ESO/PAOS progress meeting Leiden, 29 September, 2004 Fringe Sensor Unit S. Menardi, ESO.

ESO/PAOS progress meetingLeiden, 29 September, 2004

Fringe Sensor Unit

S. Menardi, ESO

Leiden, 29 September 2004

ESO/PAOS progress meeting

Overview (1)

Star Separator 1(STS1)

PS SES PS SES

Baseline, B

s1s2

s1s2

Telescope T 1

(UT or AT)

Metrology end point E2Metrology end point E1

Star Separator 2(STS2)

Delay Line 1(DL1)

Beam A1

(SES)Beam B1

(PS)Beam B2

(PS)Beam A2

(SES)

Delay Line 2(DL2)

Differential Delay Line 1B (DDL1B)

Differential Delay Line 2B (DDL2B)

Fringe Sensor Unit B (FSUB)

MetrologySystem

Fringe Sensor Unit A (FSUA)

(or MIDI,or AMBER)

Differential Delay Line 1A (DDL1A)

Differential Delay Line 2A (DDL2A)

OPD Controller

Δ OPDController

ΔL

OPDPS mod λ0, GDPS

OPDSES mod λ0, GDSES

ΔGD

Telescope T2( )UT or AT

OPDPS mod λ0, GDPS, APS

OPDSES mod λ0, GDSES, ASES

data storage

data storage

data storage

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 λ0]

• Group Delay

• Fringe Amplitude



Overview (2)

achromaticλ/4

compensator light

from T1

light from T2

BC

p1 & s1

p2 & s2τp1 + ρp2 τs1 + ρs 2

|ϕp2 - ϕs2| = 90°

PBS

τp1 + ρp2

PBS

τs1 + ρs 2

ρp1 + τp2

ρs1 + τs 2

ρp1 + τp2 ρs1 + τs 2

Φ0

Φ2 = Φ

0 + π

A

C

Φ1 = Φ

0 + π/2

Φ

CkΦ

Φ3 = Φ

0 + 3π/2

D

B

FSU operating principle



Overview (3)

Alignment System

IRACE

BeamCombiner

PolarisingBeamsplitters

Single-modefibers

(Spatial filters)

Re-imagingOptics

Dispersiveelement

B

C

D IR ArrayDetector

Cryostat

PRIMAMetrology

(1319 nm laser)

PRIMAMetrology

(1319 nm laser)

A

ShutterSystem

FSU LCU 2

PRIMA WS

FSU LCU 1

OPD Controller

FSU main components



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



Overall Configuration (1)

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




Shutter System and Alignment System



K-PRISM and Compensator Assembly




Beam Combiner Assembly





Polarising Beamsplitters, Doublets and Fibers




K + H

band



Dimensions in mm

FSU Optics (1)

Glass compensatorGlass compensator

Description:Description: Infrasil® plano parallel plates with suitable thicknesses

Task:Task: Compensation of LAD introduced by differential air path (+/- 120 meters, 5 x 48 m regions)



FSU Optics (2)

Alignment Unit MirrorsAlignment Unit Mirrors

Description:Description: 2 actuated flat mirrors on each beam(2 x 5 degrees of freedom)

Task:Task: Alignment of input beams (pupil, image, OPD)w.r.t. VLTI artificial source Leonardo

Dimensions in mm



FSU Optics (3)

Achromatic Retarder and CompensatorAchromatic Retarder and Compensator

Description:Description: Retarder is a K-Prism (3 internal reflections)Compensator is a parallelepiped.Both in Infrasil®

Task:Task: Create a π/2 phase delay between p and s (Retarder) and Compensate for OPL inside the K-prism (Compensator)

Dimensions in mm



FSU Optics (4)

Beam CombinerBeam Combiner

Description:Description: Beamsplitter cube 50/50, linear polarisers @ 1319 nm in the 2.5 mm central area, wedges to reflect metrology laser in different direction.

Tasks:Tasks: combines both telescope beams, introduces a π 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).Dimensions in mm



Beam combiner

In1

In2

Tr1 Re2

X

Y

a

b

φ

-Beam Splitter

Cube

Wedged

Polarizer

Z

Y

Top view

c

d

-beam splitter

cubeWedged

Polarizer

clear

aperture

Side view

Wedged

Polarizer

h

α

α

φ

2Tr

1Re_ _s MET in

_ _p MET in

_ _p MET out

_ _s MET out

X

Y

β

β

Wedged

Polarizer

Wedged

Polarizer



FSU Optics (5)

Metrology interfaceMetrology interface

Description:Description: two holed mirrors, reflect stellar beams and transmit the metrology beams

Task:Task: Inject/extract metrology beams from thestellar beams path (in central obscuration)

Dimensions in mm

Metrology and stellar beams are common-modeMetrology and stellar beams are common-modeup to beam-combinerup to beam-combiner



FSU Optics (6)

H/K DichroicsH/K Dichroics

Description:Description: Dichroic coatings on both sides, Substrate with small wedge

Task:Task: splits H band (reflected) and K band (transmitted) and reject metrology laser light

Dimensions in mm

H + K + 1319 nmK1319 nmH

1319 nmH

K



FSU Optics (7)

Polarising beam splittersPolarising beam splitters

Description:Description: 2 PBS designed in K band, with 2 corner cuberetroreflectors to minimize thermal background(the fiber “sees” its own core)

Task:Task: splits p and s polarisation components

Dimensions in mm



FSU Optics (8)

Injection doubletInjection doublet

Description:Description: Achromatic doublet for fiber injection

Manufacturing:Manufacturing: Fused Silica and Zinc Selenide

Task:Task: Injection of K band in the optical fiber

Dimensions in mm



FSU Optics (9)

Optical fibersOptical fibers

Description:Description: Optical fibers for K band (LeVerre Fluore).Fiber positioners (New-Focus)

Manufacturing: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

Task:Task: Spatially filter the combined beams,transport flux inside the cryostat

Dimensions in mm



Cryostat



Cryostat (2)



FSU Optics (10)

Cold collimatorCold collimator

Description:Description: Achromatic doublet Fused silica and Zinc Selenide

Task:Task: Collimation of fiber output beams



FSU Optics (11)

Cold prismCold prism

Description:Description: Fused silica prism, Wedge ~ 12º,dichroic coatings

Task:Task: disperse A, B, C, D beamsreject λ>2.5 um (cold K filter)reject MET laser straylight (1/500)



FSU Optics (12)

Cold cameraCold camera

Description:Description: Single aspheric lens

Manufacturing:Manufacturing: Zinc SelenideFront surface is aspherical, rear surface is spherical

Task:Task: Projects dispersed spots on the array detector



Cryostat (3) Cold plate



Detection Algorithm Software Architecture

TAC Standard blockstacTIMBlock: IRACE Timing and Algorithm Scheduling Probe: Algorithm results storage by callback function , max rate 8 KHzMonitor: Real Time Display of last computed quantities up to 100Hz

Custom blockstacIRVMEBlock: manage the detector raw data in CDS and NDRO readout modetacOPDAmpBlock: produce OPD and squared Amps at up to 8 kHz tacGDBlock: implement the algorithm for GD estimate at user selectable rate up to 200HztacFluxBlock: 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.



Detector readout modes

time

time

Pixel Charge

RSTTrigger by TIM

DIT DIT

IRQ IRQ

Sequencenr loop Run-Time

TAC Algorithm Run-Time < Tr

Algorithm ResultsAvailable

RD1 RD2 RD1 RD2 RD1 RD2

RST

Tr= Readout Intervalmin Tr=0.125 ms

IRQ

DWT

time

time

RST RST

RD1

DIT DIT

IRQ IRQ

Sequencenr loop Run-Time

TAC Algorithm Run-Time < Tr

RD2

RDn-1RDn

RD1

RD2

RDn-1RDn

Algorithm resultsavailable

Pixel Charge

Trigger by TIM

Tr= Readout Interval

min Tr=0.125 ms

RDn

IRQ

DWT

Read-Reset-Read Multiple Non Destructive Readouts



Measurement algorithms: OPD

OPD estimate:

iterative procedure of least square fit of measured data sk to nominal data fk

First step: linear range identification

∑ −=

k k

kk xfszxe

2

2

1

))((),(

σ

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’ )(

)('1

n

nnn zh

zhzz −≅+

2' /)(2)( nnn xfxg σ⋅−= weight function

∑ ⋅=n

nn xgxsxl )()()( bias function

signature function )()()( xlxgsxhk

kk −⋅=∑Linear iterations required: 3

Template resolution: 1 - 5 nm



Simultation results Source = 3500 K

Evaluated vs. signal + background photon noise, read-out noise

OPD performance – Noise

Integration time [ms] 0.25 0.25 0.25 0.25 2 2

Assumed Readout noise [e rms] 11 11 11 11 4 4

Magnitude (UTs) [mag] 7 8 10 11 13 14

Req. on OPD noise [nm] 11.1 19.0 71.1 159 72 140

OPD noise, p = 0 m [nm] 6.2 11.2 44.5 109 46 98

OPD noise, p = 30 m [nm] 6.2 11.3 47.0 114 48 105

OPD noise, p = 60 m [nm] 6.8 12.3 50.8 124 49 110

OPD noise, p = 90 m [nm] 7.4 13.3 54.6 133 55 121

OPD noise, p = 120 m [nm] 8.5 15.3 61.6 163 64 147



OPD performance – Linearity

OPD mod λ measurement linearity - direct I/O comparison

%101 ≤−∂∂ddmRequirement:

OPD derivative – 1 Mean [%]

Peak [%]

RMS [%]

p = 0 m 0.000 0.063 0.028

p = 30 m 0.000 0.212 0.052

p = 60 m 0.000 0.158 0.044

p = 90 m 0.000 0.133 0.035

p = 120 m 0.000 0.114 0.026

Fulfilled from zenith to 120 m air path in delay line

Simulation results:



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 m, 6.1 m]

4067 cases uniformly distributed - data resolution 3 nm

Source: point-like, T = 3’500 K and T = 25’000 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:

z1±λ, z1±2λ, z1±3λ z2

adjust local minimum around z2



Group Delay – Noise

Integration time [ms] 5 5 200 200 2000 2000

Readout noise [e rms] 5.1 5.1 0.8 0.8 0.3 0.3

Magnitude UTs 10 12 14 16 16 19

Req. on GD noise [nm] 900 3300 800 1900 600 2300

GD noise, p = 0 m [nm] 5.8 346 5.8 398 4.7 641

GD noise, p = 30 m [nm] 5.9 370 6.0 383 4.9 551

GD noise, p = 60 m [nm] 68 504 237 592 132 698

GD noise, p = 90 m [nm] 500 799 771 811 796 917

GD noise, p = 120 m [nm] 1171 1208 1012 1104 964 1186

Simultation results Source = 3500 K

Evaluated vs. signal + background photon noise, read-out noise



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 m, 6.1 m]

Point-like source @ T = 3’500 K

%201 ≤−∂∂ddmRequirement:

GD derivative – 1 Mean [%]

Peak [%]

RMS [%]

p = 0 m 0.000 0.036 0.009

p = 30 m 0.000 0.033 0.009

p = 60 m 0.000 0.026 0.009


Template resolution: 5 nm



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 [-λ0/2, λ0/2]

Point-like source @ T = 3’500 K

Template resolution: 5 nm

2/,5.2 00 λ≤−≤− ddnmddm

Requirement: GD bias Mean [nm]

Peak [nm]

RMS [nm]

p = 0 m 0.000 0.014 0.006

p = 30 m 0.000 0.013 0.005

p = 60 m 0.000 0.010 0.004


Restricted GD range derived from recent definition of GD bias specification on central fringe



Group Delay – Bias

p = 0 m p = 60 m



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 band2% on single 100 nm spectral region

Required knowledge of the phase spectral distribution: 1º over full K band5º on single 100 nm spectral region



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 m 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 m fibre misalignment is acceptable



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.



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



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 λ



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 @ T = 800 K



FSU calibration (3)

On-sky calibration sequence for FSU B (FSU A tracking)

1. Configure both STSs in calibration mode (+ telescope pointing etc.)

2. Acquire and centre stellar fringes on FSU A and FSU B independently

3. Close fringe tracking loop on FSU A: DL A driven by FSU A

4. Reset PRIMA metrology

5. 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

8. Fourier transform of FSU B output

9. Removal of source spectrum

10. Computation of transmission (modulus) and phase (argument) distributions

Procedure verified vs. photon + readout noise on FSU B



FSU calibration (4)Plot of Fsu output A, 100 m OPD scan, T = 6000 K source

Requirements for good spectral sampling (31 points) : 300 m scan

Exposure requirements by Montecarlo evaluation of noise on measured transmission (req. < 0.5%) and effective λ (req. < 0.5e-4)

K = 10 mag, TI = 400 ms 240 s total (100 ms OPD step actuation)

Nearly independent from source spectral type



On-line diagnostics

Colour index

Spectral resolution of FSU detection system photometric diagnostics

Spectral changes in measurement conditions: channel balance variation

...,11111,2

13100 DCBAI

I

IICI +++=

−=

Colour index variation in 7 transmission perturbation cases (sensitivity analysis) and cold camera alignment, 1 s equivalent integration:

Colour index Mean nominal

RMS Mean perturbed

Variation [σ]

1 T 0.92 0.015 0.97 3.15

2 T 0.92 0.013 0.96 3.03

3 T 0.92 0.015 0.93 0.66

+1 μm 0.92 0.015 -2.65 -241.14

-1 μm 0.92 0.015 4.44 237.64



SVF Configuration:

System Verification Facility (Alenia)



Collimator

FSU Calibration Facility (Paranal)

Fiber Head



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

Achieved milestones and next steps

ESO/PAOS progress meeting Leiden, 29 September, 2004 Fringe Sensor Unit S. Menardi, ESO.

Documents

Transcript of ESO/PAOS progress meeting Leiden, 29 September, 2004 Fringe Sensor Unit S. Menardi, ESO.