PRIMA Review

The PRIMA facility: Phase-Referenced Imaging and Micro-arcsecond Astrometry

Plan • PRIMA Principle • Scientific objectives • Physical limitations – Off-axis angle – Limiting magnitude

• Requirements – Group delay measurement accuracy – Fringe stabilisation

• • • •

Difficulties PRIMA system & sub-systems Observation / calibration / operation strategy Data reduction

PRIMA motivation • Main limitation of ground interferometers = atmospheric turbulence => – Fast scrambling of the fringes => snapshots => short integration time (~ 50 ms in K) => low limiting magnitude (VINCI => K~8 on UT) – Impossibility to measure the absolute position / phase of the fringes accurately • Fringe position (introduced OPD) astrometry • Fringe phase imaging

• Solutions: – “Adaptive optics for the piston term” => increase the limiting magnitude – Find a phase reference (as quasars in radio astronomy) => phase-referenced imaging and differential astrometry

u-v plane and reconstructed PSF • Image intensity: Iim(a) = IFT ( G(u1 -u2) ) (inverse the Fourier transform) with u1 -u2 = baseline vector and G = complex visibility • Good “synthetic aperture reconstruction” if good u-v coverage u-v coverage (UT 8 hours d=-15º)

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4 milli arcsec

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This is NOT the u-v plane

This IS the u-v plane Reconstructed PSF 8 milli arcsec K-band

Airy disk UT

Narrow-angle differential astrometry • Observe two stars simultaneously • Slightly different pointing directions => DOPD to be introduced in the interferometer, between the two beams to get the fringes

a

DOPD = B . sin a

DOPD T1

B

T2

• Moreover, the differential astrometric piston introduced by the atmosphere is several order of magnitude lower than the full piston => these perturbation (of the measured angle) average to zero rapidly ~ 30 min for 10” separation and 200 m baseline

Phase-referencing + astrometry Faint Science Object

Bright Guide Star DS < 60 arcsec

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Pick up 2 stars in a 2 arcmin field – bright star for fringe tracking – faint object / star

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DOPD = DS.B +  + OPDturb + OPDint – OPDint measured by laser metrology – OPDturb mean tends to 0 – DOPD measured by VINCI / AMBER / MIDI / FSU – DS => object position => astrometry –  => object phase => imaging

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complex method but very powerfull – many baselines => many nights

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synthetic aperture imaging @ 2mas resolution astrometry @ 10 mas precision

OPD(t) OPD(t)

B

OPD(t) -OPD(t) = DS B +  + OPDturb + OPDint

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The scientific objectives • General • Stellar environments – young stars – evolved stars – binaries

• AGNs • Planets => – differential astrometry – gravitational microlensing

PRIMA goals • 3 Aims: – faint object observation (by stabilizing the fringes) • dual-feed / dual-field : 2’ total FoV (2” FoV for each field) • K= 10? 13 (guide star) - K= 18? 20 (object) on UTs • K= 10 16 (object) on ATs 8? (guide star) - K= 15?

– phase-referenced imaging • accurate (1%) measurement of the visibility modulus and phase • observation on many baselines • synthetic aperture reconstruction at 2 mas resolution at 2.2 µm and 10 mas resolution at 10 µm

– micro-arcsecond differential astrometry • very accurate extraction of the astrometric phase: 10 µas rms • 2 perpendicular baselines (2D trajectory)

Scientific objectives - imaging Accretion disks / debris disks Structures of 1AU scale can be observed: - up to 1kpc at 2.2 µm and - up to 100 pc at 10 µm See O. Chesneau’s & F. Malbet’s talks Lynne Allen and Javier Alonso

Stellar ~1 magnetosphere Accretion disk

radius (AU) ~50 ~100

Planetesimals

Scientific objectives - AGNs • •

• •

Observation of central core elongation, jets, dust torus... Currently ~7 objects observable with MIDI (e.g. NGC 1068), 0-1 with AMBER With PRIMA: hopefully >~50 with each => better sample, better spectral coverage See W. Jaffe’s talk

Jaffe et al. (2003)

Scientific objectives: Sgr A* • IR imaging of the matter around the black hole (see J-U. Pott’s poster) • 10 µas astrometry of the stars in the central cusp • See J-U. Pott’s and H. Bartlo’s talks

Distance R0 = 7.62 +/- 0.32 kpc

QuickTime™ and a YUV420 codec decompressor are needed to see this picture.

Scientific objectives: GC flares •

10 µas astrometry of the galactic center flares – PRIMA can only give partial information on them (1D measurements 1 baseline) – if PRIMA can reach the appropriate limiting magnitude (UTs needed, also because of confusion) and accuracy in 30 min (time scale of flare) – a better instrument for it would be Gravity courtesy: F. Eisenhauer (MPE)

Scientific objectives: planets

G. Marcy

• Reflex motion of the star due to planet presence • Wobble amplitude proportional to: – planet Mass – ( star mass )-2/3 – ( planet period )2/3 – 1 / distance to the star – amplitude does not depend on orbit inclination • Complementarity with radial velocities: – better for large planets at large distances – not sensitive to sin(i) – applicable to (almost) all star types

• Need of long-duration survey programmes to characterise planets far from the star • Need to maintain the accuracy on such long periods ! • See R. Launhardt’s talk

Scientific objectives: micro-lensing •

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Difference in amplification on both images => – displacement of total photocenter Example: M = 10 Msun, impact parameter = 1mas, rE = 3.2 mas – maximum photocenter displacement = 1.2 mas – NOT maximum at closest approach In case of planet around the lens:

Einstein radius = 3.2 mas lens source

– secondary photometric peak and – more complex shape (3 to 5 images) => imaging and astrometry

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But has to work on alerts & needs high limiting magnitude (K~15-16 on secondary object)

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The physical limitations and The scientific requirements • Physical limitations (more in M. Colavita’s talk) – Atmospheric anisoplanatism – Sky coverage

• Scientific requirements – OPD accuracy for imaging / astrometry – OPD stabilization for fringe tracking

Atmospheric anisoplanatism 1 slope -2/3

Kolmogorov spectrum

slope -8/3 slope +4/3

Balloon measurements at Paranal slope -2/3 slope -17/3

slope -8/3

Seeing = 0.66” at 0.5 µm

 = 10 ms at 0.6 µm

Atmospheric anisoplanatism 2 • Off-axis fringe tracking anisoplanatic differential OPD  OPDmeasurement  370.B 2 / 3.

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 Tobs

for narrow angles ( < 180” UT or 40” AT) and long total observation time Tobs >> ~100s

for Paranal seeing = 0.66” at 0.5µm, 0 = 10 ms at 0.6µm (L. d’Arcio) Factor = 300 for Mauna Kea (Shao & Colavita, 1992 A&A 262)

– Increases with star separation – Decreases with telescope aperture (averaging) – High impact of seeing quality

• Translates into off-axis maximum angles to limit visibility losses (< 50 to 90%): – K-band imaging (2 µm)

  2 V = V0.exp2. . residual_ OPD       

• Bright fringe guiding star within 10-20”

– N-band imaging (10 µm) • Bright fringe guiding star within 2’

Anisoplanatism AT

Anisoplanatism UT

Sky coverage • Sky coverage limiting magnitude

Accuracy requirements • Phase-referencing measurable: difference of group delay DOPD = DS.B +  + OPDturb + OPDint Fringe sensor astrometry imaging

atmosphere Internal metrology

• Astrometric requirement – For 2 stars separated by 10” - 0.8”seeing - B=200m => Atmosphere averages to 10µas rms accuracy in 30 min – 5nm rms measurement accuracy

• Imaging requirement => – dynamic range is important (ratio between typical peak power of a star in the reconstructed image and the reconstruction noise level) – DR ~ √M .   D where M = number of independent observations – DR > 100 and M=100 D   < 0.1 60nm rms in K

• Ability to do off-axis fringe tracking

The problems / difficluties More in M. Colavita’s talk • • • • •

Air refractive index (ground based facility) Phase reference stars and calibrators Time evolving targets Fringe tracking is not easy Other instrumental problems

Dispersion and H2O seeing • Transversal & longitudinal dispersion • Fringe tracking and observation at different  • Air index of refraction depends on wavelength => – phase delay ≠ group delay – group delay depends on the observation band – fringe tracking in K does not maintain the fringes stable in J / H / N bands

• Air index varies as well with air temperature, pressure & humidity – overall air index dominated by dry air – H2O density varies somewhat independently – H2O effect is very dispersive in IR (between K and N)

• Remedy: spectral resolution

Refractive index of water vapor (©R. Mathar)

H-band L-band

K-band

N-band

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Proper phase references • We want to do imaging => – usually the scientific target is faint => • Reference star must be bright (K High probability that your guide star is: • resolved => low visibility • with resolved structures => non-zero phase

• Phase-referencing cannot disentangle between target phase and reference phase • Remedies: – baseline bootstrapping – characterize your reference star (stellar type, spectrum, interferometry) as much as possible prior to observation – find a faint star close to the reference one to calibrate it

Time and evolving targets • Phase-referencing works with 2 telescopes at a time => Measurements of different u-v points are taken at different epochs • Changing the baseline takes time (one day but not done every day) • If the object evolves, it is a problem • Remedies: – relocate more often (but overheads increase) – if the “evolution” is periodic (Cepheid, planet), plan the observations at the same ephemeris time – have more telescopes and switch from one baseline to another within one night

• No snap-shot image like with phase cl osure but better limiting magnitude

Fringe tracking problems • See Monday’s talk • Injection stability:

Solutions:

 fast tip-tilt sensing close to the instrument – Use of monomode optical  optimize injection before starting fibers as spatial filter => wavefront corrugations and  affects limiting magnitude and efficiency tip-tilt are transformed into or you accept a not-perfect fringe lock photometric fluctuations – Strehl ratio is not stable at 10 ms timescales – To measure fringes with enough accuracy for fringe tracking, one needs ~ 100 photons at any moment

• Telescope vibrations: – fast and strong sinusoidal variations of OPD – difficult to correct with the normal OPD loop

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“Vibration tracking” (predictive control) “Manhattan 2” (accelerometers) laser metrology active / passive damping

Other instrumental problems • Baseline calibration: – baseline should be known at better than < 50µm – experience on ATs: • calibration at better than 40µm • stability ca be better than 120µm

– dedicated calibrations are needed – stability with time and telescope relocation to be verified

• Telescope differential flexures: – not seen by the internal metrology – their effect on dOPD must be very limited or modeled – differential effect of 2nd order (2 telescopes - 2 stars)

• Mirror irregularities & beam footprints – non-common paths (metrology/star) to be minimized – bumps on mirrors should be avoided and mapped

PRIMA Facility • PRIMA general scheme • Sub-systems – Star Separators – Differential Delay Lines – Fringe Sensor Units – Calibration source MARCEL – End-to-end Metrology – Control Software and Instrument Software (PACMAN)

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PRIMA Scheme

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SES

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Baseline, B Telescope T1

Metrology end

Metrology end

Star Separator 1

Telescope T2

Star Separator 2 OPD Controller System

Delay Line 1

(tracking)

Delay Line 2

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B, LgB, AB Differential Delay Line

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Fringe Sensor Unit B (PS)

Differential Delay Line

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B, LgB, AB Metrology System

DL Data storage

DL Differential Delay Line

(tracking)

Fringe Sensor Unit A (or MIDI or AMBER) (SES)

A, LgA, AA dOPD Controller System

Differential Delay Line

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A, LgA, AA

4 sub-sytems DS

PRIMA System

Instr.

Star Separators (2 AT & 2 UT) Fringe Sensor Units (2) PRIMA Metrology (1) Differential Delay Lines (4)

Star Separators • •

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Star separation: from PSF up to 2’ Each sub-field = – 1.5” (UT with DDL - AMBER & PACMAN) – 2” (UT without DDL - MIDI) – up to 6” (AT) Independent tip-tilt & pupil actuators on each beam 10Hz actuation frequency (could be pushed to 50 Hz) Pupil relay to tunnel center (same as UT) Chopping / counter-chopping for MIDI Star splitting for calibration step: 40% - 40% Star swapping for environment drift calibrations Symmetrical design for easing calibrations High mechanical & thermal stability But: many additional reflections (+8 on AT, +4 on UT) Installed on AT#3 and AT#4. Under commissioning.

Differential Delay Lines • • • • • • • • • •

To be used with PACMAN and AMBER, not with MIDI > 200 Hz bandwidth, < 350 µs pure delay Push the lab pupil to FSU (4m further than now) Very stringent requirement on pupil lateral motion Cat’s eye (3 mirrors, 5 reflections) 2 stage actuator (coarse step motor + piezo on M3) Internal metrology M3 can be actuated also in tip-tilt (pupil correction ?) under vacuum Preliminary Acceptance Europe: beginning of June

PRIMA Metrology • • • •

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Super-heterodyne incremental metrology ( =1.3µm) Propagation in the central obstruction, from the instrument to the STS (Retro-reflection behind M9) Output measurement (dOPD and OPD on one of the stars) written on reflective memory for the OPD/dOPD controller Laser frequency stabilization on I2 at d/ ~100nm fringe stabilization => 5nm measurement accuracy => 10-11 ratio to propagation length

• PRIMA challenges: – very complex system (reliability) – differences to be done cleanly – 10µas accuracy requires stability & data logging • • • •

PRIMA can control some things but not the environment need to measure / calibrate what is not controlled need to minimize by operation what cannot be calibrated need of adapted data analysis and reduction software (PAOS = PRIMA Astrometric Observation & Software) for long term trends

Critical PRIMA calibrations • Swapping beams (astrometry) => – is needed to reject longitudinal differential effects between both beams and to “zero” the incremental metrology – no interruption of PRIMA metrology is allowed

• Injected flux and fiber alignment => – no photometric channels is a weakness of the FSU – relative stability of the 4 FSU fibers has to be measured

• FSU / VLTI spectral calibration => – fundamental for the group delay bias / stability

• Baseline calibration => – to be known with an accuracy better than 50µm – dedicated observations / calibrations are needed

• Polarization calibration of the VLTI => – potential cyclic errors => dedicated observation mode

Examples of long term trends • Long term trends = effects than cannot be calibrated in advance nor measured with enough accuracy • Telescope repositioning - baseline calibration – Need to know the differential baseline at ~50µm for astrometry at 10µas level

• Telescope differential flexures not monitored by the PRIMA metrology – Currently: everything above M9 – Very difficult to model at nm levels

• Mirror irregularities & beam footprints – PRIMA metrology should follow as close as possible the star path

• Longitudinal dispersion of air in tunnels:

Astrometric PrEparation Software developed by the DDL-PAOS Consortium

Data Reduction Software and Analysis Facility

PAOS Consortium

• Pipeline – Correction of detector effects + data compression – Gives an approximate DOPD

• “Morning-after” off-line processing – Correction of daily effects (dispersion) using an “old” calibration matrix – Narrow-baseline calibration – Gives a better DOPD and angle

• Data Analysis Facility (end of 6-month period) – Fitting of long term trends & better fitting of daily trends – Computation of an accurate calibration matrix

• Off-line processing (end of 6-month period) – Idem as morning after but with updated calibration matrix

Conclusions • PRIMA is aimed at boosting VLTI performances (limiting magnitude, imaging) + bringing new feature (astrometry) • PRIMA is making VLTI more complex but brings also solutions to current problems • PRIMA challenges: – fringe tracking and limiting magnitude – long term stability

• Scientific objectives are worth the effort • ESO will provide tools to reduce data and prepare observations (see summerschool next year) • => do not be discouraged and enjoy the challenge !