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Introduction GBT Specifics Results

Out-of-focus Holography Application at the Green Bank Telescope
B. Nikolic
The Cavendish Lab, University of Cambridge, UK

Green Bank, September 2007

B. Nikolic

OOF Workshop­GBT


Introduction GBT Specifics Results

Outline

1

Introduction

2

GBT Specifics

3

Results

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Introduction GBT Specifics Results

Simulated beams

In-Focus

-ve De-Focus

+ve De-Focus

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Introduction GBT Specifics Results

Observed (minimally processed) data

In-Focus
2.0 0.20 1.5 0.15 1.0 0.10

-ve De-Focus
0.20 0.15 0.10

+ve De-Focus

0.05 0.5 0.05 0.00

Tb

Tb

0.0

0.00

Tb
-0.05 -0.10 -0.15 -0.20 4.5 5.0 5.5 6.0 6.5 7.0 7.5 x1e-3 -0.25 0.008

-0.5

-0.05

-1.0

-0.10

-1.5

-0.15

-2.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

3.5 x1e-3

-0.20 4.0

0.009

0.009

0.009

0.010

0.011

0.011

0.011

Time (h)

Time (h)

B. Nikolic

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Introduction GBT Specifics Results

Observed (minimally processed) data: In-Focus
2.0

1.5

1.0

0.5

Tb

0.0

-0.5

-1.0

-1.5

-2.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

3.5 x1e-3

B. Nikolic

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Introduction GBT Specifics Results

Observed (minimally processed) data: -ve Focus
0.20

0.15

0.10

0.05

Tb

0.00

-0.05

-0.10

-0.15

-0.20 4.0

4.5

5.0

5.5

6.0

6.5

7.0

Time (h)

7.5 x1e-3

B. Nikolic

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Introduction GBT Specifics Results

Observed (minimally processed) data: +ve Focus
0.20

0.15

0.10

0.05

0.00

Tb
-0.05 -0.10 -0.15 -0.20 -0.25 0.008

0.009

0.009

0.009

0.010

0.011

0.011

0.011

Time (h)

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Introduction GBT Specifics Results

Why the GBT?

The GBT is not exactly homologous:
The active surface can fully correct for non-homologous deformation (NHD) if it is known Initially NHD estimated using a Finite-Element model Gain-elevation cur ve is cur ved at high frequencies Use OOF holography to compute an adjustment to the FEM model

A fully active, continuously adjusted, primary surface = instant application of corrections Large collecting area = high signal to noise

B. Nikolic

OOF Workshop­GBT


Introduction GBT Specifics Results

Outline

1

Introduction

2

GBT Specifics

3

Results

B. Nikolic

OOF Workshop­GBT


Introduction GBT Specifics Results

GBT-specific considerations
Asymmetric off-axis design
Phase change of wavefront due to de-focus is asymmetric and a somewhat complex function of position in aper ture No aper ture blockage to consider (although these are easily dealt with)

Dual beam differencing scheme Expensive to fully sample the measured beams
Due to a combination of few pixels and relatively low telescope agility.

Possibility of tracking/pointing errors Difficult to measure absolute and relative telescope efficiency

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Introduction GBT Specifics Results

GBT Geometry

F1
B. Nikolic

F0
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Introduction GBT Specifics Results

The wavefront phase due to de-focus

Calculated using an analytical approximation to ray-tracing
B. Nikolic OOF Workshop­GBT


Introduction GBT Specifics Results

De-focused Beams (In-Focus)

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Introduction GBT Specifics Results

De-focused Beams (-35 mm De-Focus)

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Introduction GBT Specifics Results

De-focused Beams (35 mm De-Focus)

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Introduction GBT Specifics Results

Differenced Beams (In-Focus)

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Introduction GBT Specifics Results

Differenced Beams (-35 mm De-Focus)

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Introduction GBT Specifics Results

Differenced Beams (35 mm De-Focus)

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Introduction GBT Specifics Results

Sample GBT Observation

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Introduction GBT Specifics Results

Sample GBT Observation

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Introduction GBT Specifics Results

Sample GBT Observation

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Introduction GBT Specifics Results

Sample GBT Observation: The Retrieved Surface

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Introduction GBT Specifics Results

Outline

1

Introduction

2

GBT Specifics

3

Results

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Introduction GBT Specifics Results

Experiments at the GBT

Retrieval of known deformation (bump) Retrieval and correction of surface errors during both night-time and day-time conditions Closure ­ repeated measure-correct-measure cycles to measure consistency and random error of technique Derivation of a refinement for the gravitational deformation model

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Introduction GBT Specifics Results

Closure
Measurement Set of OOF Maps Analysis Actuator ajustments

-ve De-Focus Map

Translate Finite Element Model

+ve De-Focus Map Map of wavefront errors Active surface adjustment OOF Analsyis

In-Focus Map

Pointing, Focus

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Introduction GBT Specifics Results

Closure: benign conditions

WRMS 150 µm

WRMS 100 µm

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Introduction GBT Specifics Results

Closure: Daytime

WRMS 340 µm

WRMS 210 µm

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Introduction GBT Specifics Results

Modelling Gravitational Deformation
Measurement -ve De-Focus Map

Finite Element Model

+ve De-Focus Map Active surface adjustment In-Focus Map

Pointing, Focus

Set of OOF Maps

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Introduction GBT Specifics Results

Modelling Gravitational Deformation
Obtained 37 measurements over three sessions covering a range of elevations Fit a sin() + b cos() + c to each Zernike coefficient individually
8

6

N

4

2

0 0 20 40 (deg) 60 80

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Introduction GBT Specifics Results

Gravitational Model: Ver tical Coma

1.5 1 Phase (rad) 0.5 0 -0.5

n = 3, l = -1
-1 20 40 Elevation (deg) 60 80

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Gravitational Model: Horizontal Coma

1.5 1 Phase (rad) 0.5 0 -0.5

n = 3, l = 1
-1 20 40 Elevation (deg) 60 80

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Gravitational Model: Trefoil

1.5 1 Phase (rad) 0.5 0 -0.5

n = 3, l = -3
-1 20 40 Elevation (deg) 60 80

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Introduction GBT Specifics Results

Gravitational Model: Trefoil

1.5 1 Phase (rad) 0.5 0 -0.5

n = 3, l = 3
-1 20 40 Elevation (deg) 60 80

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Introduction GBT Specifics Results

Gravitational Model: Astigmatism

1.5 1 Phase (rad) 0.5 0 -0.5

n = 2, l = -2
-1 20 40 Elevation (deg) 60 80

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Introduction GBT Specifics Results

Gravitational Model: Astigmatism

1.5 1 Phase (rad) 0.5 0 -0.5

n = 2, l = 2
-1 20 40 Elevation (deg) 60 80

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OOF Workshop­GBT


Introduction GBT Specifics Results

Gravitational Model
1.5 1 Phase (rad) Phase (rad) 0.5 0 -0.5 1.5 1 0.5 0 -0.5 Phase (rad) 1.5 1 0.5 0 -0.5

n = 4, l = -4
-1 20 40 Elevation (deg) 1.5 1 Phase (rad) Phase (rad) 0.5 0 -0.5 1.5 1 0.5 0 -0.5 60 80 -1 20 40

n = 4, l = -2
-1 60 80 20 40 Elevation (deg) 1.5 1 Phase (rad) 0.5 0 -0.5

n = 4, l = 0
60 80 Elevation (deg)

n = 4, l = 2
-1 20 40 Elevation (deg) 1.5 1 Phase (rad) Phase (rad) 0.5 0 -0.5 1.5 1 0.5 0 -0.5 60 80 -1 20 40

n = 4, l = 4
-1 60 80 20 40 Elevation (deg) 1.5 1 Phase (rad) 0.5 0 -0.5

n = 5, l = -5
60 80 Elevation (deg)

n = 5, l = -3
-1 20 40 Elevation (deg) 60 80 -1 20 40

n = 5, l = -1
-1 60 80 20 40 Elevation (deg)

n = 5, l = 1
60 80 Elevation (deg)

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Introduction GBT Specifics Results

Gravitational Model: Efficiency

FEM Only
0.55 0.5 0.45 a a 0.4 0.35 0.3 0.25 0 20 40 E (degrees) 60 80 0.55 0.5 0.45 0.4 0.35 0.3 0.25

FEM and OOF gravitational model

0

20

40 E (degrees)

60

80

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