Radio Telescopes and Interferometry
Transcript of Radio Telescopes and Interferometry
Instrumentation Class Nov. 29, 2007
Radio Telescopes: Some Technical Aspects• Radio Telescope: - Consists of two main components - Telescope (antenna) itself with control system - Receiver plus associated detection electronics• Antenna: 1) Input impedence - a measure of how efficiently signal transformed into a wave 2) Polarization 3) Power pattern or gain function g(θ, φ) 4) Effective area Ae (θ,φ) where g(θ,φ) = 4π/λ2Ae
TA = 1/4π g(θ,φ) TB (θ,φ) dΩ = Antenna Temperature: convolution of source andantenna properties - imbed antenna in Blackbody at T TA = T/4π g(θ,φ) dΩ = T
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Instrumentation Class Nov. 29, 2007
• Two basic types of antennas 1) Single Dish: Pencil beam antennas with circular apertures 2) Arrays• Single Dish - Produces electric field of uniform phase - Equivalent to a plane wave incident on a hole (dhole = dantenna)• Consider diffraction pattern of telescope - g(θ,φ) ∝ J1
2
- gain pattern is Airy pattern - first null at 1.22λ/d: “diffraction-limited” - describes HPBW of antenna - presence of side lobes (< 2%) - mitigate by picking good illumination pattern (feed illumination from detector)
Instrumentation Class Nov. 29, 2007
• Aperture Efficiency ηA
- Response to a point source - TA = g(θ = 0) TB dΩ - Ae(0) = ηA A - maximum of gain function - ηA ~ 0.5 - a measure of surface accuracy: Ruze formula
ηA = K0 exp(-2πδ/λ) - δ = rms phase displacement Main Beam Efficiency ηB
- Percent of power in main beam vs. side lobes - look at extended source: TA = 1/4π? gTB dΩ ~ <TB> - ηB = g dΩ ~ 0.7 – 0.9
- response to an extended sourceMain beam
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Instrumentation Class Nov. 29, 2007
• Intermediate source size - convolve source distribution with gain function - convolve two Gaussians: θ R2 = θ b2 + θ s2
• Optical properties - Cassegrain systems - F = f/D ratio of primary is F ~ 0.4 -0.6• Arrays - much more complicated - no longer measure the brightness distribution - measure a visibility function V(u,v) - degraded by a phase error
Instrumentation Class Nov. 29, 2007
RX1 RX2
Nutating Subreflector
Primary Reflector
Quaternary Mirror
Central Selection Mirror(Rotatable to allow access toany one of four receiver bays)
Radio signals comeFrom sky
Signalsreflectedfrom primary
Radio TelescopeOpticsDirected to
Sub-reflector
To central selection mirrorInto a radioReceiver
Instrumentation Class Nov. 29, 2007
DewarwindowLens
Feedhorn
Coupler
Mixer
Bias
Isolator
HEMTamplifier
HETERODYNE RECEIVERS withMULTIPLEXING SPECTROMETERS• Sky signal (νsky) arrives at detector (in adewar, cooled to 4.2 K
• Local oscillator (LO) signal (νLO) is mixedwith sky signal in semiconductor chip “mixer”• Generates a signal at frequency difference(intermediate frequency), νIF ⇒ νIF = νsky ± νLO.• IF frequency detected, amplified by HEMTamplifier• IF Signal sent to the spectrometer (backend)• Not single signal but range νIF ± 0.5 GHz =νsky ± 0.5 GHz
Radio Telescope Detectors
νLO
νsky
νIFTo spectrometerbackendCOMPLEX SYSTEMS
Instrumentation Class Nov. 29, 2007
• Dewar and all components cooled to 4.2 K - SIS mixers and amplifiers• Easy to switch frequencies/retune receiver/switch bands• At ARO: rotate a few mirrors
Receivers Frequency Versatile…
One Dewar withdifferent mixers(inserts) for variousfrequency(waveguide) bandsARO SMT RXR
inserts
Instrumentation Class Nov. 29, 2007
• Multi-junction mixers: increase in dynamic range(photon-step width); wider RF/IF performance• RF and IF quadrature hybrids⇒separate sidebands within mixer: USB from LSB• 8 GHz total IF Bandwidth: 4 GHz per sideband• No moving parts; no room temperature grids (no M.P.)
ALMA Band 6 Mixers: ARO’s Newest DevelopmentMixer Block
Insert in Dewar
Mixerdesign
Instrumentation Class Nov. 29, 2007
Excellent Image Rejection• Typical rejection: > 20 db in LSB; > 15 db in USB• Tested by placing strong lines from image in signal sideband
13CO in LSB (signal sideband)
12CO image from USB
NGC 7027
12CO: J=2-1line in NGC7027: TA*~8 K;reduced to0.1K in image ⇒ 20.6 dbrejection
Instrumentation Class Nov. 29, 2007
• 4 GHz per sideband, for a total of 8 GHz• Steerable IF in 4-8 GHz range ⇒ pick USB and LSB lines (separation of 8 -16 GHz)• Useful for spectral line surveys, extragalactic lines, and multiple line studies
30 min. position-switching: 1.5 GHz BW instantaneous observation
Wide, Steerable IF Bandwidths
SgrB2(N)
Instrumentation Class Nov. 29, 2007
IF Systems• Radio Telescopes: MULTIPLEX ADVANTAGE• Simultaneously collect data over whole BW of IF Amplifier• Must have electronics to process IF signals
Frequency
steering
AOS
A,B,C
Filterbanks
Rx switch/
Total power/
Attenuators
IF System Block Diagram:SMT
Channel
steering
345Rx
490Rx
NewRx
1.5G
Rx
switch RightFlange
Rx
BEswitch
1.5->5G
Converter 5G
Rx
switch
Right Rx roomLeft Rx room
Computer room
Instrumentation Class Nov. 29, 2007
Spectrometer “Backends”: Another Important Aspect
• Filter banks: Complex set of capacitors, filters, resistors, etc.• Acousto-optic systems• Correlators: digital devices
Backend separates out signalas a function of frequency⇒ A spectrum is created… ν = 178.323 MHz
Filter Banks at the SMT
Instrumentation Class Nov. 29, 2007
Filterbank Racks – Front View: SMT
Power supply
IF processor 0
PC
Digital crate
1 MHzFilter crate 0
Power supply
IF processor 1
LCD monitor
Power supply
IF processor 2
Chirp Spec.
Noise source
250KhzFilter crate 4
Clock distribution amp
Keyboard
1 MHzFilter crate 2
1 MHzFilter crate 1
1 MHzFilter crate 3
Instrumentation Class Nov. 29, 2007
Filter Crate Block Diagram (one half shown)
Second
Converter
Third
Converter
Fourth
Converter
MuxcardFilter
Card
Fourth
Converter
FilterCard
FilterCard
FilterCard
Third
Converter
Fourth
Converter
FilterCard
Fourth
Converter
FilterCard
FilterCard
FilterCard
Instrumentation Class Nov. 29, 2007
LPF
Second Converter Card Block Diagram(one channel shown)
VCOPLL PS
PS
PS
LO Mon
IF in IF out
to other channel
PS = power splitter
Instrumentation Class Nov. 29, 2007
Mux
Filter Card Block Diagram(one channel)
BPF
ZeroDAC
Square law detector
Integrator
Instrumentation Class Nov. 29, 2007
Telescope Control Systems• Both ARO telescopes use NRAO 12m control system• Fast data acquisition/ processing• Distributed nature of system⇒ Each task controlled by separate computers ⇒ Computer for telescope tracking, focus position, each backend, monitor systems, etc.⇒ Efficient, synchronous operation• Also includes On-the-Fly (OTF) mapping• Remote Observing ⇒ Trained operators at site ⇒ Routinely used by ASIAA
SMT Control System
Instrumentation Class Nov. 29, 2007
One sub-system:“Tracker-Servo”
at SMT
New computers for control system: SMT
Instrumentation Class Nov. 29, 2007
• Before you take spectra… calibrate the position of telescope: POINTING• Also position of optics (sub-reflector): FOCUSING• Pick a strong radio source to do this…
Pointing and Focusing
“CatalogTool”at ARO
Instrumentation Class Nov. 29, 2007
Pointing scan or continuum 5-point: done on planet Jupiter
Establish pointing constants in az and elv
Instrumentation Class Nov. 29, 2007
FOCUSscan onJupiter
Determine optimal position of sub-reflector
PointingandFocusingreadilydone withone dish
Instrumentation Class Nov. 29, 2007
•Background noise subtracted out with a switching technique (single-dish) 1) Position switching⇒ Switch telescope position between the source and blank sky (“off position”: 10-30 arcmin away in azimuth)⇒ Subtract “ON – OFF/OFF” to remove background⇒ Calibrate the intensity scale (voltage) by measuring voltage on sky (Tsky) and on ambient load (Tamb) ⇒ Called a “Cal scan” :Tscale=TA
*( in K) 2) Beam-switching ⇒ Nutate sub-reflector to get ON/OFF positions 3) Frequency switching ⇒ Change frequency of LO ± 1-2 MHz
Signal Collection & Processing
Molecularcloud
Blanksky
ON-OFF/OFF and calibrationall done instantly in software
Instrumentation Class Nov. 29, 2007
Sensitivity Limits:
Trms =2Tsys
ηspec ∆νt int
• Tsys = total system temperature• For a noise level of 0.5 mK, signalaverage for ~100 hours (Tsys ~ 300 K)• Requires telescope systems to be verystable over long periods of time ⇒ can be accomplished with single dish
rms = 2mk at 12+ hrs
rms = 1 mK at 25 hrs
rms = 0.5 mK at 100 hrs
Extensive Signal-Averaging • Collect data over 5-6 min as a single “scan”• Written to computer disk• Average many scans for weak lines, high S/N
Radiometer Equation
Instrumentation Class Nov. 29, 2007
Spectrum after 15 hoursTrms = 0.0014 KMOSTLY NOISE
Spectrum after 30 hoursrms = 0.0010 KMAYBE A LINE ???
Spectrum after 60 hoursrms = 0.0007 KLINES APPEAR
• Searching for KCN: new molecule• J(Ka,Kc) = 16(0,16) → 15(0,15) at 150.0433 GHz
Signal Averaging: An IllustrationIRC+10216
KCN UU
Instrumentation Class Nov. 29, 2007
Dual Polarization: YOUR DATA ADVANTAGE
J=2-1 line of HCO+ near 178 GHzOrthogonallinearpolarizationsfor receivers:Twoindependentmeasurementsof the spectra
Then averagetwo spectratogether forincreased S/N
Instrumentation Class Nov. 29, 2007
• Data obtained immediately calibrated with background subtracted• No further reduction needed (only cosmetic: baseline subtraction, “bad channels”, etc)• Look at data and ON-LINE decisions⇒Very interactive observing• Observer makes decisions in real time⇒ Change frequency⇒ Change source⇒ Change program: different rxr• Optimize data return• Flexibility for new discoveries • And its FUN !
Interactive Nature of Single-Dish Observing
Fun at the telescope
Instrumentation Class Nov. 29, 2007
0.6
0.4
0.2
0.0
T R* (K
)
226800226600226400Frequency (MHz)
J = 1.5 - 1.5hf components
J = 1.5 - 0.5hf components
J = 2.5 - 1.5hf components
G 1 9 . 6 G i a n t M o l e c u l a r C l o u d C N R a d i c a l (N = 2 - 1)
C N
N =2-1 rotational transition: 15 hyperfine components Identification of CN• Optical depth in lines• Molecular Abundance
FINAL PRODUCT: SPECTRA• Heterodyne receivers/multiplexing spectrometers • Obtain spectral resolution of 1part in 108 -109
• ?/∆? ~ 106 -107
Instrumentation Class Nov. 29, 2007
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
T R* (K
)
232000231800231600231400231200231000Frequency (MHz)
13C
S
OC
S
C2H
3CN
C2H
3CN
C2H
3CN
CH
3CH
O +
C2H
3CN
CH
3CH
O +
C2H
5CN
C2H
3CN
C2H
3CN
HC
OO
CH
3 + C
2H3C
N
C
2H3C
N
C2H
5CN
C2H
5CN
C2H
5CN
C2H
5CN
HC
OO
CH
3
HC
OO
CH
3
HC
OO
CH
3
HC
OO
CH
3
HC
OO
CH
3
CH
3CH
O
C2H
3CN
+ C
2H5C
N
C2H
5OH
C2H
5OH
+ (C
H2O
H) 2
CH
3CH
O
C2H
5OH
CH
3NH
2
HN
CO
CH
3CH
O
CH
3CH
O
HC
OO
H
C2H
5OH
(CH
3)2O
(CH
3)2O
NH
2CH
O
37 Indentified Features35 Unidentified Features~6 lines per 100 km/sTRMS = 0.003 K (theoretical)
U
U U
U U
U
U
U
U
37 Identified Features35 Unidentified FeaturesAt Confusion Limit !
SgrB2(N) 231 GHz