Advanced Topic in Astrophysics, Lecture 1
Radio Astronomy - Antennas
Phil Diamond, University of Manchester
Course Structure
•Modules
– Module 1, lectures 1-6 (Phil Diamond, Maria Rioja)
• Mon March 1, 8, 15: 3 double lectures
• Radio astronomy: antennas, interferometry and VLBI
– Module 2, lectures 7-12 (J-P Macquart)
• Radiation Mechanisms I: 3 double lectures, dates TBD
– Module 3, lectures 13-18 (J-P Macquart)
• Radiation Mechanisms II: 3 double lectures, dates TBD
– Module 4, lectures 19-24 (Ken Freeman)
• Dynamics of the Milky Way
• Dates TBD
Resources
•ATNF Synthesis Imaging workshops 2003, 2006
– http://www.atnf.csiro.au/whats_on/workshops/synthesis2006/prog.h
tml
•Texts
–Radio Astronomy: Kraus
–Tools of Radio Astronomy: Rohlfs & Wilson
–Interferometry and Synthesis in Radio Astronomy,
Thompson, Moran & Swenson.
–Very Long Baseline Interferometry and the VLBA: Napier,
Diamond & Zensus., ASP Conf series, Vol 82, 1995.
Outline
•Imaging
–Resolution
–Direct and indirect imaging
•Telescopes
–Parabolic and hyperbolic
•The Antenna
–Horns and receivers
•Radio astronomy fundamentals and tools of the trade
–Brightness temperature
–Flux density
–Radiometer equation
Direct imaging onto a focal plane
Resolution Δθ
2’’
2”
1’
10’
Diffraction limits
Δθ=1.22 λ/D
Δθ=1”
λ
D
Optical
500 nm
125 mm
Radio
20 cm
50 km
Direct and indirect imaging
•Direct imaging
–Normal imaging method whereby an image is projected onto
a detector.
–Examples: camera, telescope, single-dish radio telescope
•Indirect imaging
–Used where we cannot form a direct map of the object on the
focal plane.
–We infer the properties of the object from certain
characteristics of the received electromagnetic field.
–Examples: interferometry, NMR, ultrasound, PET, speckle.
Direct Imaging: Single dish radio
telescope
Parabolic reflector:
A parabolic reflector adds all the fields from a surface
(aperture) at a single focal point.
Direct Imaging - Angular Resolution
•Angular Resolution (radians)
θ=
D
λ
D
•Wavelength, λ=0.21m, D=64m
–⇒θ=0.003 rad, or 11 arcmin
•Such a radio telescope only matches the resolution of the
human eye at its shortest wavelength 1-2cm.
A radio telescope at Parkes, NSW
(prime focus)
Hyperbolic reflector
.
.
A
B
Light from the point A reflecting off the hyperbola
appears to come from point B
Parabolic
reflector
Prime Focus
Hyperbolic
reflector
Parabolic
reflector
Classical Cassegrain
AT antenna:
•Cassegrain configuration
•22-m diameter primary
main reflector
•2.75-m secondary reflector
Reflector types
Prime Focus
Cassegrain Focus
Offset Cassegrain
Naysmith
Beam Waveguide
Dual offset
Reflector types
Prime Focus
e.g. GMRT
Cassegrain Focus
e.g. Mopra (AT),
e-MERLIN
Offset Cassegrain
e.g. VLA and
ALMA
Naysmith
e.g. OVRO
Beam Waveguide
e.g. NRO
Dual offset
e.g. ATA
Signal path:
Signal path:
The antenna collects the E-field over the aperture at the focus
The focus is a fixed spot at all frequencies.
The reflector antenna is achromatic.
EM-wave electric field oscillations
induce voltage oscillations at the
antenna focus, in a device called a feed.
Feeds are ‘compact’ and ‘corrugated’ horns
The inner profile is curved
Cross-section
of a horn
The inner surface has grooves in
order to increase the surface
impedance, so that the wave does
not set up voltages in the surface
material, but is channelled into a
dipole at the end of the horn.
Brightness Temperature
Radio photons are pretty wimpy:
Radio photons are too wimpy to do very much - we cannot detect
individual photons
e.g. optical photons of 600 nanometre => 2 eV or 20000 Kelvin
(hv/kT)
e.g. radio photons of 1 metre => 0.000001 eV or 0.012 Kelvin
Photon counting in the radio is not an option, must think classically in
terms of electric fields, i.e. the best we can do is for the incoming
EM-wave electric field oscillations to induce voltage oscillations in a
conductor.
Brightness Temperature
Solid angle of
source, Ω
Radio photons are pretty wimpy:
Rayleigh-Jeans Law:
Iν α ν2
1 Jansky (Jy) = 10-26 W m-2 Hz-1
•In radio astronomy T is known as the brightness temperature.
•Radiation mechanisms in radio astronomy are often non-thermal, but
this does not stop astronomers talking about the “brightness
temperature” of a source: i.e. the equivalent or effective temperature that
a blackbody would need to have in order to be that bright!
Brightness Temperature
Examples of brightness temperatures:
“Blank” sky ~ 2.73 K (big bang BB radiation)
Sun at 300 MHz, 500000 K
Orion Nebula at 300 GHz ~ 10-100 K (“warm”
molecular clouds)
Quasars at 5 GHz ~ 1012 K (synchrotron)
Brightness temperature
Orion H2O super-maser: Tb > 1018 K
• However, the giant pulses observed from the Crab
Pulsar have Tb ~ 1034K
Antenna Performance
The antenna aperture efficiency
= Power collected by feed
Power incident on antenna
There are many different potential loss factors:
~ 0.4
<==
Surface Aperture
Feed
Misc. - other
Feed
efficiency blockage spillover illumination minor losses
~ 0.8
efficiency efficiency efficiency ~ e.g feed mis~ 0.8
~ 0.8
match.
0.8
Spillover past the sub-reflector (ray A)
Feed
Blockage of antenna surface by sub-reflector
Spillover past main reflector (ray C)
Feed does not illuminate all of
antenna surface equally
Antenna Temperature and Gain
Solid angle of
antenna pattern
ΩA
• Antenna temperature is same as brightness temperature only if the
source fills the antenna beam, e.g.
Ta ~ Tb x beam filling factor
•Examples
– Parkes 64-m antenna (η=60%): G=0.7 K Jy-1
– SKA 1km2: G=362 K Jy-1
Antenna surface efficiency
According to the Ruze (1966) formula, the surface efficiency of a paraboloid is well
described by:
where sigma is the r.m.s. error in the surface of the antenna.
Or re-arranging:
e.g. For a surface efficiency of 0.7 (typical target value), the
required surface error (r.m.s.) is ~ lambda/20.
==> at 7 mm (43 GHz) the surface accuracy must
be ~ 350 micron.
==> many different forces acting on an antenna and its
surface...
Appreciating the scale of large radio
telescopes....
How can we possibly achieve a 350 µm accuracy (the thickness of three human
hairs) – over a 100 metre diameter surface – an area equal to 2 football fields!
==> “active surface”.
GBT Surface has 2004 panels
average panel rms: 68µm.
2209 precision actuators are
located under each set of surface
panel corners
Actuator Control Room (left):
26000 control and supply
wires terminate in this room!
How big can parabolic telescopes be?
As the size (diameter) of a radio telescope increases, the gravitational and wind loads on
the structure become difficult to manage. The worst problem is the problem of surviving
a gale-force wind. The degree of wind distortion between paraboloids of different
diameters (D) scales as D3.
The cost of antennas also scales roughly as D3.
Telescopes like the Jodrell Bank Mark V
(right) with a diameter of ~ 305 metres
(1970), will probably always remain in
model form!
‘System’ Temperature, Tsys
•System Temperature is the equivalent noise power produced
in the antenna receiver from sources other than the object
being observed.
•It is the same noise that would be produced by a perfect
resistor in a heat bath of temperature Tsys.
•
Tsys=Trx+Tspill+Tatm+Tcmb+Tgal+…
–Trx, receiver: internal noise in electronics (4-20K at low
frequency)
–Tspill, spillover: reflected ground emission (few % of 300K)
–Tatm, atmosphere: frequency dependent emission (2-300K)
–Tcmb, cosmic microwave background (3K)
–Tgal, Galactic background (10K at 1 GHz, Tgal ∝ ν -2.7)
Temperature sensitivity
•Radiometer equation
T
ΔT =
τΔν
ΔT = Temperature sensitivity (K); T = System temperature (K);
τ= Integration time (s); Δν = Bandwidth (Hz)
•T=20K, τ=7200 sec, Δν=200 MHz ⇒ ΔT=50μK !
•System temperature describes the signal a telescope ‘sees’ if looking
at a blackbody of temperature T which fills the field of view. NOT
physical temperature.
p.32
Other parts of telescope system
Radio Telescope Block Diagram
Radio Source
Receiver
Antenna
Heterodyne system with LNAs,
mixers etc
Frequency
Conversion
Signal
Processing
Signal
Detection
Computer
Post-detection
Processing
Bolometers, e.g. SCUBA-2
Other Antenna types
Yagi
Dipoles
Log-periodic
Helix
Some less conventional (weird!)reflector types
The Jodrell Bank Mark 2 telescope (1964). Was
considered to be a prototype of the then planned
giant 300-metre MkIV. The aperture is elliptical the idea was that a 300-metre would require an
elliptical surface in order to reduce the height of the
structure off the ground.
The off-axis cylinder radio
telescope at Ooty. India (1970)
The big-ear antenna built by Kraus:
Other similar examples: Ratan 600 - Russia
Nancay (France):
Cross antennas
Instead of building an entire parabola, Cross
Antennas employ only a narrow section of a
parabola, we get a beam narrow in the antenna’s
wide direction, and broad in the other direction: .
By observing a source with two orthogonal
beams, we can get a 2-d image of the sky.
The cross antenna response is similar to
the crossing point of the two beams (but
with very high side-lobes):
The first Cross
telescope - was built
by Bernie Mills in
Australia.
The Mills cross:
Molonglo cross
Northern cross (Bologna)
The 305-m Arecibo telescope is fixed in the ground. A spherical
reflector is therefore employed:
Northern cross (Bologna)
While a parabola has a single focus point, a spherical reflector focus
the incoming radio waves on a line:
Northern cross (Bologna)
By having a moving secondary a spherical reflector can be pointed in different (but still
somewhat limited) directions on the sky.
Note that only part of the total surface area is useable for any given direction.
Part of the primary:
Northern cross (Bologna)
Looking on to the surface: quite a lot of litter!
Northern cross (Bologna)
While a parabola has a single focus point, a spherical reflector focus the incoming radio
waves on a line:
Northern cross (Bologna)
By having a moving secondary a spherical reflector can be pointed in different (but still
somewhat limited) directions on the sky.
Note that only part of the total surface area is useable for any given direction.
Arecibo is built in a karst depression. The Gregorian secondary hangs
on cables that are supported by 3 large towers:
Northern cross (Bologna)
Horn antennas:
The reflecting “ear” reflects the incoming radio
waves towards a horn or bare dipole.
The Horn Antenna combines several ideal
characteristics: it is extremely broad-band, has
calculable aperture efficiency, and the sidelobes are
so minimal that scarcely any thermal energy is
picked up from the ground. Consequently it is an
ideal radio telescope for accurate measurements of
low levels of weak background radiation.
A very famous example is the horn antenna located at Bell Telephone Laboratories in Holmdel,
New Jersey, used by Penzias and Wilson to detect the relic radiation of the big bang.
Horn antennas have many practical applications - they are used in short-range radar systems, e.g.
the hand-held radar “guns” used by policemen to measure the speeds of approaching or retreating
vehicles.
The Bell Telephone Laboratories horn in
Holmdel, New Jersey. Note the rotation
axis that permits the horn to be directed
at different points in the sky.
Things I have not touched upon
• Array receivers, focalASKAP
plane phased arrays, aperture
arrays
chequer
board
servo
control,
array
• Antenna pointing,
holography, receiver
stability, radomes, dielectric lenses, frequency
conversion, time and frequency standards, ADCs, signal
processing, data transmission
• Post-processing: pulsar searches, pulsar timing,
spectroscopy, continuum, single-dish imaging
• RFI, spectrum management, radio quiet zones
• + …..
Things I have not touched upon
• Array receivers, focalASKAP
plane phased arrays, aperture
arrays
chequer
board
servo
control,
array
• Antenna pointing,
holography, receiver
stability, radomes, dielectric lenses, frequency
conversion, time and frequency standards, ADCs, signal
processing, data transmission
• Post-processing: pulsar searches, pulsar timing,
spectroscopy, continuum, single-dish imaging
• RFI, spectrum management, radio quiet zones
• + …..