Transcript Astronomy makes things happen

Basic Detection Techniques
1b (2009/09/04): Single pixel feeds
Theory:
Brightness function
Beam properties
Sensitivity, sky noise, system noise, Aeff/Tsys
Receiver systems, mixing, filtering
Case study: the LOFAR Low Band Antenna
Reference frequency
Bandwidth
Beampattern
BDT Radio – 1b – CMV 2009/09/04
EM waves
Directionality (RA, dec, spatial resolution)
Time (timing accuracy, time resolution)
Frequency (spectral resolution)
Flux (total intensity, polarization properties)
I 
Q 
   f (t , , l , m,  )
U 
 
V 
BDT Radio – 1b – CMV 2009/09/04
Coherent detectors
Responds to electric field ampl. of incident EM waves
Active dipole antenna
Dish + feed horn + LNA
Requires full receiver chain, up to A/D conversion
Phase is preserved
Separation of polarizations
Typically narrow band
But tunable, and with high spectral resolution
For higher frequencies: needs frequency conversion schemes
BDT Radio – 1b – CMV 2009/09/04
Sensitivity
Key question:
What’s the weakest source we can observe
Key issues:
Define brightness of the source
Define measurement process
Define limiting factors in that process
BDT Radio – 1b – CMV 2009/09/04
Brightness function
Surface brightness:
Power received /area /solid angle /bandwidth
Unit: W m-2 Hz-1 rad-2
Received power:
Power per unit bandwidth:
Power spectrum: w(v)
Total power:
Integral over visible sky and band
Visible sky: limited by aperture
Band: limited by receiver
BDT Radio – 1b – CMV 2009/09/04
Point sources, extended sources
Point source: size < resolution of telescope
Extended source: size > resolution of telescope
Continuous emission: size > field of view
Flux density:
Unit: 1 Jansky (Jy) = 10-26 W m-2 Hz-1
BDT Radio – 1b – CMV 2009/09/04
Reception pattern of an antenna
Beam solid angle (A = A/A0)
Measure of Field of View
Antenna theory: A0 Ωa = λ2
BDT Radio – 1b – CMV 2009/09/04
Black-body radiation
General: Planck’s radiation law
Radio frequencies (hf << kT):
Rayleigh-Jeans law (or rather: approximation)
B = 2kT/λ2
BDT Radio – 1b – CMV 2009/09/04
Antenna temperature, system temperature
Express noise power received by antenna in terms of
temperature of resistor needed to make it generate the
same noise power.
Spectral power:
w = kT/λ2 Aeff Ωa = kT
Observed power:
W = kT Δv
Observed flux density: S = 2kT / Aeff
Tsys = Tsky + Trec
Tsky and Tant: what’s in a name
After integration:
BDT Radio – 1b – CMV 2009/09/04
Tsky  Trec
T 
B
Sensitivity
Source power from Ta:
P  G k Ta 
Source power from flux:
P  12 Gha A S 
Antenna area A, efficiency ha
Rx accepts 1/2 radiation from
unpolarized source
Define scaling factor K
K is antenna’s gain or
“sensitivity”
unit: degree Jy-1
BDT Radio – 1b – CMV 2009/09/04
Ta ha A
K

S
2k
System Equivalent Flux Density
Covers Tant, not Tsys
Define SEFD:
SEFD 
Tsys
K

2k Tsys
ha A
What’s in Tsys?
3K background and Galactic radio emission
Atmospheric emission
Spill-over from the ground and other directions
Losses in feed and input waveguide
Receiver electronics
At times: calibration source
BDT Radio – 1b – CMV 2009/09/04
Tbg
Tsky
Tspill
Tloss
Trx
Tcal
[ Jy ]
Example: VLA and MFFE
BDT Radio – 1b – CMV 2009/09/04
Receiver chain at WSRT
west Recv.
Recv.
Equal.
Equal.
IVC
IVC

ADC
ADC
Recv. east
Recv.
Equal.
Ref. Equal.
Ref.
LO
LO

VLBI
VLBI
PuMa
PuMa
TADU
TADU

Correlator
Correlator
BDT Radio – 1b – CMV 2009/09/04
LO
LO

IVC
IVC
ADC
ADC

Horn antennas
BDT Radio – 1b – CMV 2009/09/04
Inside the MFFE
UHFlow UHFhigh 92
49
13
18/21
6
3.6
Cryogenic
sw
sw
1 GHz
1 GHz
1200-2200 MHz
LO1low
low
LO1
Synth.
Synth.
LO2
LO2
Synth.
Synth.
IF1
IF1
sw
sw
sw
sw
1 GHz
2200-9600 MHz
900 MHz
BDT Radio – 1b – CMV 2009/09/04
LO1high
high
LO1
Synth. .
Synth
2x IF out
100 ± 80 MHz
BDT Radio – 1b – CMV 2009/09/04
Wire antennas, vivaldi
BDT Radio – 1b – CMV 2009/09/04
RCU Board
A/D
Filter

Filter

RSP Board
Beamformer
Central Processing Facilities
WAN
Filter
Backplane
& RF Shield


Output
control
Distributed
Beamforming
GbE switch fabric (231 outputs)
Filter
Station GbE switch (24 ports)
GbE
RSP board 24
calibration
Sync.
Correlator /
Beamformers
(Blue Gene /L)
Storage
Buffering
Image creation
Delay
Ionosphere
Calibration
RFI Mitigation
User applications
WAN fibre
connections
Station 77
BDT Radio – 1b – CMV 2009/09/04
Archive
Export and
GRID
on/off
delaystep
on/off
delaystep
1..16
BDT Radio – 1b – CMV 2009/09/04
to receiver
BDT Radio – 1b – CMV 2009/09/04
Sampling
observation
mode I
10 - 90
observation
mode II
110 - 190
optional
observation
mode III
170 - 230
30
10
90
I : 0-80
BDT Radio – 1b – CMV 2009/09/04
Filters
160 MHz clock
Nyquist Zones
110
II: 80 - 160
I: 0 - 100
0
observation
mode IV
210 - 250
II: 100 - 200
100
200 MHz clock
Nyquist Zones
III: 160 - 240
III: 200- 300
200
300 frequency [MHz]