Metrology for Phase-referenced Imaging and Narrow-Angle Astrometry - - PowerPoint PPT Presentation

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Metrology for Phase-referenced Imaging and Narrow-Angle Astrometry with the VLTI

Samuel Lévêque

European Southern Observatory sleveque@eso.org

SPIE 4006-45

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Introduction

The “Phase-Referenced Imaging and Micro-arcsecond Astrometry” (PRIMA) facility [1] of the VLTI is based on the simultaneous coherent observation of two celestial objects in which the two interferometric signals are tied together by an internal metrology system. The role of this metrology system is to monitor the PRIMA instrumental optical path errors to possibly reach a final instrumental phase accuracy limited by atmospheric piston anisoplanatism[2].

Requirements and Constraints

∆OPD =B.(S2 - S1)=+ ∆L OPD1 =B. S1=+ L1 OPD2 =B. S2=+ L2

Accuracy Goal: 5 nm (driven by Astrometry at 10

µarsec accuracy, B=100m)

Range: 60 mm

• Long propagation paths: L1,2 >276m (1 way)
• The beams are relayed through air as opposed to vacuum
• Metrology beam and the stellar beams must share the same internal path down to the beam combiners
• Careful management of the interfaces with all VLTI sub-systems is required including possible straylight

contamination on existing detectors

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Implementation Baseline

Laser Heterodyne Assembly Control Electronics Light Source Beam Relay Beam injection Beam Launcher Optics Mechanics Metrology end points Beam extraction Relay Opto-mechanics Beam Combiner(s) Detection Signal conditioning Signal Processing Control HW/SW Phase meter Metrology System

• Telescope Pair-wise configuration (L1 and L2 are individually monitored)
• Incremental Heterodyne interferometry with zero point calibration using stellar reference source
• Super-heterodyne phase detection
• Nd-Yag laser compatible with 500nW detected power, stability imposed by ∆L only
• Common mode injection using central obscuration
• Metrology end points installed in the image of the telescope’s central obscuration

Sub-system Breakdown

4 Allocation of heterodyne frequencies

• Cross-talk minimized by operating with different heterodyne frequencies fi=1,2 separated by ∆ν
• Li=1,2 coded at frequency fi=1,2
• δfi=1,2 given by dynamic requirements driven by the PRIMA differential delay lines

ν ν ν ν ∆ν ∆ν ∆ν ∆ν= = = = 2 MHz f1

1 1 1=

= = = 650 kHz f2

2 2 2=

= = = 450 kHz ν+ ν+ ν+ ν+f1

1 1 1

ν+ ν+ ν+ ν+∆ν ∆ν ∆ν ∆ν+f2

2 2 2

ν+ ν+ ν+ ν+∆ν ∆ν ∆ν ∆ν f1

1 1 1=

= = = 650 kHz f2

2 2 2=

= = = 450 kHz Cross-talk noise δ δ δ δf2=±25 kHz δ δ δ δf1=±35 kHz 200 kHz ∆ν ∆ν ∆ν ∆ν=2 MHz Intensity noise

5 Phase Meter

Up/down counter

• Decrement
• Increment
• utput

Counter

• Start
• Stop
• clock
• utput

PLL xN

Adder i f

• Sum. Start
• Sum. Stop

Integer Nb. Read signal Probe:

Probe 1 L1(φ1) ref 1 ref.2 Integer fringe counter δφ Probe 2 L2 (φ2) f1

δf1

f2

δf2

f1-f2

δf Ref:

cos [2.π π π π(f1-f2).t+φ φ φ φ1-φ φ φ φ2] cos 2.π π π π(f1-f2).t

f1

δf1

f2

δf2

f1-f2

δf Fractional Nb.

• Direct measurement of ∆L using super-heterodyne detection [7]
• Digital phase meter: Phase difference given by counting number of clock cycles
• Clock generated from reference signal using PLL to avoid phase drifts

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Laser beam propagation simulation

☛Gaussian Beam superposition algorithm [8] used to simulate diffraction effects

induced on laser beam propagating through overall VLTI optical train (return way):

• Characteristics of injected laser beam:

Mode: Gaussian TEM00 Polarization: Linear (W-direction) Wavelength: 1 µm Power: 1mW Waist size: 4.1 mm (image of central obscuration for the UT’s)

• Propagation distance : 177 mx2= 354m (return way)
• “Perfect” Retro-reflector located at the center of the Telescope’s secondary mirror

intensity [W m–2] –0.1 –0.05 0.05 0.1 0.01 0.02 0.03 v [m] BEAM CLIPPED AT RETROREFLECTOR Retroreflector Ø = 120 mm w –u POLARIZATION MODE AT EXIT PUPIL Injected beam linearly polarized in w-direction

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Returned intensity and phase map of laser beam after 354 m propagation (return way) through the VLTI optical train

Conclusion:

Polarized heterodyne interferometry using telescope’s central obscuration feasible from diffraction point of view

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Identification of error sources

Error sources on ∆ ∆ ∆ ∆L Layout errors Instrumental errors

• Beam

routing (OPL

• ffsets

and misalignments) Retro-reflector Beam injection/combination VLTI optical train Active mirror Air turbulence Mechanical stability Thermal effects

• Wavefront distortion

Deformable mirror Internal air turbulence

• Figuring errors associated with beam walk
• Field dependent errors
• Laser head

Frequency stability Power stability

• Electronics

Detection noise Signal conditioning noise Demodulation noise

• Optical cross-talk
• Metrology Wavelength dependent

errors Chromatic errors on coatings Air dispersion

• Drift of "zero" point (dead path)

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• 1

10 10

1

10

2

10

3

10

• 2

10

• 1

10 10

1

10

2

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Received Optical power in microWatts OPD error in nm

V=1 V=0.5 V=0.1 V=fringe visibility

Example of detection noise for a given laser fringe visibility

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Conclusion

The PRIMA metrology system must clearly meet an ambitious accuracy goal. A baseline for this metrology system has been identified, including a phase demodulation architecture. The next steps will include the consolidation of the metrology error budget. The development of a prototype of the phase meter is planned in the course of this year and measurements will be performed at Paranal to characterize in more detail the effect of internal turbulence in the context of PRIMA. The author would like to thank U.Johann, E.Manske, R,Sesselmann from Dornier Satellitensystem GmbH for fruitful discussion about the metrology system during the PRIMA feasibility Study, as well as Y.Salvadé, A.Courteville, and R.Dändliker from the Institute of Micro-Technology in Neuchâtel, who conducted the PRIMA metrology rider study. The contribution of R.Wilhelm, who simulated the gaussian beam propagation, is gratefully acknowledged.

Acknowledgments

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References

• 1. F. Delplancke et al. "Phase-referenced imaging and micro-arcsecond astrometry with the VLTI", these

proceedings.

• 2. L. D’Arcio, "Selected aspects of wide-field stellar interferometry”, Ph.D. Thesis, University of Delft, Nov. 1999,

ISBN 90-6464-016-6.

• 3. Ph.Gitton, B.Koehler, S.Lévêque, A.Glindemann, " The VLT Interferometer-Preparation for first fringes", these

proceedings.

• 4. O.von der Luehe, A.Quirrenbach,B.Koehler, "Narrow Angle Astrometry with the VLT Interferometer", in

Science with the VLTI, ESO Astrophysics symposia, editors J.R.Walsh and I.J.Danziger, ISBN 3-540-59169-9, 1995

• 5. U.Johann et al, "Prima Feasibility Study", Dornier Satellitensysteme GmbH, ESO technical report VLT-TRE-

DSS-15700-0001, July 1999.

• 6. Y.Salvadé, A.Courteville, R.Dändliker, "PRIMA metrology rider study", Institute of Micro-Technology of

Neuchâtel, ESO technical report VLT-TRE-IMT-15700-0001, January 2000.

• 7. Y.Salvadé, A.Courteville, R.Dändliker, "Absolute metrology for the Very Large Telescope Interferometer",

these proceedings.

• 8. R. Wilhelm, B. Koehler, "Modular toolbox for dynamic simulation of astronomical telescopes and its application

to the VLTI", these proceedings