Tuesday June 4, 2002 Frédéric Derie, Françoise Delplancke, Andreas Glindemann, Samuel Lévêque,

PPhase hase RReferenced eferenced IImaging and maging and MMicro-arsec icro-arsec AAstrometrystrometry(PRIMA)(PRIMA)

an introduction to technical description and an introduction to technical description and developmentdevelopmentTuesday June 4, 2002

Frédéric Derie, Françoise Delplancke, Andreas Glindemann, Samuel Lévêque,

Serge Ménardi, Francesco Paresce, Rainer Wilhelm, K. Wirenstrand

European Southern Observatory (ESO)Karl Schwarzschild Strasse 10, D-85748 Garching München

(Germany) Email:[email protected]

F. Derie GENIE Workshop Leiden 04.06.02

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ContentsContents

Introduction Main Scientific Objectives of PRIMA Overall Description and Principles Four Sub-Systems of PRIMA GENIE and PRIMA Development in three Phases Planning Conclusion


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IntroductionIntroduction PRIMA is designed to make use of the Dual feed capabilityDual feed capability of the VLTI for

both UT and AT and is compatible with VINCI, AMBER, MIDI Gain of about six magnitudessix magnitudes in H, K and N. PRIMA is designed to perform:

– faint Objectfaint Object Observation in J, H, K and N Bands– high accuracy (10 µas) narrow angle astrometrynarrow angle astrometry in band H or K, – aperture synthesisaperture synthesis (phase reference imaging) and model constrained

imaging in J, H, K and N In the astrometric mode, the prime observable is the optical differential optical differential

delaydelay between the object and a reference. In Imaging mode, the observable are phase and amplitudephase and amplitude of the object

visibilityvisibility. Observation on a number of baselines :minimum two for Astrometry, more

for Imaging. PRIMA is composed of four major sub-systems: Star separator, Differential Star separator, Differential

Delay Lines, Metrology, Fringe Sensor UnitsDelay Lines, Metrology, Fringe Sensor Units and one overall control system and software


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Main Scientific ObjectivesMain Scientific ObjectivesDetection and Characterization of extra-solar planetsextra-solar planets and their birth environment. astrometric and the imaging capability of PRIMA.astrometric and the imaging capability of PRIMA.

The astrometric capability will determine the main physical parameters of planets orbiting around nearby stars already found by radial velocity (RV) techniques including their precise mass, orbital inclination and a low resolution spectrum (with Amber).

Another key objective for VLTI with PRIMA will be the exploration of the nuclear nuclear regionsregions of galaxiesgalaxies including our own

High precision astrometry with PRIMA might even go so far as to probe the central BH to a few times its Schwarzschild radius (2.5*10-7 pc). H

High resolution imaging with the VLTI of the nearest AGN (Cen-A is at ~ 3.5Mpc) has a crucial role to play in this area especially with MIDI at 10 and 20.


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Main Scientific ObjectivesMain Scientific Objectives

Astrometry allows the search for planets to stars that cannot be properly covered by the RV technique, (e.g. particular pre-main sequence (PMS) stars).

The crucial exploration of the initial conditions for planetary formation in the stellar accretion disk as a function of age and composition will be finally possible.

The reach of the VLTI in the local star forming regions where the expected magnitude of the reflex motion of the object specified is plotted as a function of distance.


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Main Scientific ObjectivesMain Scientific Objectives

The simultaneous image at mas resolution or better of the accretion disk from which any particular planet is born will add a new and exciting dimension to our understanding of both planetary and stellar formation mechanisms since the complex accretion disk is expected to be the cradle of these objects. The expected disk structure open to VLTI with PRIMA might look something like:

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PRIMA DiagramPRIMA Diagram

AT

?

FSU A/B

Delay lines


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Overall Description and Overall Description and PrinciplesPrinciples

The recorded distance between white fringes of the reference and the object is given by the sum of four terms:

(ΔS . B) the Angular separation (< 1 arcmin) times Baseline;+ (Ф ) the Phase of Visibility of Object observed for many

baselines;+ (ΔA) the Optical Path Difference caused by Turbulence

(supposed averaged at zero in case of long time integration);+ (ΔC) the Optical Path Difference measured by Laser

Metrology inside the VLTI.

n.b. For astrometry both Objects are supposed to have the Phase of their complex visibility = zero (point source object)

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Main System PerformanceMain System Performance Faint Objects ObservationFaint Objects Observation

Gain of about 6 MagnitudesGain of about 6 MagnitudesModel Independent Imaging (Aperture Synthesis)Model Independent Imaging (Aperture Synthesis)Phase referenced imaging on faint objects with AMBER, MIDI, VINCI

Visibility accuracy < 1%Visibility accuracy < 1%10µas astrometry10µas astrometryPhase and Group Delay measurement with FSU A and Metrology,

Angular resolution < 10 µarcsecAngular resolution < 10 µarcsecBaselines from 8 to 200mBaselines from 8 to 200m

Limiting Magnitude with tracking with FSU B(in band K)Limiting Magnitude with tracking with FSU B(in band K)

UT ATObject in Band H or K 19 16Object in Band N 11.5 (no IR counterpart) 8.3Reference in Band K 13/15 10/12

2 UT 3 UT 4 UT 2 AT/29 stationsRange 45, 56, 60, 86, 99, 130m Same as Same as 8 to 200 mIndependentBaselines

1 3 6 259 (with 8 DL)229 (with 6 DL)

Main System Main System Performance(con’t)Performance(con’t)

Sky Coverage Sky Coverage

Probability to find a star of MK within field of x arcsec

A. Robin Model Obs Besançon, stellar formation in the galaxy

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Anisoplanatic OPD and visibility lossAnisoplanatic OPD and visibility loss Fringe visibility is attenuation is related to off-axis observation. This

visibility loss has two effects on the observation of the faint object: it reduces the limiting magnitude on the faint object it reduces the accuracy with which the visibility of the object is measured.

The visibility loss can be calibrated on reference stars.

Anisoplanatic OPD noise as a function of the angle between the stars for the AT (left) for a wavelength of 2.2m.

Fringe visibility (normalised to V0=1) as a function of theoff-axis angle for the AT at 2.2m (K-band) and 10m (N-band).

R0=1m

R0=0.5m

R0=1m

R0=0.5m

NK

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Optical Path Difference Tracking ResidualOptical Path Difference Tracking ResidualOPD noise dominated by Fringe tracking noises (sensor and loop

noises)linked to band pass and atmospheric turbulanceCurrent DL have 44.6 Hz Band Pass, OPD residual about 100 nmOPD residual about 100 nm.

For small angular separation the DDL with high sampling rate can optimise the OPD residual but only for < 3” in K and bright stars.

Normalised fringe visibility as a function ofthe fringe sensor measurement noise, K-band

F. Derie 13


Total Observation TimeTotal Observation TimeShort exposure visibility and phase measurements averages incoherently,for Paranal typical atmospheric condition 18µas/hr,

baseline 200m, 10” separation:10µas astrometryastrometry accuracy : about 3 hr per baseline3 hr per baseline1% accuracy in imagingimaging : about 40 min40 min per baseline and in K bandSingle Exposure TimeSingle Exposure Time

Variation of the residual OPD (left) and of the fringe visibility at 2.2m (right) as a function of theintegration time, for the AT (D=1.8m, B=200m) and UT (D=8m, B=120m) cases.


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STS Function/PerformanceSTS Function/Performance

The Star Separator is located at Coudé Focus of UT and AT, it is in charge of: picking up 2 stars in the Coudé field of view up to 120 arcsec ØCoudé field of view up to 120 arcsec Ø, collimating them in two beams of 80mm diameter80mm diameter separated by 240mm, compensate for field rotationfield rotation, adjusting and stabilizing the beam tip-tiltbeam tip-tilt and the output pupil pupil

transversaltransversal position, allowing PRIMA calibrationPRIMA calibration by injecting the same star in both feeds, transmitting the metrology and not interrupting it, especially at the

calibration step. A possibility of (counter-)choppingcounter-)chopping is also foreseen for the operation @

10µm. Tracking accuracy 10 mas and resolution 2masTracking accuracy 10 mas and resolution 2mas, Output beam and pupil stability and OPD stability as for other VLTI sub-

system


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DDL DDL Function/PerformanceFunction/Performance

Differential Delay Line (one per beam and per arm), is used to compensate the relative OPD introduced between the the primary and of the secondary objects.

Range differential OPD 70 mm70 mm

OPD accuracy 5 nmaccuracy 5 nm

StabilityStability (OPD noise) 14 nm RMS over 8 ms14 nm RMS over 8 ms

Sampling Rate 8 kHz8 kHz

Pupil relayPupil relay


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FSU FSU Function/PerformanceFunction/Performance

The Fringe Sensor Unit (FSU) is where the stellar beam combination is taking place. The FSU comprises two identical channels (A and B) handling respectively the secondary and the primary objects.

The role of each FSU channel is to: Detect the fringesDetect the fringes (DL and DDL for the secondary object

are scanning the OPD. Deliver at a high ratehigh rate the fringe pattern positionfringe pattern position (phase

delay and group delay) to the OPD controller, in charge of closing the various OPD control loops.

Store the measuredStore the measured data over the whole observation time.


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FSU Function/Performance FSU Function/Performance (con’t)(con’t)

In imaging modeimaging mode, FSU BFSU B performs the Fringe TrackingFringe Tracking for both objects, by feeding-back its measurements to the main DL. The secondary object can be observed by MIDI or AMBER with long integration times and its phase measured, for several interferometric baselines, with respect to the “constant point” provided by FSU B, thanks to the PRIMA Metrology System.

In astrometric modeastrometric mode, both FSUboth FSU channels are operated and one OPD control loop is closed for each object, at a frequency which depends on the object magnitude. The FSU records the residual group delay for each object, which is processed off-line to derive the astrometric data.

FSU FSU FSU FSU Function/Performance Function/Performance

(con’t)(con’t)

The selected instrument concept uses co-axial beam combinationco-axial beam combination, which consists of adding the incident beams amplitudes at a semi-reflective surface, close to a pupil plane. The beam combiner provides simultaneously four interferometric outputs (optical beams resulting from the superimposed incident beams), with /2, and 3/2 relative phase shifts. This enables using the so-called ABCD algorithmABCD algorithm to derive the phase delay, without introducing any temporal OPD modulation.

PRIMA metrology beams are inserted in (and extracted from) the stellar path after the FSU beam combiner. Therefore the OPD is monitored by the metrology system over the whole FSU internal path

ΦΦk

Φ

BCk

A

D

C


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MET PRIMA MetrologyMET PRIMA Metrology

A highly accurate metrology system is required to monitormonitor the PRIMA instrumental optical path errorsinstrumental optical path errors to ultimately reach a final instrumental phase accuracy limited by atmospheric piston anisoplanatism. The metrology system must measure the internal differential delay, L, between both stars in both interferometer arms with a 5 nm accuracy5 nm accuracy requirement over over typically 30 mintypically 30 min. This accuracy requirement is driven by the PRIMA astrometric mode.

OPD =B.(S2 - S1k L

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MET PRIMA Metrology MET PRIMA Metrology RequirementsRequirements

Range and accuracyMax. propagation path (return way) 552 mIndividual OPD L1, L2(return way) 240 mL=L1-L2 range (=1 arcmin) 60 mmL Accuracy Goal (µas accuracy) < 5 nmL Resolution < 1 nm

Main dynamic phase variations (= 1 µm)on individual OPD Typical valueTracking of DL & STS ( L t = 11 mm/s) 22 kHzOPD correction (atmospheric, “Internal”) “Slow” in comparaisonon differential OPDTracking of DDL & STS ( L t = 10 µm/s) 20 HzSlewing of DDL & STS ( L t= 15 mm/s) 30 kHz


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MET ConceptMET Concept

Two heterodyneheterodyne interferometers with different heterodyne frequencies f1 and f2

Frequency offset between the two interferometers for suppressing cross talks in calibration mode

DirectDirect measurement of L after frequency mixing

Super Heterodyne Laser interferometry:

L coded in phase of (f1-f2) carrier signal No need for 2 independent high bandwidth and highly synchronised measurements of L1 and L2

Digital Phase meter using principle of time interval measurementwith on-board averaging capability

MET Super-Heterodyne MET Super-Heterodyne Laser interferometerLaser interferometer

I1(t) = cos(2πf1t + 1)

I2(t) = cos(2πf2t + 2)1.4

1 Lc

2)(42 L

c

Lc

Lc

Lc

4 44

221

Imeas(t) = I12cos[2π(f1-f2).t+ 1- 2)

1.1 µm (Si)<<1.45 µm (H band) Lc >> 500 m < 10-8

f 450 kHz f 650 kHz

38.65Mhz

38Mhz

40Mhz

39.55Mhz

0

f 650 kHzf 450kHz

f2=50kHz f1=50 kHz

200 kHzIntensitynoise

Crosstalk noise80MHz

BP650kHz

BP450kHz

BP450kHz

BP650kHz

BP200kHz

BP200kHz

Lim.Amp.

Lim.Amp.

DigitalPhaseMeter

VLTIOpticalTrain

Nd-Yag Laser

Acousto optics

Telescope 1

Telescope 2VLTIOpticalTrain

Ref.1 Ref.2 Meas.2 Meas.1Photodetection &Amplification

Ref. Meas.

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MET Prototyping and MET Prototyping and testingtesting

Nd-Yag, 1319nm

+38.65 MHz+38.MHz-39.55 MHz-40 MHz

A.O.M’s

ESO/IMT collaborationBeam Launcher/combiner (1 channel)

25mm

MET Testing at ParanalMET Testing at Paranal

Objectives:•Check the performance of the concept & prototype hardware under representative operating conditions•Characterise Internal OPD fluctuations •Check interfaces with VLTI

0 5 10 15 20 25 30 35-80-60

-40

-20

0

20

40

Time [min]

OP

D [m

icro

ns]

Arm length= 520m (return way)

Optical configuration in the VLTI optical lab

MET Main ResultsMET Main Results

ReferencesS. Lévêque,Y. Salvadé,R. Dändliker,O.Scherler, “High accuracy laser metrology enhances the VLTI”, Laser Focus World, April 2002.Y. Salvadé, R. Dändliker, S. Lévêque, “Superheterodyne Laser Metrology for the Very Large Telescope Interferometer", ODIMAP III, 3rd topical meeting on Optoelect. Distance / Displacement Meas..and Appl., University of Pavia,20–22/09/01

Fibre pigtailed photodetector: •NEP=0.2pW/Hz (Johnson noise)•10MHz bandwidthPhase measurement: •Resolution: 2p/1024 rad or 0.64 nm in double-pass (using 200MHz clock )•Max. Sampling Frequency: Fsmax=200kHz•Electronic noise < 0.64nm rms for Popt>100nW per arm and V=70%•Accuracy (contribution of photodetection & phase measurement chain):

2p/800 rad or 0.8 nm in double-pass for Popt>20nW per arm, V=70% and 50kHz Bandwidth

OPD MeasurementsReliable L and DL Meas. along 520m arm length(return way) during 30min (PRIMA integration time)Polarisation measurements (return way, l=1319nm )TVLTI(s)=17.1% ; TVLTI(p)=19.4% ; fp- fs =10.5 deg


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GENIE and PRIMAGENIE and PRIMA

GENIE can benefit from PRIMA technology

GENIE requirements for OPD control accuracy (closed loop):

– 20 nm RMS for nulling in N band

– 5–7 nm RMS for nulling in L band (preferable because of background in N)

Need for a very fast OPD control systemfast OPD control system

Strongest benefit: Fringe Tracking on Reference StarFringe Tracking on Reference Star


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GENIE and PRIMA (2)GENIE and PRIMA (2) Two-stage fringe tracking concept:

“Classical” VLTI fringe tracking loop, plus aFast “GENIE fringe tracking loop”

Sensor:PRIMA FSU for both loops, plusPRIMA Laser Metrology for GENIE loop to monitor OPD,

e.g., due to telescope vibrations Actuator:

Main DL for classical VLTI loopFast GENIE DL for GENIE loop; 2 possibilities:

GENIE-internal delay line (Piezo actuator) PRIMA DDL with up-graded performance


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Based on Recommendations of the Ad-Hoc Committee for VLTI (Tiger Team, Nov 2001)

– Phase 1Phase 1:: 2002-2004 Astrometry (10 µas, Band K) and Phase Referencing Imagery (Bands H, K, N) with 2 ATs (ML 15/12 in K, 8 in N), . [2 STS AT, MET, FSU A/B, CTL S/W]

– Phase 2Phase 2:: 2005-2008 Phase Referencing Imagery (ML 20/15 in K, 11 in N),

and Astrometry with 2 UTs (50 µas, K Band)[2 STS UT, upgrade MET & CTL]

– Phase 3Phase 3:: 2008-2010 Phase Referencing Imagery and 10 µas Astrometry with any pair of UTs in Band K and H [2 more STS UT, 4 DDL, Upgrade MET & FSU A/B & CTL]

PRIMA Three PhasesPRIMA Three Phases


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Phase 1(only)

– Kick Off June 2002 (low level kick Off STS, FSU June 2002)– PDR January 2003 (low level PDR STS, FSU, MET Dec 2002)– PDR CTL S/W May 2003– FDR July 2003 (low level FDR STS, FSU, MET June 2003)– FDR S/W December 2003– PAE STS, FSU, MET November 2004– Assembly and Commissioning on site Q1 and Q2 2005– PRIMA Operation May 2005

PRIMA PlanningPRIMA Planning


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– PRIMA will enable VLTI to perform high precision narrow-angle astrometryhigh precision narrow-angle astrometry down to the atmospheric limit of 10 μas10 μas, real imaging of objects fainter than K~14imaging of objects fainter than K~14 and nullingnulling at contrast levels of ~10-4.

– The first goal is to implement 10 µas astrometry in 200410 µas astrometry in 2004, in order to search for exoplanets. In parallel, ESO should make sure that the first images of extragalactic objects can be obtained and that the necessary actions be taken to get to faint objects in 2006faint objects in 2006.

– PRIMA is driven by the scientific objectives and competitive situation, but it also provides for the most logical sequence of technical developments.

– GENIE can benefit on PRIMA developmentGENIE can benefit on PRIMA development : FSU, DDL, MET, STS UT.

ConclusionsConclusions

Documents

Tuesday June 4, 2002 Frédéric Derie, Françoise Delplancke, Andreas Glindemann, Samuel Lévêque,