Phase Referenced Imaging and Micro-arsec Astrometry (PRIMA) an introduction to technical description and development Tuesday 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 Garching München (Germany)

F. DerieGENIE Workshop Leiden Contents + + Introduction + + Main Scientific Objectives of PRIMA + + Overall Description and Principles + + Four Sub-Systems of PRIMA + + GENIE and PRIMA + + Development in three Phases + + Planning + + Conclusion

F. DerieGENIE Workshop Leiden Introduction n Dual feed capability n PRIMA is designed to make use of the Dual feed capability of the VLTI for both UT and AT and is compatible with VINCI, AMBER, MIDI n six magnitudes n Gain of about six magnitudes in H, K and N. n n PRIMA is designed to perform: –faint Object –faint Object Observation in J, H, K and N Bands –narrow angle astrometry –high accuracy (10 µas) narrow angle astrometry in band H or K, –aperture synthesis –aperture synthesis (phase reference imaging) and model constrained imaging in J, H, K and N n optical differential delay n In the astrometric mode, the prime observable is the optical differential delay between the object and a reference. n phase and amplitudevisibility n In Imaging mode, the observable are phase and amplitude of the object visibility. n n Observation on a number of baselines :minimum two for Astrometry, more for Imaging. n Star separator, Differential Delay Lines, Metrology, Fringe Sensor Units n PRIMA is composed of four major sub-systems: Star separator, Differential Delay Lines, Metrology, Fringe Sensor Units and one overall control system and software

F. DerieGENIE Workshop Leiden Main Scientific Objectives extra-solar planets Detection and Characterization of extra-solar planets and their birth environment. þ 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). nuclear regions galaxies Another key objective for VLTI with PRIMA will be the exploration of the nuclear regions of galaxies 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 .

F. DerieGENIE Workshop Leiden Main 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.

F. DerieGENIE Workshop Leiden Main 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:

7 PRIMA Diagram AT ? FSU A/B Delay lines

F. DerieGENIE Workshop Leiden Overall Description and Principles 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)

9 Main System Performance Faint Objects Observation Gain of about 6 Magnitudes Model Independent Imaging (Aperture Synthesis) Phase referenced imaging on faint objects with AMBER, MIDI, VINCI Visibility accuracy < 1% 10µas astrometry Phase and Group Delay measurement with FSU A and Metrology, Angular resolution < 10 µarcsec Baselines from 8 to 200m Limiting Magnitude with tracking with FSU B(in band K)

Main System Performance(con’t) Sky Coverage Probability to find a star of MK within field of x arcsec A. Robin Model Obs Besançon, stellar formation in the galaxy

11 Main System Performance(con’t) Anisoplanatic OPD and visibility loss n n 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. n n The visibility loss can be calibrated on reference stars. R 0 =1m R 0 =0.5m R 0 =1m R 0 =0.5m N K

12 Main System Performance(con’t) Optical Path Difference Tracking Residual OPD noise dominated by Fringe tracking noises (sensor and loop noises)linked to band pass and atmospheric turbulance OPD residual about 100 nm Current DL have 44.6 Hz Band Pass, OPD 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.

F. Derie13 Main System Performance(con’t) Total Observation Time Short exposure visibility and phase measurements averages incoherently, for Paranal typical atmospheric condition 18µas/hr, baseline 200m, 10” separation: astrometry3 hr per baseline 10µas astrometry accuracy : about 3 hr per baseline imaging40 min 1% accuracy in imaging : about 40 min per baseline and in K band Single Exposure Time

F. DerieGENIE Workshop Leiden STS Function/Performance n n The Star Separator is located at Coudé Focus of UT and AT, it is in charge of: -Coudé field of view up to 120 arcsec Ø -picking up 2 stars in the Coudé field of view up to 120 arcsec Ø, -80mm diameter -collimating them in two beams of 80mm diameter separated by 240mm, -field rotation -compensate for field rotation, -beam tip-tiltpupil transversal -adjusting and stabilizing the beam tip-tilt and the output pupil transversal position, -PRIMA calibration -allowing PRIMA calibration by injecting the same star in both feeds, - -transmitting the metrology and not interrupting it, especially at the calibration step. n counter-)chopping n A possibility of (counter-)chopping is also foreseen for the 10µm. n Tracking accuracy 10 mas and resolution 2mas n Tracking accuracy 10 mas and resolution 2mas, Output beam and pupil stability and OPD stability as for other VLTI sub-system

F. DerieGENIE Workshop Leiden DDL Function/Performance q q 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 mm - Range differential OPD 70 mm - accuracy 5 nm - OPD accuracy 5 nm - Stability14 nm RMS over 8 ms - Stability (OPD noise) 14 nm RMS over 8 ms - 8 kHz - Sampling Rate 8 kHz - Pupil relay

F. DerieGENIE Workshop Leiden FSU Function/Performance n n 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. n n The role of each FSU channel is to: n Detect the fringes n Detect the fringes (DL and DDL for the secondary object are scanning the OPD. n high ratefringe pattern position n Deliver at a high rate the fringe pattern position (phase delay and group delay) to the OPD controller, in charge of closing the various OPD control loops. n Store the measured n Store the measured data over the whole observation time.

F. DerieGENIE Workshop Leiden FSU Function/Performance (con’t) n imaging modeFSU BFringe Tracking n In imaging mode, FSU B performs the Fringe 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. n astrometric modeboth FSU n In astrometric mode, both 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 Function/Performance (con’t) co-axial beam combination ABCD algorithm The selected instrument concept uses co-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 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 Φ ΦkΦk Φ B CkCk A D C

F. DerieGENIE Workshop Leiden MET PRIMA Metrology monitorinstrumental optical path errors 5 nm accuracyover typically 30 min 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 anisoplanatism. The metrology system must measure the internal differential delay,  L, between both stars in both interferometer arms with a 5 nm accuracy requirement over typically 30 min. This accuracy requirement is driven by the PRIMA astrometric mode.  OPD =B.(S 2 - S 1  k   L

20 MET PRIMA Metrology Requirements

F. DerieGENIE Workshop Leiden MET Concept heterodyne q Two heterodyne interferometers with different heterodyne frequencies f 1 and f 2  Frequency offset  between the two interferometers for suppressing cross talks in calibration mode Direct  Direct measurement of  L after frequency mixing Super Heterodyne Laser interferometry:  L coded in phase of (f 1 -f 2 ) carrier signal No need for 2 independent high bandwidth and highly synchronised measurements of L 1 and L 2  Digital Phase meter using principle of time interval measurement with on-board averaging capability

MET Super-Heterodyne Laser interferometer I 1 (t) = cos(2πf 1 t +  1 ) I 2 (t) = cos(2πf 2 t +  2 ) I meas (t) = I 12 cos[2π(f 1 -f 2 ).t+  1 -  2 ) 1.1 µm (Si)< <1.45 µm (H band) Lc >> 500 m  < f   450 kHz f   650 kHz  38.65Mhz  38Mhz  40Mhz  39.55Mhz 0 f   650 kHzf   450kHz  f 2 =50kHz  f 1 =50 kHz 200 kHz Intensity noise Crosstalk noise  80MHz

23 MET Prototyping and testing Nd-Yag, 1319nm MHz +38.MHz MHz -40 MHz A.O.M’s ESO/IMT collaboration Beam Launcher/combiner (1 channel) 25mm

MET Testing at Paranal Objectives: Check the performance of the concept & prototype hardware under representative operating conditions Characterise Internal OPD fluctuations Check interfaces with VLTI Time [min] OPD [microns] Arm length= 520m (return way) Optical configuration in the VLTI optical lab

MET Main Results References éS. Lévêque,Y. Salvadé,R. Dändliker,O.Scherler, “High accuracy laser metrology enhances the VLTI”, Laser Focus World, April éY. 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 bandwidth Phase measurement: Resolution: 2p/1024 rad or 0.64 nm in double-pass (using 200MHz clock ) Max. Sampling Frequency: Fs max =200kHz Electronic noise 100nW per arm and V=70% Accuracy (contribution of photodetection & phase measurement chain): 2p/800 rad or 0.8 nm in double-pass for P opt >20nW per arm, V=70% and 50kHz Bandwidth OPD Measurements Reliable L and DL Meas. along 520m arm length(return way) during 30min (PRIMA integration time) Polarisation measurements (return way, l=1319nm ) T VLTI (s)=17.1% ; T VLTI (p)=19.4% ; f p - f s =10.5 deg

F. DerieGENIE Workshop Leiden GENIE and PRIMA n n GENIE can benefit from PRIMA technology n n 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) fast OPD control system  Need for a very fast OPD control system n Fringe Tracking on Reference Star n Strongest benefit: Fringe Tracking on Reference Star

F. DerieGENIE Workshop Leiden GENIE and PRIMA (2) * * Two-stage fringe tracking concept: + + “Classical” VLTI fringe tracking loop, plus a + + Fast “GENIE fringe tracking loop” * * Sensor: + + PRIMA FSU for both loops, plus + + PRIMA Laser Metrology for GENIE loop to monitor OPD, e.g., due to telescope vibrations * * Actuator: + + Main DL for classical VLTI loop + + Fast GENIE DL for GENIE loop; 2 possibilities: + + GENIE-internal delay line (Piezo actuator)   PRIMA DDL with up-graded performance

F. DerieGENIE Workshop Leiden Based on Recommendations of the Ad-Hoc Committee for VLTI (Tiger Team, Nov 2001) –Phase 1: –Phase 1: Astrometry (10 µas, Band K) and Phase Referencing Imagery (Bands H, K, N) with 2 ATs (M L 15/12 in K, 8 in N),. [2 STS AT, MET, FSU A/B, CTL S/W] –Phase 2: –Phase 2: Phase Referencing Imagery (M L 20/15 in K, 11 in N), and Astrometry with 2 UTs (50 µas, K Band) [2 STS UT, upgrade MET & CTL] –Phase 3: –Phase 3: 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 Phases

F. DerieGENIE Workshop Leiden 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 – –FDRJuly 2003 (low level FDR STS, FSU, MET June 2003) – –FDR S/W December 2003 – –PAE STS, FSU, METNovember 2004 – –Assembly and Commissioning on site Q1 and Q – –PRIMA Operation May 2005 PRIMA Planning

F. DerieGENIE Workshop Leiden –high precision narrow-angle astrometry 10 μasimaging of objects fainter than K~14nulling –PRIMA will enable VLTI to perform high precision narrow-angle astrometry down to the atmospheric limit of 10 μas, real imaging of objects fainter than K~14 and nulling at contrast levels of ~ –10 µas astrometry in 2004 faint objects in 2006 –The first goal is to implement 10 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 – –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 development –GENIE can benefit on PRIMA development : FSU, DDL, MET, STS UT. Conclusions