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)

B. Nikolic (University of Cambridge) Introductory talk February 2010 4 / 34

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

B. Nikolic (University of Cambridge) Introductory talk February 2010 6 / 34

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

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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]

B. Nikolic (University of Cambridge) Introductory talk February 2010 13 / 34

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)

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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

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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

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)

B. Nikolic (University of Cambridge) Introductory talk February 2010 32 / 34

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

B. Nikolic (University of Cambridge) Introductory talk February 2010 33 / 34

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

B. Nikolic (University of Cambridge) Introductory talk February 2010 34 / 34

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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