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

Metrology for Phase-referenced Imaging and Narrow-Angle Astrometry with the VLTI Samuel Lévêque European Southern Observatory sleveque@eso.org SPIE 4006-45 1

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 1 =B. S 1 = + L 1 ∆ OPD =B.( S 2 - S 1 ) = + ∆ L OPD 2 =B. S 2 = + L 2 Accuracy Goal: 5 nm (driven by Astrometry at 10 µ arsec accuracy, B=100m) • Long propagation paths: L 1,2 >276m (1 way) Range: 60 mm • 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 2

Implementation Baseline • Telescope Pair-wise configuration (L 1 and L 2 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 Metrology System Light Source Beam Launcher Metrology end points Beam Combiner(s) Phase meter Laser Beam Relay Optics Beam extraction Detection Heterodyne Beam injection Mechanics Relay Signal Assembly Opto-mechanics conditioning Control Signal Electronics Processing Control HW/SW 3

Allocation of heterodyne frequencies • Cross-talk minimized by operating with different heterodyne frequencies f i =1,2 separated by ∆ν • L i =1,2 coded at frequency f i =1,2 • δ f i= 1,2 given by dynamic requirements driven by the PRIMA differential delay lines ν ν ν+ ν+ f 1 ν+ ν+∆ν ∆ν ν+ ν+∆ν ∆ν +f 2 ν ν ν+ ν+ ν+ ν+ ∆ν ∆ν ν+ ν+ ∆ν ∆ν 1 2 1 1 2 2 ∆ν ∆ν ∆ν ∆ν= = = = 2 MHz 1 = = 650 kHz = = 2 = = = 450 kHz = f 1 f 2 1 1 2 2 2 = = 450 kHz 1 = = 650 kHz = = = = f 2 f 1 2 2 1 1 ∆ν ∆ν =2 MHz ∆ν ∆ν Intensity noise 200 kHz δ δ δ δ f 1 =±35 kHz δ f 2 =±25 kHz δ δ δ 0 Cross-talk noise 4

Phase Meter • 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 Up/down counter ref 1 f 1 Integer Nb. f 1 -f 2 cos 2. π π (f 1 -f 2 ).t π π output δ f 1 Ref: •Decrement δ f •Increment ref.2 f 2 cos [2. π π (f 1 -f 2 ).t+ φ π π φ 1 - φ φ φ φ φ φ 2 ] Probe: Read signal δ f 2 Counter L 1 ( φ 1 ) Fractional Nb. output f 1 •Start Probe 1 f 1 -f 2 δ f 1 •Stop Adder i •clock PLL L 2 ( φ 2 ) δ f f xN f 2 δφ δ f 2 Probe 2 Sum. Start Integer fringe counter Sum. Stop 5

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 TEM 00 Polarization: Linear (W-direction) Wavelength: 1 µ m 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 BEAM CLIPPED AT RETROREFLECTOR POLARIZATION MODE AT EXIT PUPIL 0.03 Injected beam linearly polarized in w-direction intensity [W m –2 ] 0.02 –u 0.01 Retroreflector Ø = 120 mm 0 –0.1 –0.05 0 0.05 0.1 v [m] 6 w

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 7

Identification of error sources Error sources on ∆ ∆ ∆ ∆ L Layout errors Instrumental errors • Beam • Laser head routing (OPL offsets and misalignments) Frequency stability Retro-reflector Power stability • Electronics Beam injection/combination VLTI optical train Detection noise Active mirror Signal conditioning noise Air turbulence Demodulation noise • Optical cross-talk Mechanical stability • Metrology Wavelength dependent Thermal effects • Wavefront distortion errors Deformable mirror Chromatic errors on coatings Internal air turbulence Air dispersion • Figuring errors associated with beam walk • Drift of "zero" point (dead path) • Field dependent errors 8

Example of detection noise for a given laser fringe visibility 3 10 V=0.1 2 10 V=0.5 OPD error in nm 1 10 V=1 0 10 -1 10 V=fringe visibility -2 10 -2 -1 0 1 2 3 10 10 10 10 10 10 Received Optical power in microWatts 9

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. Acknowledgments 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. 10

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 11