Transcript IMPLEMENTATION for VLTI PRIMA

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-85748 Garching München (Germany)
Email:[email protected]
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. Derie
GENIE Workshop Leiden 04.06.02
2
Introduction

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

Gain of about six magnitudes in H, K and N.

PRIMA is designed to perform:
– faint Object Observation in J, H, K and N Bands
– high accuracy (10 µas) narrow angle astrometry in band H or K,
– aperture 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 delay between the
object and a reference.

In Imaging mode, the observable are phase and amplitude of the object visibility.

Observation on a number of baselines :minimum two for Astrometry, more for Imaging.

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. Derie
GENIE Workshop Leiden 04.06.02
3
Main Scientific Objectives
Detection and Characterization of extra-solar planets and their birth environment.
 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 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. Derie
GENIE Workshop Leiden 04.06.02
4
Main Scientific Objectives
Astrometry allows the search for planets to
stars that cannot be properly covered by
the RV technique, (e.g. particular premain 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. Derie
GENIE Workshop Leiden 04.06.02
5
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:
F. Derie
GENIE Workshop Leiden 04.06.02
6
PRIMA Diagram
FSU A/B
Delay lines
?
AT
7
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)
F. Derie
GENIE Workshop Leiden 04.06.02
8
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
Range
Independent
Baselines
2 UT
45, 56, 60, 86, 99, 130m
1
3 UT
Same as
3
4 UT
Same as
6
2 AT/29 stations
8 to 200 m
259 (with 8 DL)
229 (with 6 DL)
Limiting Magnitude with tracking with FSU B(in band K)
Object in Band H or K
Object in Band N
Reference in Band K
UT
19
11.5 (no IR counterpart)
13/15
AT
16
8.3
10/12
9
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
Main System Performance(con’t)
Anisoplanatic 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.
N
K
R0=0.5m
R0=1m
R0=1m
R0=0.5m
Anisoplanatic OPD noise as a function of the angle between
the stars for the AT (left) for a wavelength of 2.2m.
Fringe visibility (normalised to V0=1) as a function of the
off-axis angle for the AT at 2.2m (K-band) and 10m (N-band).
11
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
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.
Normalised fringe visibility as a function of
the fringe sensor measurement noise, K-band
12
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:
10µas astrometry accuracy : about 3 hr per baseline
1% accuracy in imaging : about 40 min per baseline and in K band
Single Exposure Time
Variation of the residual OPD (left) and of the fringe visibility at 2.2m (right) as a function of the
integration time, for the AT (D=1.8m, B=200m) and UT (D=8m, B=120m) cases.
F. Derie
13
STS 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 Ø,
 collimating them in two beams of 80mm diameter separated by 240mm,
 compensate for field rotation,
 adjusting and stabilizing the beam tip-tilt and the output pupil transversal position,
 allowing PRIMA 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-)chopping is also foreseen for the operation @ 10µm.
Tracking accuracy 10 mas and resolution 2mas,
Output beam and pupil stability and OPD stability as for other VLTI sub-system
F. Derie
GENIE Workshop Leiden 04.06.02
14
DDL Function/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 mm
 OPD accuracy 5 nm
 Stability (OPD noise) 14 nm RMS over 8 ms
 Sampling Rate 8 kHz
 Pupil relay
F. Derie
GENIE Workshop Leiden 04.06.02
15
FSU Function/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:



F. Derie
Detect the fringes (DL and DDL for the secondary object are scanning the
OPD.
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.
Store the measured data over the whole observation time.
GENIE Workshop Leiden 04.06.02
16
FSU Function/Performance
(con’t)

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.

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.
F. Derie
GENIE Workshop Leiden 04.06.02
17
FSU FSU Function/Performance
(con’t)
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.
C
B
k
A
C
D
Φ Φ
Φ
k
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
MET PRIMA Metrology
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, DL, 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.
DOPD =B.(S2 - S1)+ f/k + DL
F. Derie
GENIE Workshop Leiden 04.06.02
19
MET PRIMA Metrology
Requirements
Range and accuracy
Max. propagation path (return way)
552 m
Individual OPD L1, L2(return way)
240 m
DL=L1-L2 range (=1 arcmin)
60 mm
DL Accuracy Goal (µas  accuracy)
< 5 nm
DL Resolution
< 1 nm
Main dynamic phase variations ( = 1 µm)
on individual OPD
Tracking of DL & STS (L
Typical value
t
= 11 mm/s)
OPD correction (atmospheric, “Internal”)
22 kHz
“Slow” in comparaison
on differential OPD
Tracking of DDL & STS (DL
Slewing of DDL & STS (DL
t
t
= 10 µm/s)
= 15 mm/s)
20 Hz
30 kHz
20
MET Concept
Super Heterodyne Laser interferometry:
 Two heterodyne interferometers with different heterodyne frequencies f1 and f2
 Frequency offset Dn between the two interferometers for suppressing cross talks in calibration mode
 Direct measurement of DL after frequency mixing
 DL coded in phase of (f1-f2) carrier signal
No need for 2 independent high bandwidth and highly synchronised measurements of L 1 and L2
 Digital Phase meter using principle of time interval measurement
with on-board averaging capability
F. Derie
GENIE Workshop Leiden 04.06.02
21
MET Super-Heterodyne Laser
interferometer
Telescope 1
VLTI
Optical
Train
Acousto optics
Telescope 2
1.1 µm (Si)<<1.45 µm (H band)
Lc >> 500 m
D/ < 10-8
n+38.65Mhz
n40Mhz
f1 650 kHz
f2 450kHz
0
f1 650 kHz
200 kHz
Df2=50kHz
Photodetection &
Amplification
n+38Mhz
n39.55Mhz
Intensity
noise
Nd-Yag Laser
Ref.1 Ref.2 Meas.2 Meas.1
n
f2 450 kHz
VLTI
Optical
Train
Crosstalk noise
80MHz
Df1=50 kHz
BP
650
kHz
BP
450
kHz
BP
450
kHz
BP
650
kHz
BP
200
kHz
BP
200
kHz
Lim.
Amp.
Lim.
Amp.
Ref.
Digital
Phase Meas.
Meter
I1(t) = cos(2πf1t + f1)
f 
4
n .L
1
c
f 
4
(n + Dn ) L
2
c
1
I2(t) = cos(2πf2t + f2)
2
Imeas(t) = I12cos[2π(f1-f2).t+ f1f2)
4n
4Dn
4n
f1  f2 
DL 
L2 
DL
c
c
c
MET Prototyping and testing
ESO/IMT collaboration
Beam Launcher/combiner (1 channel)
A.O.M’s
+38.65 MHz
+38.MHz
-39.55 MHz
-40 MHz
Nd-Yag, 1319nm
25mm
23
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
40
OPD [microns]
20
Arm length= 520m (return way)
0
-20
-40
-60
-80
0
Optical configuration in the VLTI optical lab
5
10
15
20
25
30
35 Time [min]
MET Main Results
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: 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 Measurements
Reliable 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
References
S. 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
GENIE 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 system
Strongest benefit: Fringe Tracking on Reference Star
F. Derie
GENIE Workshop Leiden 04.06.02
26
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. Derie
GENIE Workshop Leiden 04.06.02
27
PRIMA Three Phases
Based on Recommendations of the Ad-Hoc Committee for VLTI
(Tiger Team, Nov 2001)
– Phase 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 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 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]
F. Derie
GENIE Workshop Leiden 04.06.02
28
PRIMA Planning
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
F. Derie
May 2005
GENIE Workshop Leiden 04.06.02
29
Conclusions
– 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 ~10-4.
– 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 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 development : FSU, DDL, MET, STS UT.
F. Derie
GENIE Workshop Leiden 04.06.02
30