II. B. The Laser Metrology System
	HOME INDEX SEARCH HELP NEWS

I. Introduction A. The goals of PRIMA B. Interferometry Tutorial C. Atmospheric and physical Constraints   II. Description A. The STS B. The PRIMET C. The FSU D. The DDL E. PRIMA Software III. Applications   IV. The Team   V. The Partners   VI. References, links

Table of contents of the PRIMET part: 1) Objectives of PRIMET 2) Technical requirements on PRIMET 3) Principle 4) Next milestones 5) Publications about PRIMET   This system ties together the two interferometric signals obtained by the simultaneous coherent observation of two celestial objects with PRIMA. The role of this Metrology System is to monitor the PRIMA instrumental optical path errors to ultimately reach a final instrumental phase accuracy limited by atmospheric piston anisoplanatism. To explain its framework, we will see precisely the real objectives of this system before precising what are the PRIMA requirements for its realisation (concerning the laser quality etc...), then we will describe the principle of the metrology and finally we will conclude by precising the next milestones. PRIMET is located in the laboratory and sends laser beams that are travelling all along the VLTI optical train (cf fig.1):

Fig. 1: Link between PRIMET and PRIMA

Objectives of PRIMET

A highly accurate metrology system is required to monitor the PRIMA instrumental optical path errors to ultimately reach a final instrumental phase accuracy limited by atmospheric piston anisoplanetism. The metrology system must measure the internal differential delay, OPD int, between both stars in both interferometers toward the VLTI optical train (cf fig.2) arms with a 5 nm accuracy over typically 30 min. This accuracy is driven by the PRIMA astrometric mode.

Fig.2: The optical train of the VLTI traversed by the two pairs of laser beam

Requirements on PRIMET

a) Accuracy:

PRIMET has an accuracy goal of 5 nm rms (root mean square for 30 min) for the astrometric mode (for an accuracy goal of dq = 10 µarcsec and a baseline of B = 100 m), and 100 nm for the imaging mode.

b) Resolution:

The resolution on OPD measurements shall be compatible with the above accuracy equipment. As a goal the resolution shall be less than 1 nm.

c) Range:

The main requirements concerning the range performances can be resumed in the following table:

Principle of PRIMET

a) The residual OPD:

In order to place the metrology system in the context of PRIMA, two celestial objects of vector coordinates S1 and S2 are considered. These objects are simultaneously observed on two independent beam combiners (A and B), i.e using a dual-feed configuration. By stabilising the interference fringes on the so-called bright object S1, the residual Optical Path Difference (cf fig.3) DOPD, seen by S2 is given by:

DOPD=B. (S2-S1)+ f/k+ dA + DL (1)

where:

• B is the vector baseline of the interferometer,
• f is a phase factor inherent to the nature of the observed objects and is the observable for phase-referenced imaging, k being the wave number,
• dA represents the differential OPD (or piston) anisoplanatism which contributes to the residual fringe motion seen by S2. For the maximum angular separation S2-S1 of PRIMA of 1 arcmin, the standard deviation of dA is about 2 µm rms, i.e. 10 times lower than the open-loop atmospheric fringe motion imposed on S1. A has a zero mean and can be averaged- out down to 5 nm rms by successive measurements of OPD in a typical 30 min. time frame for a 10 arcsec star separation, and 100m baseline
• DL=L2-L1 represents the difference between the internal OPD's of each object. DL (Delay lines) can thus be seen as the instrumental contribution to OPD.

Fig. 3: Differential Optical Path Length

b) The differential OPD:

Knowing the baseline vector B and measuring independently DOPD and DL, Eq.(1) shows that one can estimate either the factor f/k for a known star separation (Phase-referenced Imaging mode), or inversely the star separation for a known f (Astrometric mode). The bottom line being that the implementation of PRIMA is intimately linked with the ability to trace back the "differential" internal OPD between the two objects, DL.

c) General principle:

In the VLTI, the light captured by two telescopes follows a train of 25 mirrors distributed along a subterranean path of approximately 200 meters, before being coherently combined. Inside the VLTI, the fringe signals are affected by static optical path differences and also by time-varying optical path fluctuations introduced by the motion of the Delay lines and of the Differential Delay Lines, by vibrations of mechanical structures, and by air turbulence. The PRIMA Metrology system is designed to monitor these instrumental disturbances, which are included in the variable DL, with an ultimate accuracy goal of 5 nm.

The concept of the PRIMA Metrology System is based on "super-heterodyne laser interferometry", where two heterodyne Michelson interferometers are operating simultaneously and have common optical paths with both observed stars through the VLTI optical train (see RD and RD), i.e. from the interferometric laboratory to the metrology "end-points" located inside the telescopes. The disturbance to be monitored, DL, corresponds to the difference between the path variations recorded by the two Michelson interferometers.

Because such a system is purely incremental (i.e. counting the number of 2p phase variation while DL is varying), the estimation of the absolute value of DL implies an accurate calibration of the metrology "zero" point, i.e. when DL=0. For this calibration, the solution selected for the first two phases of PRIMA (2002-2008) consists in the simultaneous observation of the bright celestial object on both PRIMA channels, for which L must be zero by definition (S2=S1, dA=0, f=0 for a point-like bright object). Starting from this calibration mode, the metrology system is zeroed and DL is continuously monitored while the faint object is acquired and tracked on the Channel A of PRIMA. In the third phase of PRIMA (2008-2010), this calibration scheme will not be required any more. Indeed the metrology will be upgraded to an absolute metrology system (i.e. not purely incremental). It will simplify the overall PRIMA calibration sequence at the expense of a more complex metrology system. The implementation of such an absolute metrology system for PRIMA is clearly more ambitious than an incremental one since all problems related to incremental metrology must be previously solved.

d) Principle of the sub-systems:

The PRIMA Metrology system can essentially be seen as a "sensor" which outputs the quantity DL upon request from the PRIMA Workstation and make it available on the PRIMA Reflective memory Network, as shown in fig.4 . The Metrology sub-system breakdown is shown in Fig. 4. The present design document is structured accordingly.

Fig. 4: Figure 1 Prima Metrology System: an OPD "sensor" for PRIMA

Light Source:

This sub-system includes the laser head, the heterodyne assembly and its control HW/SW. Its role is to provide to the metrology system all necessary laser beams characterised by their wavelength, frequency stability, optical power, polarization state, and the relevant heterodyne frequencies. The light source sub-system and its associated electronics is located in the VLTI storage room.

Beam launchers/Beam combiner:

The role of the beam launcher sub-system is to inject the laser beam(s) provided by the light source sub-system into each stellar channel, with the appropriate optical characteristics. The beam combiner is the location where the metrology beams of each channel interfere. There will be one beam launcher/combiner on each of the the following optical tables: FSU#A, FSU#B, AMBER and MIDI.

Metrology end-points:

The metrology end-points terminate the internal optical path monitored by the metrology system by retro-reflecting the metrology beams back to their injection points.

Phase meter:

The role of the phase meter is to detect and process the metrology interference signals to retrieve the quantity L and make it available to the PRIMA control system.

Control HW/SW:

This sub-system includes the hardware and software required to control the metrology system and exchange data/status/diagnostics with the PRIMA Control System.

The Light source, the Phase meter and the Control Hardware sub-systems are all located inside the VLTI storage room.

e) Hardware/Software part of PRIMET:

An overview of the location of the metrology hardware is shown in fig.5:

Fig. 5: Overview of the location of the metrology hardware inside the interferometric laboratory and inside the storage room

The light of a frequency stabilized Nd-Yag laser (Laser Assembly) is split into four frequency shifted laser beams (Heterodyne Assembly). The pair of beams attributed to Channel B is relayed towards the Beam Launcher/Combiner of the FSU#B through optical fibers (Beam Relay). Similarly, the pair of beams attributed to Channel A is relayed towards the beam launcher/combiner of FSU#A, or MIDI or AMBER. In the first phase of PRIMA, this fiber "multiplexing" will be performed manually from the VLTI storage room, according to Table . In order to simplify fig.5 , only two beam launcher/combiners are shown. The metrology beam launcher/combiner for AMBER and MIDI are independent of the MIDI and AMBER optics. For the metrology Beam Launcher/Combiner of FSU#A and FSU#B, the beam splitters represented in fig.5 with a dot pattern are physically located in the Beam Combiner of each FSU's.
In each metrology channel, the metrology beams are first superimposed to create a reference signal and then launched separately in the stellar paths. After a round trip through the VLTI, the beams are recombined to form a probe signal. Both reference and probe signals are relayed through optical fibers towards the Phase Meter where they are detected and processed to generate the quantity L.
The metrology control electronics is integrated inside an electronic cabinet located inside the VLTI storage room. The metrology cabinet hosts:

• 2 LCU crates
• 1 Phase meter crate

The electronics of the Light Source composed of the Heterodyne assembly and of the electronics of the Laser Assembly.

All the hardware located inside the storage room (control electronic, light source, beam relay) is common to all metrology configurations. The differences only concern the beam launcher/combiners, thus the way the metrology beams are physically injected and extracted in/from the various stellar paths.

Next milestones

The preliminary design of the metrology has been presented and tested in Paranal in Chile in 2002. The final design review should be finished at the end of 2003. All the milestones can be resumed in the following table:

More informations about PRIMET

I. Introduction A. The goals of PRIMA B. Interferometry Tutorial C. Atmospheric and physical Constraints   II. Description A. The STS B. The PRIMET C. The FSU D. The DDL E. PRIMA Software III. Applications   IV. The Team   V. The Partners   VI. References, links

"PRIMA Technical Description and Implementation", F.Derie, F.Delplancke,A.Glindeman,S.Leveque, S.Menardi,F.Paresce,R.Wilhelm,K.Wirenstrand,Workshop "Hunting for Planets ", Lorentz center, Leiden University, 3-6 June 2002. Slides of the presentation. "Towards nanometer accuracy laser metrology for phase-referenced interferometry with the VLTI", S.Leveque, R.Wilhelm, Y. Salvadé,O.Scherler, R.Daendliker, SPIE conference Interferometry for Optical Astronomy, August 2002, SPIE Vol.4838. "Superheterodyne Laser Metrology for the Very Large Telescope Interferometer", Y. Salvadé, R.Daendliker, S.Leveque, ODIMAP III, 3rd topical meeting on Optoelectronic Distance/Displacement Measurements and Applications,University of Pavia , 20-22/09/01. Slides of the presentation "High accuracy laser metrology enhances the VLTI", S.Leveque,Y. Salvadé,O.Scherler, R.Daendliker, Laser Focus World, April 2002. “Metrology for phase-referenced imaging and narrow-angle astrometry with the VLTI” S. Leveque, SPIE conference: "Astronomical Telescope and Instrumentation 2000", Munich (Germany), 25-31 March 2000 , [4006-45] , slides version, “Absolute metrology for the Very Large Telescope Interferometer (VLTI)”, Y. Salvadé, A. Courteville, R. Dandliker,SPIE conference: "Astronomical Telescope and Instrumentation 2000", Munich (Germany), 25-31 March 2000 , [4006-49] For the other publications, see the part: References and links.

Send comments to <fdelplan@eso.org> Last update: Jul.31 2003