Technologies for Precise Distance and Angular Measurements In Space M. Shao JPL.

Technologies for Precise Distance and Angular Measurements

In Space

M. Shao JPL

Technology and Flight Hardware Development of Optical Metrology

• Components and subsystems for precise distance measurements, applicable to SIM, and future relativity missions.– Moderate power lasers with long lifetimes.– Metrology source (freq shifters etc.)– Beam launchers (metrology gauges)– Types of errors at the picometer level

• Angular measurements at the microarcsec level, the SIM technology program. – End to end demonstration of micro-arcsec astrometric precision

• Astrometry as a tool to study dark matter in our galaxy, and the local group.

Metrology Source• Two major components

– Laser– Frequency shifters and fiber

distribution systems• Laser

– SIM’s laser is a NPRO diode pumped YAG laser, designed with redundant pump laser diodes to achieve > 99.7% probability of working for 5 years in space. (SIM has a spare laser, )

• The acousto-optic frequency shifters provide the optical signals needed for heterodyne interferometry.

• The major activity here is not developing new technology but engineering components for flight.

Engineering model built and tested (Shake, thermal vac) in 2007.

Instrumental Errors in Long Distance Metrology

• Pointing /diffraction• Beam walk (imperfect optics)• Laser freq stability• Transmissive optics (dN/dT)

5

Pointing ErrorsIf the outgoing wavefront is not properly pointed at the other spacecraft the optical phase of the wavefront may not represent the distance between the two fiducials.

This is minized if the outgoing wavefront is spherical, centered on the fiducial.

Light hitting a retroreflector reverses the direction of the laser. The optical path measured is separation of the fiducials*cos(). A 10m distance and a 1 urad pointing error yields a 5 picometer distance error.

For very long distances, a collimated laser beam through diffraction will turn into a spherical wavefront. As a rough estimate, the pointing error applies to the path where the wavefront hasn’t become spherical. (D2/)

Defining a Retroreflector’s Vertex

The vertex of a CC is where the three planes intersect. The plane as defined by where metrology beam samples the CC. One has to be careful if we want a definition more precise than the fabrication of the surfaces. (/100 ~ /1000)

Cat’s eye retroreflector

If the footprint of the interrogating laser beam moves by 1% of the beam dia, and the surface is perfect to l/1000, one would expect the vertex position to be stable to ~/100,000

A cat’s eye retro will interrogate a few micron spot on the mirror at focus. The vertex definition is only as good as the quality of the surface.

Metrology Beam Launchers

• Beam launcher, designed with critical alignment components fixed on a zerodur optical bench.

• Launcher includes provision for pointing the beam with 1urad accuracy.

Engineering model built and tested (Shake, thermal vac) in 2007.

Laser pointing should be parallel toa vector joining the vertices of the two CC’s

Optical Fiducials in Optical Trusses

• Several missions make use of precise (sub nanometer) optical trusses. – SIM (optical truss to connect

several stellar interf)– Beacon (test of relativity)– LISA?– Optical trusses requires that

multiple lasers reference the same optical fiducial.

• Dual corner cube, optically contacted construction.

/20 p-v wavefront to 1mm/edge• Common vertex to ~1um• Measure vertex offset to ~1nm.

Precise Measurement of Angles Between Stars

External Delay – Internal Delay0

DetectedIntensity

An interferometer measures (B·s) the dot product of the baseline vector & a unit vector to the star,

The peak of the interference pattern occurs when [Internal delay] = [External delay]

Internal Path Delay

or, the projection of the star vector in the direction of the baseline

S

B

Delay line

Detector

Beam Combiner

telescope 2telescope 1

External Path Delayx = B cos()+C

x

SIM Technology FlowComponent Technology

Subsystem-Level Testbeds

Picometer

KnowledgeTechnology

Nanometer

ControlTechnology

Numbers before box labels indicate HQ Tech Gate #’s

(1 through 8)

System-Level

2: STB-3 (three baselinenanometer testbed)

3, 5, 6, 7: MAM Testbed (single baseline picometer testbed) Narrow & Wide Angle Tests

4: Kite Testbed (Metrology Truss)

STB-1 (single baselinenanometer testbed)

Optical Delay Line

1: Beam Launchers

HexapodReaction Wheel

Isolator

Metrology Source Absolute Metrology

High Speed CCD Fringe TrackingCamera

Multi-Facet Fiducials

1999

4:Oct2002

3:Sep2002; 5:Mar20036:Sep2003; 7:Jun2004

8:Jul2005

1:Aug2001

2:Nov2001

2001

1999

1998

1998

1998 2000

1999

TOM Testbed(distortion of front end optics)

8: Overall system Performance via

Modeling/TestbedIntegration

All 8 Completed

STB-3 on 9-meter Flexible Structure

The Micro Arcsec Metrology Testbed

IIPSMAM

Interferometer

Laser metrology measures the position of the IIPS.

Test is to compare metrology to whitelight (starlight) fringe position.

Wide Angle Astrometry

SIM goal is 4uas global astrometry (end of mission) Single epoch accuracy ~ 10uas.

Wide angle test sequence looks at ~60 stars over a 15 deg field of regard. (~1hr test)

Instrumental error vs position inthe field of regard.Met milestone ~4 uas error (end of mission)~10uas single epoch error.

Dominated by field dependent biases and thermal drift over 1 hr (versus 90sec for NA)

Narrow Angle Astrometry

1 uas total error0.7 to photon noise 0.7 to instrument0.5 to science interf

0.5uas ~25 pm

Meet 25pm in 8 chops

Each dot is an 8 chop average

MAM test: 4 ref stars, 1 target star, (T, R1, T, R2, T, R3, T, R4 …. Repeat)

~20 runs conducted over ~1 week.

Thermal Drift, 1/f type noise

• Thermal drift will change optical pathlengths. But most thermal drift on SIM is benign, because it’s accurately monitored by laser metrology. (accurate means accurate at the few picometer level)

• Astrometric errors occur when the alignment of the starlight and metrology light diverge. Since both starlight and metrology light are actively control, this happens when the alignment sensors in the ABC (astrometric beam combiner) move wrt each other.

• Dimensional instability (from thermal instability) of the ABC bench can cause star-light and metrology to diverge.

• ABC bench is a box within a box. The ABC enclosure is controlled to 10mK. The ABC optical bench inside the enclosure is stable to better than 1 mK.

Thermal Stability of the Lab Testbed vs Model of SIM on Orbit

0 10 20 30 40 50 60 70 80 90 100-0.03

-0.02

-0.01

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0.01

0.02

0.03

Time, hrs

Tem

pera

ture

, o C

SIM Thermal model of ABC Bench temperatures over 100 hrs

0 10 20 30 40 50 60 70-0.01

0

0.01

0.02

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Time, hrs

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Beam Combiner Bench Temperature profiles for SCDU (Measured) and SIM (Thermal model)

SCDU Sub-Bench (Measured)

SIM ABC Bench (Model)

Multi-100 node thermal model of SIM-(lite) in solar orbit executing an orange peel. Plot is temperature on the ABC bench.

Inside Testbed Vac Tanktemperature measurement

The MAM optics in the MAM vacuum chamber was reconfiguredand the testbed called SCDU. But the thermal properties of the chamberwere overall unchanged. (Shorter ~6hr allan variance data taken showed that the new setup is slightly better than before. The plot on prior page 2 over estimates the thermal error.

•We have two squiggly lines for thermal drift. How do we compare them? We compare their power spectra.•SIM in solar orbit is expected to be more stable than the inside of the MAM vacuum tank. (Thermal instability even in the MAM tank is not the dominant error/noise source.)•The reason chopped astrometry error goes as sqrt(T) is because we’re sensitive to the noise at ~0.01 hz, (90sec chop period). The rms error of a 1000sec integration of a chopped signal is roughly a 0.001hz bandwidth around 0.01hz.

Effect of Chopping on Thermal Drift

• While the drift of the starlight-metrology optical path can be quite large over long periods of time, the chopped signal only sees changes on a time scale of ~90 sec.

Instrumental Systematic Error

• Instrumental errors in the SIM testbed (chopped) does integrate down as sqrt(T)

– At least down to ~1 picometer after 1~2x105 sec

MAM testbed March 2006

Terrestrial Planet searchSingle epoch precision 1as

Systematic error floor~ 40 nanoarcsec

Summary• The SIM technology program has demonstrated the ability to

make precise angular measurements in space.• The activities have changed from (demonstrating it can be

done) to building engineering units that can survive launch loads and operate in space, with high reliability over many years. (A series of engineering milestones have replaced the technology milestones).

• In subsequent talks at this conference S. Majewswski ,and E. Shaya will talk about how they would use SIM to study Dark Matter in our galaxy and the local group.

• The components that have been flight qualified have uses in other space missions that test relativity. (Beacon will be discussed by B. Lane later today.)