• Power WA detected by an antenna due to a source of flux
density S
SAe
WA  Ae SB
TA 
.
2k
where antenna temperature is TA , and effective aperture is Ae.
(Factor of one-half because detector is only sensitive to one
polarization.)
1
2
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System Temperature
= total noise power detected, a result of many contributions
a
Tsys  Tant  Trcvr  Tatm (1  e )  Tspill  TCMB     
Thermal noise T
= minimum detectable signal
T  k1
Tsys
  tint
For GBT spectroscopy
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Smoothing by the beam
Kraus, 1966. Fig. 3-6. p. 70; Fig. 3-5, p. 69.
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Atmospheric opacity
• The amount of absorbed radiation depends upon the number of absorbers
along the line of sight
0
Atmosphere
=1.4*0
I   I 0 , e   I 0 , e  0 sec z ,
where  is the optical depth and z is the angle from zenith.
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Beam & sidelobes
• Essentially diffraction pattern of telescope functioning as
transmitter
• Uniformly illuminated circular aperture: central beam &
sidelobe rings
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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.
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Interferometry and Aperture Synthesis
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VLA
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Constructieve Interference
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Interference Pattern
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Aperture Synthesis
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Baselines
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Optical Interferometry?
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Examples of Radio Telescopes
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Don’t Let This Happen to Your Radio Telescope
300 foot radio telescope in Green Bank, WV
9:42pm, Nov. 15, 1988
9:43pm, Nov. 15, 1988
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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.
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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
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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
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Subreflector and
receiver room
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GBT Receiver Turret
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Very Large Array (VLA)
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VLA Main Features
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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:
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A array, with a maximum antenna separation of 36 km;
B array -- 10 km;
C array -- 3.6 km; and
D array -- 1 km.
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VLA Receivers
Receivers Available at the VLA
4 Band
P Band
L Band
C Band
X Band
U Band
K Band
Q Band
Frequency (GHz)
0.073-0.0745
0.30-0.34
1.34-1.73
4.5-5.0
8.0-8.8
14.4-15.4
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
2
1
Highest resolution (arcsec)
24.0
6.0
1.4
0.4
0.24
0.14
0.08
0.05
1000-10,000.K
150-180.K
37-75.K
44.K
34.K
110.K
50-190.K
90-140.K
System Temp
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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)
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Objects in Radio
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Sagittarius A: Mystery Mass in Galaxy Center
RADIO
NIR
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Virgo A (M87): Hidden Massive Black Hole shooting out a Jet
RADIO
OPTICAL
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Molecules
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