Fringe Sensor Unit

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• Fringe Sensor Unit

ESO/PAOS progress meeting Leiden, 29 September,
2004
S. Menardi, ESO
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Overview (1)

• Scope of the contract with Alenia development of
  FSU A and B operating in K band with provisions
  for H band.
• The FSU combines the light of each object
  (Primary Star and Secondary Star), collected by
  two VLTI telescopes and delivers and records
  measurements of
• Optical Path Difference modulo ?0
• Group Delay
• Fringe Amplitude

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Overview (2)
FSU operating principle
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Overview (3)
FSU main components
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Project organisation

• Alenia Spazio, prime contractor
• opto-mechanics design and procurement
• system engineering
• system assembly, integration and verification
• Osservatorio Astronomico Torino (OATo), main
  sub-contractor
• Cryostat design and procurement
• Measurement algorithms and performance analysis
• Software development (LCU level)
• ESO furnished equipment
• 2 x PICNIC detectors IRACE systems
• Control Electronics Hardware

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Overall Configuration (1)

• FSU-A and FSU-B Overview (K band only)

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Overall Configuration (2)
Shutter System and Alignment System
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Overall Configuration (3)

• K-PRISM and Compensator Assembly

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Overall Configuration (4)

• Beam Combiner Assembly

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Overall Configuration (5)
Polarising Beamsplitters, Doublets and Fibers
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Overall Configuration (6)
K H band
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FSU Optics (1)
Glass compensator
Task Compensation of LAD introduced by
differential air path (/- 120 meters,
5 x 48 m regions)
Description Infrasil plano parallel plates
with suitable thicknesses
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FSU Optics (2)
Alignment Unit Mirrors
Task Alignment of input beams (pupil, image,
OPD) w.r.t. VLTI artificial source Leonardo
Description 2 actuated flat mirrors on each
beam (2 x 5 degrees of freedom)
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FSU Optics (3)
Achromatic Retarder and Compensator
Task Create a p/2 phase delay between p and s
(Retarder) and Compensate for OPL
inside the K-prism (Compensator)
Description Retarder is a K-Prism (3 internal
reflections) Compensator is a
parallelepiped. Both in Infrasil
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FSU Optics (4)
Beam Combiner
Tasks combines both telescope beams, introduces
a p phase delay between transmitted and
reflected beams, combines both metrology
beams, reject unwanted polarisation
component of metrology beams, reject
metrology laser stray light (angular deviation).
Description Beamsplitter cube 50/50, linear
polarisers _at_ 1319 nm in the 2.5 mm central area,
wedges to reflect metrology laser in
different direction.
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Beam combiner
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FSU Optics (5)
Metrology interface
Task Inject/extract metrology beams from
the stellar beams path (in central obscuration)
Description two holed mirrors, reflect stellar
beams and transmit the metrology beams
Metrology and stellar beams are common-mode up to
beam-combiner
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FSU Optics (6)
H/K Dichroics
Task splits H band (reflected) and K band
(transmitted) and reject metrology laser light
Description Dichroic coatings on both sides,
Substrate with small wedge
K
1319 nm
H
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FSU Optics (7)
Polarising beam splitters
Task splits p and s polarisation components
Description 2 PBS designed in K band, with 2
corner cube retroreflectors to minimize thermal
background (the fiber sees its own core)
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FSU Optics (8)
Injection doublet
Task Injection of K band in the optical fiber
Description Achromatic doublet for fiber
injection
Manufacturing Fused Silica and Zinc Selenide
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FSU Optics (9)
Optical fibers
Task Spatially filter the combined
beams, transport flux inside the cryostat
Description Optical fibers for K band (LeVerre
Fluore). Fiber positioners (New-Focus)
Manufacturing Single-mode fiber in ZrF4. On the
cold side, the 4 fibers are glued in a metallic
block to from a square array (3 um accuracy). NA
0.17
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Cryostat
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Cryostat (2)
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FSU Optics (10)
Cold collimator
Task Collimation of fiber output beams
Description Achromatic doublet Fused silica
and Zinc Selenide
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FSU Optics (11)
Cold prism
Task disperse A, B, C, D beams reject ?gt2.5 um
(cold K filter) reject MET laser straylight
(1/500)
Description Fused silica prism, Wedge
12º, dichroic coatings
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FSU Optics (12)
Cold camera
Task Projects dispersed spots on the array
detector
Description Single aspheric lens
Manufacturing Zinc Selenide Front surface is
aspherical, rear surface is spherical
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Cryostat (3) Cold plate
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Detection Algorithm Software Architecture
TAC Standard blocks tacTIMBlock IRACE Timing and
Algorithm Scheduling Probe Algorithm results
storage by callback function , max rate 8
KHz Monitor Real Time Display of last computed
quantities up to 100Hz Custom
blocks tacIRVMEBlock manage the detector raw
data in CDS and NDRO readout mode tacOPDAmpBlock
produce OPD and squared Amps at up to 8 kHz
tacGDBlock implement the algorithm for GD
estimate at user selectable rate up to
200Hz tacFluxBlock provide the flux estimates
in the 3 spectral channels tacRTNBlock deliver
OPD, GD and squared Amps to the OPD Controller at
the rates applicable to each quantity.
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Detector readout modes
Read-Reset-Read
Multiple Non Destructive Readouts
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Measurement algorithms OPD
OPD estimate iterative procedure of least
square fit of measured data sk to nominal data fk
First step linear range identification
Selection of minimum error position among three
initial points (x1, x2, x3) in the fringe period
Subsequent steps iterations of zero crossing
estimate formula using tabulated functions f, g,
l, h
signature function
Linear iterations required 3 Template
resolution 1 - 5 nm
weight function
bias function
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OPD performance Noise

• Simultation results Source 3500 K
• Evaluated vs. signal background photon noise,
  read-out noise

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OPD performance Linearity
OPD mod l measurement linearity - direct I/O
comparison
Fulfilled from zenith to 120 m air path in
delay line
Requirement
Simulation results
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Measurement algorithms Group Delay
GD estimate by least square fit of measured data
sk to nominal data fk
Montecarlo simulation over GD range -6.1 mm,
6.1 mm 4067 cases uniformly distributed - data
resolution 3 nm Source point-like, T 3500 K
and T 25000 K Nominal FSU configuration -
Spatial template resolution 5 nm Fringe jumps
included (not removed)

• Implementation approach
• find local minimum of error in central fringe
  using OPD algorithm ? z1
• find global minimum by error comparison over 6
  fringes
• z1l, z12l, z13l ? z2
• adjust local minimum around z2

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Group Delay Noise

• Simultation results Source 3500 K
• Evaluated vs. signal background photon noise,
  read-out noise

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Group Delay Linearity
GD measurement linearity - direct I/O comparison
Noiseless FSU GD output computed for a set of
2500 input GD values, uniformly distributed in
the range -6.1 mm, 6.1 mm
Point-like source _at_ T 3500 K
Template resolution 5 nm
Requirement
Fulfilled from zenith to 60 m air path in
delay line
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Group Delay Bias
GD measurement bias - direct I/O comparison
Noiseless FSU GD output computed for a set of
1000 input GD values, uniformly distributed in
the range -l0/2, l0/2
Point-like source _at_ T 3500 K
Template resolution 5 nm
Requirement
Restricted GD range derived from recent
definition of GD bias specification on central
fringe
Fulfilled from zenith to 60 m air path in delay
line
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Group Delay Bias
p 60 m
p 0 m
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Sensitivity analysis
Method modify FSU parameters (A beam only)
evaluate FSU output variation
Required knowledge of the transmission spectral
distribution 0.5 on transmission over full K
band 2 on single 100 nm spectral region
Required knowledge of the phase spectral
distribution 1º over full K band 5º on
single 100 nm spectral region
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Sensitivity analysis warm fiber end alignment

• Questions How well shall A, B, C, D fibers be
  mutually aligned?
• What is the differential instant coupling
  efficiency (for a given misalignment)?
• Results
• Differential coupling efficiency for 1 mm
  misalignment
• 0.5 average/PTV, 0.1 RMS
• Conclusion
• Assuming a uniform distribution of transmission
  perturbation of 0.26 (independent for each
  fiber), the fraction of configurations
  exceeding the specified 2.5 nm peak GD error is
  below 5 (acceptable).

? 0.5 mm fibre misalignment is acceptable
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Sensitivity analysis Cold Camera Alignment
Fiber alignment along dispersion is critical, as
it affects the spectral response of the FSU A,
B, C, D channels.
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Sensitivity analysis Cold Camera Alignment (2)
Simulation 1117 GD values in -1.11 ?m, 1.11 ?m
- resolution 2 nm
Peak I/O discrepancy 2.42 nm RMS 1.26 nm
Linear discrepancy variation in apparent
effective wavelength Peak value compatible with
GD bias requirement Conclusion 1 ?m alignment
stability of cold camera is acceptable
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FSU calibration (1)
FSU Calibration procedure Rationale detailed
characterisation of instrument parameters Global
approach includes VLTI optical train and
average atmosphere Method FSU A B in
calibration mode, OPD scan (Fourier Transform
Spectroscopy) FSU A on Fringe Tracking loop, FSU
B measuring for self-calibration A ? B roles
(tracking / calibration) exchanged for
calibration of FSU A
Requirements FSU A B AT (UT) DL STS MET
known bright star Purpose FSU spectral
response Target effective l
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FSU calibration (2)
Source requirements knowledge / stability
temperature or K magnitude
Requirements more relaxed for higher temperature
sources Not unreasonable for coldest stars few
10 K, 0.02 mag Requirements on lab source 1 K
_at_ T 800 K
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FSU calibration (3)
On-sky calibration sequence for FSU B (FSU A
tracking)

• 1. Configure both STSs in calibration mode (
  telescope pointing etc.)
• Acquire and centre stellar fringes on FSU A and
  FSU B independently
• Close fringe tracking loop on FSU A DL A driven
  by FSU A
• Reset PRIMA metrology
• DL B driven by FSU A MET to cancel internal
  dOPD OPD scan offsets
• 6. FSU B outputs recorded during OPD scan
• 7. OPD noise on FSU A and on MET supposed to
  average down to nm level
• Fourier transform of FSU B output
• Removal of source spectrum
• Computation of transmission (modulus) and phase
  (argument) distributions

Procedure verified vs. photon readout noise on
FSU B
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FSU calibration (4)
Plot of Fsu output A, 100 mm OPD scan, T 6000 K
source
Requirements for good spectral sampling (31
points) 300 mm scan Exposure requirements by
Montecarlo evaluation of noise on measured
transmission (req. lt 0.5) and effective l (req.
lt 0.5e-4) K 10 mag, TI 400 ms ? 240 s total
(100 ms OPD step actuation) Nearly independent
from source spectral type
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On-line diagnostics
Colour index Spectral resolution of FSU
detection system ? photometric diagnostics
Spectral changes in measurement conditions
channel balance variation
Colour index variation in 7 transmission
perturbation cases (sensitivity analysis) and
cold camera alignment, 1 s equivalent
integration
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System Verification Facility (Alenia)

• SVF Configuration

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FSU Calibration Facility (Paranal)

• Collimator

Fiber Head
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Achieved milestones and next steps

• Achieved
• Contract Kick-off July 2002
• Final Design Review September 2003
• Order of fiber bundle (critical long lead item)
  Feb. 2004
• First release of Ali LCU software and FSU WS
  software May 2004
• Cryostat acceptance tests and PICNIC detector
  integration Sept. 2004
• Next steps
• Finalise procurement of Beam Combiners
  (prototypes are available)
• Complete opto-mechanical mechanical integration
  (by end 2004)
• SVF and Calibration Faciliy manufacturing
• Acceptance testing, scheduled in March 2005
• Delivery of FSU A and B, in June 2005