The Proposed Nova Fringe Tracker for the VLTI Jeff Meisner Leiden Observatory, Technical University...

The Proposed Nova Fringe Tracker for the VLTI

Jeff MeisnerLeiden Observatory, Technical University of Delft

Walter JaffeLeiden Observatory

Rudolf Le PooleLeiden Observatory, TNO Science & Industry

Perhaps you've heard this talk before at an interferometry conference?? I apologize, but......

A review: Why fringe-tracking?x

0) In order to adjust the delay lines in real-time, to allow for long coherent exposures on instruments otherwise suffering from readout noise. Sensitivity increase.

1) As a necessary component of a narrow-angle astrometric interferometer (such as one FSU of PRIMA)

2) To generate (in hardware adjustment of the delay-line and/or as an offline estimate) a phase reference for performance of coherent integration on a science target.

1 & 3: Using light from the science target itself (thus at a different wavelength)

1 & 2 & 3: Using an off-axis reference star with a dual-beam system <<< Most powerful technique(Also, variations of the NFT hardware have potential uses beyond simply fringe tracking, outside the scope of this talk.)

Is fringe-tracking “new”? You might have been lead to believe so.....Actually, first fringe tracking performed by the Mark I interferometer in 1980. Since then, “Fringe-Tracking” has gotten more difficult and more difficult to precisely describe!

Among the culprits: Wider bandwidths Larger apertures More ambitious interferometers

Timeline of NFT

2009: ESO calls for Phase-A studies to propose a concept for a Second Generation Fringe Tracker for the VLTI

Requirements: Accept beams from 4 or 6 telescopes (not just 2) either from science target itself or from off-axis reference star Measure phase for control of VLTI delay lines (fringe locking) for long coherent exposures & phase referenced imaging Sensitive to group delay for dispersion control and fringe-jump detection Tolerant of wavefront and photometric fluctuations Tolerant of different (possibly small) visibilities on some baselines Possibility of combining AT (1.8 meter telescope) with UT (8 meter telescope), 20x brighter! Rapid update rate possible (up to 2 KHz) with best possible limiting sensitivity (of course!)

Two well developed proposals submitted in response to the call by ESO:

• POPS (Planar Optics Phase Sensor) submitted by LAOG

• NFT (NOVA Fringe Tracker) submitted by the NOVA consortium

Concepts presented in May 2010 for ESO's consideration

Meeting at ESO in December 2011 to discuss and compare the concepts, with inconclusive results. It was generally agreed that: Theoretical sensitivity of NFT significantly higher. Technology used in POPS proposal already had been tested and verified in on-sky interferometry, unlike that of the NFT.It's understandable that ESO strongly wants to avoid accepting an instrument whose success is judged as “risky”It does not make sense that ESO would sacrifice the sensitivity of other VLTI instruments by selecting a less sensitive fringe tracker.

NFT Bulk optics interferometric implementation, using a novel beam-combination topology Only partial spatial filtering using pinholes, adaptable to conditions Ultra-wide-band (1.2 m – 2.4 m) Direct combination of 4 baselines, other two inferred Nominally measures single interference phase in order to generate delay-line correction

POPS Integrated optics beam-combination using a fiber-fed IO device

Full spatial filtering by virtue of using optical waveguide components IO chip built either for K or H band Direct combination of each of the 6 telescope pairs Measures both quadrature phases of interference

Similarities: Both use pair-wise interference. Both based on static measurement of interference phase (like PRIMA) rather than phase stepping/scanning (like FINITO). Both use low-resolution spectral dispersion to detect group-delay offset while co-phasing (again, like PRIMA!).

Differences:

Factors driving the design of the NFT:

Problem: a fringe tracker is needed to cophase 4 (or in the future 6) VLTI telescopes in order that the science instrument can coherently integrate (thus obtaining a good SNR on dim targets) and also to obtain visibility phase either with respect to an off-axis reference star or with respect to the target itself at the fringe-tracker's wavelength.

Therefore the ability to perform interferometry on a faint target is dependent on the fringe-tracker's ability to lock onto its phase or the phase of a reference star found nearby.

The limiting magnitude of an arbitrary interferometric instrument is the limiting magnitude of the fringe-tracker it uses!

NFT thus is maximized for sensitivity (not rms error, which isn't something you can independently specify).

Using dual feed system, interferometry on an arbitrary target is dependent on the availability of a nearby referencing star.

NFTK ~ 12

K band

Isoplanatic angle dependent on target observation wavelength

NFTK ~ 12

Portion of sky coverage, near galactic plane (from ESO):

N band

Achievement of highest limiting sensitivity (dimmest star on which reasonably reliable tracking can be performed) through: High throughput: - Use of free space optical paths rather than integrated optics - No need for spatial filtering (fiber injection losses) but still use pinholes for spatial mode reduction in order to maximize performance (as demonstrated by simulations)- Minimal number of non-cemented optical surfaces (AR coated)Resulting throughput calculated: 85%, guaranteed: 80% Minimize wavefront degradation before beam-overlap: - 3 reflecting and 3 transmitting surfaces including VLTI pickoff and PBS surfaceResult: calculated V=.977 at 2 m Minimal spectral resolution (but variable) of 4 or 5 spectral channels over 1.2 – 2.4 m, just in order to gauge dispersion and fringe-jumps In original configuration, use of only one of the two quadrature phases for interference! Full quadrature detection wastes half of the light measuring the real part of visibility which supplies no first-order information regarding OPD offset.(However doing this over a very wide band requires a more elaborate dispersion compensator than otherwise required). Partial quadrature detection available with 2011 hardware additions.

Likewise no optical power wasted on photometric monitoring which supplies no first-order information regarding OPD offset.-Variable photometric monitoring available with 2011 hardware additions. Slower detector readout rate, when possible (but ESO requests 1-2 kHz update rates).

Additional goals addressed by the NFT concept: Simplicity, low risk, based on standard optical components.

Essentially no moving parts after setup/alignment

Upgradability from 2 to 6 (or more) telescopes (next slides)

Flexibility, especially with latest additions:- Beam reconfigurability using local switchyard and polarization swapping.- Variable (partial) spatial filtering using selectable pinhole masks- Continuously variable spectral dispersion (low resolution) - Optional quadrature detection using variable amount of optical power- Optional photometric monitoring using variable amount of optical power

Achievement of photometric symmetry (detailed later) for tracking on:- Highly resolved (low V) sources- Fringes between an AT and a UT (20 x brighter)

The NFT topology is also notable for what it doesn't do:It does not try to measure visibility amplitude, thus not wasting photons on the second (unneeded) quadrature signal or photometric monitoring, or being dependent on a high degree of spatial filtering (however these are all options that can be dialed in using the 2011 hardware additions)

Incoming beams from M1 (pick-off)

PRS

M4 x1

M5 x1

Spectral prism (optional)Wollaston prismCamera lensDetector

fixed

Align, coarse OPD + piezo

/2 plates

Minimal configuration to verify components and performance on 1 baseline

fixed

align

align

pinhole

PBS

NFTUpgradability

Local Switchyard

Polarization Reversers(rotatable)

PRS M4's

M5's

Mask

Spectral prismWollaston prismCamera lensDetector

End-around combiner

Configuration for up to 4 telescopes.....

NFTUpgradability


Local Switchyard

Polarization Reversers

PRS M4's

M5's

Mask


End-around combiner

NFTUpgradability

All 6 channels now installed(unless you want even more!)


Further advantages of the technological choices adopted by the NFT: While beam combination is polarization-based, operation is less affected by input polarization mismatches than other topologies (including PRIMA). In particular, no SNR loss at all due to birefringence between vertical and horizontal polarizations (due to relay optics following telescopes), since each pairwise beam combination only uses one or the other. Beam overlap is performed very early in the optical chain, after which OPD shifts, vibrations, or poor optical surface quality has no effect on interferometric visibility or the measured phase of interference. Most of the optical train does not require interferometric precision, does not affect OPD result A 4 or 6 beam combining topology can be used at full efficiency with only N telescopes if N is even. Modest SNR loss if N is odd.

Simulations of NFT hardware performed.

• A basic linear fringe-tracking algorithm proposed, identical to what the analog electronics in the experimental setup did: Delay line motion = gain * Im{V} (too simple!).

• A complicated algorithm was developed (but this had not been a requirement of the original ESO call!) to examine the detector outputs in 2 or more spectral channels in orde to detect fringe jumps and order correction jumps. Also generates a judgement of “verified tracking intervals” needed in order for a science instrument to (properly) employ the fringe-tracker's data to the fullest. This was successfully simulated.

• Specifically in response to ESO, a method was developed for the detection of loss of tracking (actually several parallel indicators have been identified!), and successfully simulated.

The problem of “photometric crosstalk” due to a beam-combiner lacking photometric symmetry. This is a key issue addressed by the particular topology of the NFT.

An ideal 2 x 2 on-axis beam combiner:

Detector

+-

Detector

EA

EB E

1

E2

I2

I1

|V| cos()

Ideal 50/50 Beamsplitter

The problem of “photometric crosstalk” due to a beam-combiner lacking photometric symmetry. This is a key issue addressed by the particular topology of the NFT.

An actual beam combiner:

Detector

+-

Detector

Photometriccrosstalk

EA

EB E

1

E2

I2

I1

|V| cos()+ (I

A – I

B)

where:

= ½ (R - T)

Ideal 50/50 Beamsplitter

Visibility estimator I1 – I

2 inevitably includes a

photometric crosstalk term = (IA – I

B)

Naive approach to prevent photometric crosstalk:Force =0 by making the beamsplitter's T = R = ½

But: It will not generally be constant over wavelength It will almost always be different between the polarizations Moreover this is difficult/impossible to do in the first place! This can be seen to be a major problem with PRIMA (along with sensitivity to tunnel birefringence) even though the nominal design of the PRIMA FSU's was close to optimum.

Cancelling photometric crosstalk term using photometric pick-offs:Split off a fraction m of the incoming optical powers, and add them in the correct proportions to the interferometric outputs.

+-

A

I2

I1

m IA

m IB

Drawbacks:• Photometric monitoring beams rob power from the interferometric channels• Added photometric corrections m I

A and m I

B

contain their own detector noise, added to resulting visibility determination• Want cancellation in both polarizations and at all wavelengths: generally impossible!

OPDModulator

CoherentDetection

Estimate of complex VA

C B D(for instance)

Combating photometric crosstalk by chopping the phase (modulation of the OPD): This is the most common solution! For instance: ABCD phase stepping, or scanning of fringe packet Then the visibility appears as an AC signal on the photodetector. Just ignore the DC (and low frequencies) then. So measure |E

1+ E

2 exp(j )|2 + (I

A – I

B) as a function of (t), and the

photometric crosstalk term can be eliminated (if it stays constant!).

Drawbacks: Requires more than one detector readout to measure a visibility Since the photometrys and OPD are changing due to the atmosphere, these fluctuating components leak into the result. In order to reduce that effect, a faster readout speed is required, reducing sensitivity in a NIR instrument.

VV

HH

VV

OriginalPolarization

3s 4p

2s 3p

1s 2p

H 2sV 2p

V 3sH 3p

H 4sV 4p


PolarizingBeam Splitters

Polarization Recombining Stage (PRS) Consists of multiple Polarizing Beamsplitters (PBS) s polarization from telescope M is paired with p polarization of telescope M+1 into same spatial mode but a different polarization (thus a different mode) Thus the “polarization recombined beam” can proceed through various (non polarized) optical elements and both waves are affected identically Only when they finally reach the polarization analyzer are the two waves actually interfered and directed onto 2 (or more) photodetectors

TelescopeBeams

PolarizationRecombinedBeams

2

Important point:After the PRS, the “polarization recombined beams” no longer need to be treated according to interferometric standards. OPD variations/instability, wavefront degradation do not affect the visibility or rejection of photometric crosstalk!

1s 2p

45o Rotation

of Coordinate

System

A = 1s + 2p

B = 1s - 2p

Polarization Analyzer in rotated system

Differential Amplifier

+-

IA

Telescope 1

Telescope 2

Detector A

Detector B

Polarization Analyzer produces output beams implementing a balanced beamcombiner. When their powers are detected and differenced, photometric crosstalk is suppressed!

3

Detectors

IB

Beam from the polarization recombination stage (PRS) has the two telescopes' light in H and V polarizations

Analyze the polarizations in the A and B directions to get the opposite interference phases with equal photometric contributions!

Wideband interferometry over 1.2 - 2.4 microns simultaneously Spectrally resolved detection over 4 - 7 pixels (reconfigurable) Always at least 2 pixels over K band for dispersion detection Single beam combiner, all wavelengths fixed to same reference plane

Combines up to 4 (6) telescopes pairwise over 4 (6) baselines Not spatial filtering but spatial mode reduction (partial spatial filtering) for maximization of sensitivity

No fiber injection loss, hassle Wide (effective) visibility fluctuations tolerated

Basic operation: two-phase interferometric detection with fringe-locking OPD corrections based on Im{V} No need to use ½ the photons for measuring Re{V}, doubles sensitivity. Correction to OPD supplied every frame, no OPD modulation required However additional optical element inserted for detection of quadrature

phase in varying amounts. Based on the Polarization-Based Collimated Beam Combiner topology

Balanced beam combination, photometric crosstalk rejected Can track on low |V| sources, use stars well past 1st visibility null Combining AT with UT (20x brighter!): no problem

The NFT hardware design

1 meter

The NOVA Fringe Tracker (NFT)hardware overview

Local Switchyard

VLTI Beams

PolarizationReversers

PolarizationRecombiningStage

VLTI Beams

Backend:Insensitive to vibrations, optical surface errors, etc.

SpectrometerAnalyzerCameraDetector

4

3

2

1

PolarizationRecombining

Stage(PRS)

In NFT, Polarization Reversers implemented using (almost) achromatic ½ wave plates. Even channels: at 45o to reverse polarization Odd channels: at 0o, no change in polarization (but compensates for material dispersion) All are rotatable so their roles can be reversed

1

VLTI Beams frompick-off mirrors

LocalSwitchyard

Half wave plates(polarization reversing

in even beams)

M2 mirrors include very short strokeOPD adjustment + piezo, and motors for beam alignment

Local Switchyard


Polarization Reversers(rotatable /2 plates)

PRS M4's

M5's

Mask


End-around combiner

NFT: The entire optical train after VLTI pick-off (M1)

Back end of NFT,Not sensitive to OPD, No visibility loss possible due to alignment etc.

Monolithic prism block (like “integrated optics”)Reference point for beam superposition (OPD reference)

VV

HH

VV


3s 4p

2s 3p

1s 2p

H 2sV 2p

V 3sH 3p

H 4sV 4p


PolarizingBeam Splitters

Polarization Recombining Stage (PRS) Consists of multiple Polarizing Beamsplitters (PBS) s polarization from telescope M is paired with p polarization of telescope M+1 into same spatial mode but a different polarization (thus a different mode) Thus the “polarization recombined beam” can proceed through various (non polarized) optical elements and both waves are affected identically Only when they finally reach the polarization analyzer are the two waves actually interfered and directed onto 2 (or more) photodetectors

TelescopeBeams

PolarizationRecombinedBeams

2

Important point:After the PRS, the “polarization recombined beams” no longer need to be treated according to interferometric standards. OPD variations/instability, wavefront degradation do not affect the visibility or rejection of photometric crosstalk!

3

2

1

4

5

6

Constant OPD plane at 45o

2s+1p

3s+2p

4s+3p

5s+4p

s=V p=H

s=V p=H

s=V p=H

s=H p=V

s=H p=V

s=H p=V

1s

1s+(M)p

6s+5p

5s+4p

6p

End-around recombiner (set for M=4 telescopes)

NFT implementation of Polarization Recombining Stage for 6 telescopes, including end-around channelBased on giant prism block with 2 polarizing beamsplitting surfaces per channel

2

1s 2p

45o Rotation

of Coordinate

System

A = 1s + 2p

B = 1s - 2p

PolarizationAnalyzer inrotated system

A = 1s + 2p

B = 1s - 2p

WollastonPrism

CameraLens

DetectorArray

Detectors

Differential Amplifier

+-

IA

IA

IB

IB

Implementation in NFT:

Telescope 1

Telescope 2

Detector A

Detector B

Polarization Analyzer produces output beams implementing a balanced beamcombiner. When their powers are detected and differenced, photometric crosstalk is suppressed!

3

NFT Backend Implementation

ArrayDetector

PRS

M4: off-axis paraboloids

M5

Mask with turret of hole diameters at intermediate focus

Spectral Prism (zero-deviation)

Spectral resolution variable by shifting

Wollaston prism

(polarization analyzer)

Channel 1 2 3 4 5 6

blue

red

Possible layout of 2 spectra from each channel on 40 micron pixels of PICNIC detector.

All 6 polarization recombined beams pass through the same optics after diverging from their foci at the mask

(Not a spatial filter)

3

Augmented design for (optional) detection of both quadrature components, or photometric monitoring.

Involves insertion of an additional optical element in the backend beam.

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Original system: IA – I

B = Re{V}

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Detection of quadrature phase: IA – I

B = Im{V}

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Photometric detection IA = I

1 I

B = I

2

/4 plate at 0o

/2 plate at 22.5o

Modification of NFT beam combiner in order to allow for (partial) detection of quadrature visibility component and/or photometry

A = 1s + 2p

B = 1s - 2p

WollastonPrism

CameraLens

DetectorArray

IA

IB

Basic implementationIA – I

B = Re{V}

Augmented system:I'

A – I'

B = Im{V}

I''A = I

1

I''B = I

2

/4 plate with wedge

etc.

I'A

IA I

A

Proposed layout on PICNIC detector:

IB I

A etc.

IB I

B

/2 plate with wedge

I'B I'

A

Laboratory demonstration experimentImmunity to photometric crosstalk demonstrated in experiment conducted at TU Delft. Interference from He-Ne laser (left) detected using Polarization-Based Collimated Beam Combiner” topology, with added incoherent light (“photometric noise”) injected into each “telescope” beam from independently pulsating laser diodes. Cancellation of photometric noise by ratio >> 100:1 (stable over weeks) with intended interference signal retained.

Pulsating laser diodes

He-Nelaser

Piezo

Polarization analyzer (PBS) at 45o and detectors

“Telescope beam 1”

“Telescope beam 2”

Injected “Photometric noise”

Coherent “starlight” split toward both “telescope beams”

He-N

eL

aser

LD

Photodiode

Pho

todi

ode

Photodiodepreamps &

differencing circuit

PolarizingBeamsplitters

He-Ne Laser

Piezo

Stack

Laser Diodes (source of “photometric interference”)

Control loopfilter -

integrator

HVAmp

Scan

Lock

Detector assembly rotates about beam axis, nominally at 45o

(for testing)

Hot turbulent air from hair dryer

ATMOSPHERE

CORR

AnalyzerPBS

Detectors

+

PRS

NDFilterT = 3%

LD

Each detector output (left) contains an equal combination of the two photometric signals, but an opposite amount of the visibility signal!

A+B(photometry only) in red

A-B(photometry rejected) in white

Shown: actual recorded scope data of detectors (left) and sum and difference (right) while scanning the OPD. The “photometric noise” was much (~ 10x) stronger than the “starlight,” but is completely rejected in the difference signal. In locked mode (with feedback to the piezo, not shown) the same photometric rejection is present.

More experimental Results..... Inserted neutral density filter (T=.03) in one interferometric beam, simulating interference between a VLTI UT (8 meter telescope) and an AT (1.8 meter telescope). Still with laser diode mixed with each beam, pulsing at 250Hz and 1000Hz and hair dryer creating “atmospheric turbulence.” No noticeable change in tracking performance

Demonstration of insensitivity of polarization combined beam to modest optical disturbances which would be impossible if applied to one beam before the PRS Inserted a wine glass in the 15cm space between the PRS and the analyzer (beam passing through both sides of the glass) and wiggled it around. No noticeable effect on tracking, no fringe jump during entire period.


PRS

M4 x1

M5 x1

Spectral prism (optional)Wollaston prismCamera lensDetector (>= 2 pixels)

fixed

Align, coarse OPD + piezo

/2 plates

But ESO wasn't impressed by an interferometer using laser light (who would be?), so we'd like the next experiment to be one that works on starlight, perhaps using a stripped-down version of the NFT concept as shown here.

fixed

align

align

pinhole

PBS

Performance of NFT evaluated using detailed simulations.Same underlying atmospheric OPD (top, white trace) and wavefronts, but new realization of detection noiose

Signal at marginal level (70000 photons per second over K band at NFT input) but using 5ms (rather than 1ms) framerate, extends sensitivity ~ .5 mag

Performance of NFT according to simulations: Typical thresholding behaviour of performance vs. input SNR

K = 10.5 (assumes good seeing conditions, 1ms framerate)K = 12 <<< with upgrade to low-noise SELEX etc. detector

Augmented design for (optional) detection of both quadrature components, or photometric monitoring.

Involves insertion of an additional optical element in the backend beam.

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Original system: IA – I

B = Re{V}

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Detection of quadrature phase: IA – I

B = Im{V}

1s 2p

45o Rotation

of Coordinate

System


+-

IA

IB

Photometric detection IA = I

1 I

B = I

2

/4 plate at 0o

/2 plate at 22.5o

Modification of NFT beam combiner in order to allow for (partial) detection of quadrature visibility component and/or photometry

A = 1s + 2p

B = 1s - 2p

WollastonPrism

CameraLens

DetectorArray

IA

IB

Basic implementationIA – I

B = Re{V}

Augmented system:I'

A – I'

B = Im{V}

I''A = I

1

I''B = I

2

/4 plate with wedge

etc.

I'A

IA I

A

Proposed layout on PICNIC detector:

IB I

A etc.

IB I

B

/2 plate with wedge

I'B I'

A

So, adding it all up: The VLTI and its second-generation instruments (particularly MATISSE) requires an updated fringe-tracker in order to achieve its potential. The NOVA consortium submitted a concept which meets all of the requirements and achieves a high sensitivity (limiting magnitude). The proposal relies on standard optical components and techniques, is easily understood and modelled. The unique feature of the beam combiner has been tested in the laboratory and (unsurprisingly) works. A stripped-down version for only one baseline or otherwise simplified could be easily and relatively cheaply built and tested on the sky, for further verification. (A failure at that point wouldn't be an embarrassment for ESO ;-)

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The Proposed Nova Fringe Tracker for the VLTI Jeff Meisner Leiden Observatory, Technical University...

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