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ASTR 3520ASTR 3520Observations & Instrumentation II:Observations & Instrumentation II: SpectroscopySpectroscopy
Intro to Radio Astronomy
Intro to Radio Astronomy
• Concepts - Amplifiers - Mixers (down-conversion) - Principles of Radar - Radio Astronomy basics: System temperature, Receiver temperature Brightness temperature, The beam ( = / D) [ its usually BIG] Interferometry (c.f. the Very Large Array – VLA) Aperture synthesis
Detection of EM Radiation Across the Spectrum
-ray X-ray UV Visible NIR TIR sub-mm mm cm radio
>MeV 0.2 – 100 keV 4000 – 10,000A 3 – 200 m 1 – 10 mm >1 cm 100 – 4000 A 1 – 3 m 200 m – 1000m
Incoherent detection Coherent detection - Particle properties - Wave properties (photons) - Energy, arrival time - Amplitude, phase - quantum noise: Trec > h / 2
Transmission
History of Radio Astronomy (the second window on the Universe)
• 1929 - Karl Jansky (Bell Telephone Labs)• 1030s - Grote Reber• 1940s - WWII, radar - 21 cm (Jan Oort etc.)• 1950s - Early single dish & interferometry - `radio stars’, first map of Milky Way - Cambridge surveys (3C etc)• 1960s - quasars, pulsars, CMB, radar, VLBI aperture synthesis, molecules, masers (cm)• 1970s - CO, molecular clouds, astro-chemistry (mm)• 1980s /90s – CMB anisotropy, (sub-mm)
Karl Jansky, Holmdel NJ, 1929
300 m dish at Arecibo:300 m dish at Arecibo: / D ~ 48” at = 6 cm (5 GHz)
408 MHz radio sky408 MHz radio sky
Outline• A Simple Heterodyne Receiver System
– mixers and amplification• Observing in the Radio
– resolution– brightness temperature
• Radio Interferometry• Aperture synthesis
Mixers
signal inLO
local oscillator
signal out and
A mixer takes two inputs: the signal and a local oscillator (LO).
The mixer outputs the sum and difference frequencies.
In radio astronomy, we usually filter out the high frequency (sum) component.
Mixers
frequency
sign
al LO
originalsignal
mixedsignal
0 Hz
Mixers
frequency
sign
al LO
originalsignal
mixedsignal
The negative frequencies in the difference appear the same as a positive frequency.
To avoid this, we can use “Single Sideband Mixers” (SSBs) which eliminate the negative frequency components.
0 Hz
W-band (94 GHz,4 mm) amplifier
Local oscillatorDownconverted signal
Frequency
Single sideband mixer:
Band-pass of amplifier: Intermediate frequency = IF
Local oscillator
Amplifier passband
Frequency
Lower sideband Upper sideband
FLOFIF
Double sideband mixer:
Local oscillator
Amplifier passband
Frequency
Lower sideband Upper sideband
FLOFIF
ASTR 3520ASTR 3520Intro to Radio Aastronomy:Intro to Radio Aastronomy:
• Review heterodyne & mixing Radar examples: cars, Venus (review)• Single dish spectroscopy Orion nebula CO example• Radiometer equation (noise vs, exposure time)• Interferometer basics The U, V plane => Dirty beams, Fourier Inversion => Dirty maps De-convolve Dirty Beam => CLEAN maps • The VLA, VLBA, and ALMA
Freflect = f trans + / - f f = 2 f (V/c) f = 10 GHz V = 100 km/h = 27.8 m /s c = 3 x 1010 cm/s = 3 x 105 km/s f = 1850 Hz
Radar:
f = 1850 Hz
f trans
Freflect = f trans + / - f
Mixing: Adding waves together
Local oscillatorDownconverted signal
Frequency
Single sideband mixermixer:
f = 10 GHz
F + f = 10 GHz + 1850 Hz
1850 Hz f = fIF
Band-pass of amplifier: Intermediate frequency = IF
Planetary Radar imaging: Doppler shift + time delay = 2D map
Radar PulseEarly echo
Late echo
BlueshiftRedshift
VenusVenusUV
Radar
Radar
VenusVenusRadar
A Simple Heterodyne Receiver
low noiseamplifier
filter
receiver horn
LO
tunablefilter
signal @ 1420 MHz
1570 MHz
1420 MHz
tunableLO
~150 MHz
Analog-to-DigitalConverter
Computer
+ +
outputs a power spectrum
150 MHz
Amplification• Why is having a low noise first amp so
important?– the noise in the first amp gets amplified by all
subsequent amps– you want to amplify the signal before subsequent
electronics add noise• Amplification is in units of deciBells (dB)
– logarithmic scale• 3 dB = x2• 5 dB = x3• 10 dB = x10• 20 dB = x100• 30 dB = x1000
)(log10dB2
110 VV
Observing in the Radio I
• We get frequency and phase information, but not position on the sky– 2D detector
• A CCD is also a 2D detector (we get x & y position)
Observing in the Radio II:Typical Beamsize (Resolution)
• i.e. The BURAO 21 cm horn (D ~ 1 m)
o1210021
≈cc
Observing in the Radio II• i.e. The NRAO GBT (D ~ 100 m)
'2.710000
21≈
cc
''2.6'10.010000
3.0≈
cc
at 21cm = 1.420 GHz
at 0.3 cm = 100 GHz
Observing in the Radio II• i.e. The Arecibo Telescope (D ~ 300 m)
'4.230000
21≈
cc
''1.230000
3.0≈
cc
at 21cm = 1.420 GHz
at 0.3 cm = 100 GHz
300 m dish at Arecibo300 m dish at Arecibo
Transmission (and brightness of the Atmosphere) depends on H2O!
Observing in the Radio III:Brightness Temperature
Flux: erg s-1 sr-1 cm-2 Hz-1 (1023 Jy)B(T): erg s-1 sr-1 cm-2 Hz-1 (1023 Jy)
We can use temperature as a proxy for flux (Jy)
Conveniently, most radio signals have h/kT << 1, so we can use the Raleigh-Jeans approximation
B(T) = 2kT/2
Thus, flux is linear with temperature
Antenna TemperatureTB = F2/2k
• Brightness temperature (TB) gives the surface temperature of the source (if it’s a thermal spectrum)
• Antenna Temperature (TA):– if the antenna beam is larger than the source, it will
see the source and some sky background, in which case TA is less than TB
TA ~ TB s/b
System Temperature
• Noise in the system is characterized by the system temperature (Tsys)– i.e. you want your system temperature
(especially in the first amp) to be low
Radiometer Equation• Trms = Tsys / ( t) ½
– Trms = r.m.s. noise in observation– a ~ (2)1/2 since you have to switch off-source = position switch
off-frequency = frequency switch
– Tsys = System temperature– = bandwidth, frequency range observed– t = integration time (how long is the exposure?)
Spectral Resolution• The spectral resolution in a radio
telescope can be limited by several issues:– integration time (signal-to-noise)– filter bank resolution (if you’re using a filter
bank to generate a power spectrum in hardware)
•Single Dish line & continuum basics • = / D ~ arc minutes 10 m @ 1 cm => 250”
• Brightness temperature: B(T) => I T
• Radiometer equation: TRMS = Tsys / (
Ex: = 100 GHz, = 1 MHz (= 3 km/s), = 1 sec, Tsys = 1000 K TRMS = 1 (Kelvin)
Orion B
Orion A
Orion Nebula
Orion MolecularClouds
13CO 2.6 mm
350 m
13CO J=6-5 661 GHz
(Wilson et al. – in prep)
12CO J=9-8(discovery)
12CO J=9-8
OMC1OMC1-S(Kawamura et al. 2002)
THz spectra!
Milky Way all-sky: Visual wavelengths
Milky Way all-sky: Infrared wavelengths (COBE)
408 MHz radio sky408 MHz radio sky
21 cm HI all-sky map21 cm HI all-sky map
Dame et al. CO map of Milky WayDame et al. CO map of Milky Way
Dame et al. CO map of Milky WayDame et al. CO map of Milky Way
R Cor Aust.
-Ophiucus
Galactic Center
Multi-transitionAnalysis ofOutflow massspectra
Multi-transition analysis of outflow mass spectra
L1551 J=2-1 CO
[SII] with COL1551
Radio Interferometry
+
East
posit
ional
phas
e dela
yto
sourc
e
Two Dish Interferometry• The fringe pattern as a function of time
gives the East-West (RA) position of the object
• Also think of the interferometer as painting a fringe pattern on the sky– the source moves through this pattern,
changing the amplitude as it goes
Extended Sources
+ -+ - -+ + • Spacing of the fringes is a function of the projected baseline
• The area under the fringes determines the amplitude of the signal (positive fringes add, negative fringes subtract)
• The projected baseline changes as the source rises or sets
Extended Sources
The narrow fringes (not visible here), represent the positional delay.
The broad envelope is the self-interference of the extended source.
A four hour observation of the Sun at 12 GHz using a two dish E-W interferometer.
Extended Sources• In order to determine the extended object’s
shape, we must disentangle the fringes due to the projected baseline (which we’d get from a point source) from the interference of the different parts of the source– to do this, we use delay lines– we introduce a delay between the two antennas
to compensate for the the positional delay– this leaves only the fringes from the structure of
the source
Aperture Synthesis• A two dish interferometer only gives
information on the E-W (RA) structure of a source
• To get 2D information, we want to use several dishes spread out over two dimensions on the ground
Radio Telescope ArraysThe VLA:An array of 27 antennas with 25 meter apertures
maximum baseline: 36 km
75 Mhz to 43 GHz
Very Large Array radio telescope (near Socorro NM)Very Large Array radio telescope (near Socorro NM)
The VLAThe VLA
An amplifier
The U-V Plane• Think of an array as a partially filled aperture
– the point source function (PSF) will be have complicated structure (not an airy disk)
– the U-V plane shows what part of the aperture is filled by a telescope
– this changes with time as the object rises and sets
– a long exposure will have a better PSF because there is better U-V plane coverage (closer to a filled aperture)
The U-V plane
a snapshot of the U-V plane(VLBA)
U-V coverage in a horizon to horizon exposure
Point Spread FunctionThe dirty beam : the diffraction pattern of the array
Fourier plane samplingUV plane covered over 6 hrs Amplitude of fringes on a source
Examples of weightingDirty Beams:A snapshot (few min) Full 10 hrs VLA+VLBA+GBT
Image Deconvolution• Interferometers have nasty PSFs• To get a good image we “deconvolve” the
image with the PSF– we know the PSF from the UV plane coverage– computer programs take a PSF pattern in the
image and replace it with a point– the image becomes a collection of point
sources
UV Plane Coverage and PSF
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
UV Plane Coverage and PSF
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
Image Deconvolution
images from a presentation by Tim Cornwell (given at NRAO SISS 2002)
The Orion Nebula (at 1.4 GHz)
SNR Cassiopeia AContinuum in 3 colors:1.4 GHz (L band)5.0 GHz (C band)8.4 GHz (X band)
Jupiter andits magnetosphere
W50 SNR: SS 433 (accreting neutron star + 0.27c jets)
21 cm HI map21 cm HI mapof M33of M33
Centaurus A (the nearest radio galaxy)
PH 227PH 227
Radio galaxyand jets
Radio galaxyRadio galaxy3C2863C286
VLBAVLBA
Radio Telescope ArraysALMA:An array of 64 antennas with 12 meter apertures
maximum baseline: 10 km
35 GHz to 850 GHz