bojan nikolic - university of cambridge
TRANSCRIPT
Introductory talk
Bojan Nikolic
Cavendish Laboratory/Kavli Institute
February 2010Kavli Institute, Cambridge
B. Nikolic (University of Cambridge) Introductory talk February 2010 1 / 34
Introduction
Outline
1 Introduction
2 Out-of-focus holography
3 Phase correction for ALMA
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Introduction
Introduction
Observer:
Source:
Wavefront:
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Introduction
Introduction
Observer:
Source:
Corrupted
Wavefront
B. Nikolic (University of Cambridge) Introductory talk February 2010 3 / 34
Introduction
Causes of wavefront errors
Observer:
Source:
Corrupted
Wavefront
Some of the causes of wavefronterrors:
Interstellar mediumEarth’s Ionosphere (primarilyat low frequencies)Earth’s Troposphere(primarily at highfrequencies)Errors in telescope optics(almost always, because wetry to do thingscost-effectively)
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Introduction
Effect of wavefront errors
Perfect Good Noise
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Introduction
Requirements for wavefront accuracy
In single dish radio astronomy “Ruze law”:
Efficiency ∝ exp
[−(
4πσ
λ
)2]
(1)
σ: Root-mean-square wavefront errorλ: Observing wavelength
σ/λ
εeff
λ
10λ
20λ
30
1
12
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Introduction
When do we “add the vectors”?
Observer:
Source:
Corrupted
Wavefront
Perfect Good Noise
Choices:Before detection: single dish telescopesAfter (coherent) detection: aperture synthesis arrays
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Introduction
The Green Bank Telescope
B. Nikolic (University of Cambridge) Introductory talk February 2010 8 / 34
Introduction
ALMA artists impression
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Introduction
ALMA current status
B. Nikolic (University of Cambridge) Introductory talk February 2010 9 / 34
Out-of-focus holography
Outline
1 Introduction
2 Out-of-focus holography
3 Phase correction for ALMA
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Out-of-focus holography
The Green Bank Telescope
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Out-of-focus holography
Limits of single dish telescopes
von Hoerner (1967), 1967AJ.....72...35V
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Out-of-focus holography
Limits of single dish telescopesGravity, thermal effects and the homology principle
101 102 103
101
102 GBT
ALMAJCMT
SMA
NRAO 140-ftNobeyama
IRAM 30m Gravity - steelGravity - CFRP
Therm
al- Stee
l
Therm
al- CFRP
Surface error (µm)
Ape
rtur
edi
amet
er(m
)
[Data partially from Radford & Woody, 2009, NA URSI meeting, Boulder]
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Out-of-focus holography
Active surfaceSolution to non-homologous gravitational and thermal deformation
From http://www.gb.nrao.edu/gallery/gbt/index.html
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Out-of-focus holography
Simulated Out-Of-Focus Beams, Perfect Telescopeor “point-spread-functions”
In-Focus -ve De-Focus +ve De-Focus
≈ −12 dB of taperDe-focus: ≈ λ of path across the aperture
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Out-of-focus holography
A surface with random large-scale errors
Receiver Response Surface Errors(Taper/Apodisation/...) (Projected to an imaginary surface)
B. Nikolic (University of Cambridge) Introductory talk February 2010 16 / 34
Out-of-focus holography
Simulated Out-Of-Focus Beams
In-Focus -ve De-Focus +ve De-Focus
≈ −12 dB of taperRandom large-scale surface error added to the surface
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Out-of-focus holography
Simulated Out-Of-Focus Beams, with noise
In-Focus -ve De-Focus +ve De-Focus
≈ −12 dB of taperSignal-To-Noise: 100:1 per pixel
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Out-of-focus holography
GBT Observation at 90 GHz with MUSTANG
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Out-of-focus holography
Night-time thermal deformation from MUSTANG
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Out-of-focus holography
OOF in action at the GBTCorrecting the thermal deformations of the telescope
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Out-of-focus holography
Summary
Current status:The model for non-homologous gravitational deformation of theGBT is derived observationally from OOF map measurementsOOF on-line surface correction is used routinely for 3 mmobserving and for some (normally daytime) 7 mm–10 mmobserving at the GBTWhen used OOF also replaces the traditional pointing and focusmeasurements
Ongoing work:Optimising the technique to minimise time taken for both acquiringthe data and processing it
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Phase correction for ALMA
Outline
1 Introduction
2 Out-of-focus holography
3 Phase correction for ALMA
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Phase correction for ALMA
3-element ALMA
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Phase correction for ALMA
Path fluctuations measured by observing a quasar
−2000
−1500
−1000
−500
0
500
δL(µ
m)
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
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Phase correction for ALMA
Phase closureAntenna 0 Vs 1 Antenna 0 Vs 2
−2000
−1500
−1000
−500
0
500
δL(µ
m)
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
0
250
500
750
1000
1250
δL(µ
m)
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
Antenna 1 Vs 2 Closure phase
−3500
−3000
−2500
−2000
−1500
−1000
δL(µ
m)
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
−1000
−500
0
500
1000
δL(µ
m)
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
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Phase correction for ALMA
Atmospheric Phase Fluctuations
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Phase correction for ALMA
Atmospheric Phase Fluctuations
B. Nikolic (University of Cambridge) Introductory talk February 2010 27 / 34
Phase correction for ALMA
Atmospheric Phase Fluctuations
B. Nikolic (University of Cambridge) Introductory talk February 2010 27 / 34
Phase correction for ALMA
Atmospheric Phase Fluctuations
B. Nikolic (University of Cambridge) Introductory talk February 2010 27 / 34
Phase correction for ALMA
Water Vapour cm/mm/sub-mm lines1 mm precipitable water vapour
0
50
100
150
200
250
300
Tb
(K)
Tb
(K)
200 400 600 800 1000
ν (GHz)ν (GHz)
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Phase correction for ALMA
The 183 GHz Water Vapour LineBlue rectangles are the production WVR filters
0
50
100
150
200
250
T b(K
)T b
(K)
175 177.5 180 182.5 185 187.5 190
ν (GHz)ν (GHz)
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Phase correction for ALMA
WVR in the ALMA receiver cabin
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Phase correction for ALMA
WVR dataThe colours represent the four channels
100
150
200
250
300
T B(K
)T B
(K)
16.8 17 17.2 17.4 17.6 17.8
t (hours UT)t (hours UT)
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Phase correction for ALMA
4th Jan, WVR channel 3
Antenna 0 Vs 1 Antenna 0 Vs 2
−2000
−1000
0
1000
δL(µ
m)
δL(µ
m)
7.1 7.2 7.3 7.4 7.5
t (hours UT)t (hours UT)
−2
0
2
4
6
∆TB
,3(K
)∆T
B,3
(K)
0
500
1000
1500
δL(µ
m)
δL(µ
m)
7.1 7.2 7.3 7.4 7.5
t (hours UT)t (hours UT)
−5
−4
−3
−2
−1
∆TB
,3(K
)∆T
B,3
(K)
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Phase correction for ALMA
4th Jan, Antenna 0 vs 1Channel 1 Channel 2
−1.5
−1
−0.5
0
0.5
1
1.5
∆TB
,1(K
)∆T
B,1
(K)
−500 −250 0 250 500
δL(µm)δL(µm)
0
5
10
15
20
25
30
−1.5
−1
−0.5
0
0.5
1
1.5
∆TB
,2(K
)∆T
B,2
(K)
−500 −250 0 250 500
δL(µm)δL(µm)
0
5
10
15
20
Channel 3 Channel 4
−1.5
−1
−0.5
0
0.5
1
1.5
∆TB
,3(K
)∆T
B,3
(K)
−500 −250 0 250 500
δL(µm)δL(µm)
0
5
10
15
20
−1.5
−1
−0.5
0
0.5
1
1.5
∆TB
,4(K
)∆T
B,4
(K)
−500 −250 0 250 500
δL(µm)δL(µm)
0
5
10
15
20
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Phase correction for ALMA
4th Jan, Bl 0-1, Channel 3
Empirical Simple model
−400
−200
0
200
400
resi
dual
δL(µ
m)
resi
dual
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
−400
−200
0
200
400
δL(µ
m)
δL(µ
m)
−400
−200
0
200
400
resi
dual
δL(µ
m)
resi
dual
δL(µ
m)
7.1 7.15 7.2 7.25 7.3 7.35 7.4 7.45
t (hours UT)t (hours UT)
−400
−200
0
200
400
δL(µ
m)
δL(µ
m)
Residual RMS: 46 µm Residual RMS: 49 µm
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Phase correction for ALMA
Summary
Current status:The prototype radiometers were designed by the Cavendish laband Onsala observatorySeries production units from industry partners are now beingdelivered to the site and work fineWe have the first version of the end-to-end software system readyInitial tests look very encouraging
Ongoing work:Development and refinement of new algorithmsTesting in ChileApplying the technique to early science in about 1.5 years time
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