Transcript Astronomy makes things happen

Basic Detection Techniques
1b (2011/09/22): Single dish systems
Theory: basic properties, sky noise, system noise,
Aeff/Tsys, receiver systems, mixing, filtering, A/D
conversion
Case study: LOFAR Low Band Antenna
BDT Radio – 1b – CMV 2009/09/04
Basic Detection Techniques
Visit to Dwingeloo for APERTIF measurements
2011/09/29
13:00-15:00
NS to Beilen: 13:31-13:58
NS fm Beilen: 16:59-17:28
Transport Beilen – Dwl vv will be arranged by ASTRON
Call 0521 595119 (Diana van Dijk) in case of problems
Host is Laurens Bakker
APERTIF System Engineer)
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
h
P d 
e
h / kT
1
d
Radio frequencies (hv << kT):
Rayleigh-Jeans law (or rather: R-J approximation)
P  k T 
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
K is only related to Tant, not to Tsys
Define SEFD:
SEFD 
Tsys
K

2k Tsys
ha A
[ Jy ]
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
Example: EVLA (slides Rick Perley, jul 2010)
BDT Radio – 1b – CMV 2009/09/04
Example: EVLA (slides Rick Perley, jul 2010)
BDT Radio – 1b – CMV 2009/09/04
Example: EVLA (slides Rick Perley, jul 2010)
BDT Radio – 1b – CMV 2009/09/04
Example: MFFE
BDT Radio – 1b – CMV 2009/09/04
Example: MFFE
BDT Radio – 1b – CMV 2009/09/04
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
Wire antennas, vivaldi
BDT Radio – 1b – CMV 2009/09/04
BDT Radio – 1b – CMV 2009/09/04
BDT Radio – 1b – CMV 2009/09/04
High time resolution data (LOFAR //
Nancay Decametric Array)
Blow-up: 0.2 seconds showing
complex structure
ESERO Docentendag - CMV 2008/11/05
on/off
delaystep
on/off
delaystep
1..16
BDT Radio – 1b – CMV 2009/09/04
to receiver
BDT Radio – 1b – CMV 2009/09/04