Transcript Optical Metrology Progress on Technology and Flight Hardware

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)
Pointing Errors
If 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(q). A 10m distance and a 1
urad pointing error yields a 5 picometer distance error.
5
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/l)
Defining a Retroreflector’s Vertex
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 ~l/100,000
Cat’s eye retroreflector
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. (l/100 ~ l/1000)
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 to
a 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.
 l/20 p-v wavefront to 1mm/edge
• Common vertex to ~1um
• Measure vertex offset to ~1nm.
Precise Measurement of Angles
Between Stars
External Path Delay
x = B cos(q)+C
telescope 2
S
telescope 1
q
B
Detector
Detected
Intensity
Beam Combiner
0
Internal
Path Delay
Delay line
Dq @ Dx/B
An interferometer measures (B·s)
 the dot product of the baseline
vector & a unit vector to the star,
or, the projection of the star vector
External Delay
in the direction of the baseline
– Internal Delay
The peak of the interference pattern occurs when [Internal delay] = [External delay]
SIM Technology Flow
Component Technology
1999
Subsystem-Level Testbeds
4:Oct2002
2001
Metrology Source
System-Level
8:Jul2005
Absolute Metrology
4: Kite Testbed (Metrology Truss)
1999
Picometer
Knowledge
3:Sep2002; 5:Mar2003
6:Sep2003; 7:Jun2004
8: Overall system
Performance via
Modeling/Testbed
Integration
Multi-Facet Fiducials
Technology
Numbers before box
labels indicate HQ
Tech Gate #’s (1
1:Aug2001
1: Beam Launchers
1998
2000
3, 5, 6, 7: MAM
High Speed CCD
Fringe Tracking
Camera
Testbed
(single baseline picometer
testbed) Narrow & Wide
Angle Tests
TOM Testbed
(distortion of front
end optics)
through 8)
All 8 Completed
2:Nov2001
Nanometer
Control
Optical
Delay Line
1998
Technology Hexapod
Reaction Wheel
Isolator
1998
1999
STB-1 (single baseline
nanometer testbed)
2: STB-3 (three baseline
nanometer testbed)
STB-3 on 9-meter Flexible
Structure
The Micro Arcsec Metrology Testbed
Laser metrology measures the
position of the IIPS.
Test is to compare metrology to
whitelight (starlight) fringe
position.
IIPS
MAM
Interferometer
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 in
the 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
MAM test: 4 ref stars, 1 target star, (T, R1, T, R2, T, R3, T, R4 …. Repeat)
~20 runs conducted over ~1 week.
1 uas total error
0.7 to photon noise
0.7 to instrument
0.5 to science interf
0.5uas ~25 pm
Meet 25pm in 8 chops
Each dot is an 8 chop
average
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 starlight 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
Beam Combiner Bench Temperature profiles for SCDU (Measured) and SIM (Thermal model)
SIM Thermal model of ABC Bench temperatures over 100 hrs
0.08
0.03
0.07
0.06
0.05
0.01
o
D Temperature, C
Temperature, oC
0.02
0
SCDU Sub-Bench (Measured)
SIM ABC Bench (Model)
0.04
0.03
0.02
-0.01
0.01
-0.02
0
-0.03
-0.01
0
10
20
30
40
50
60
Time, hrs
70
80
90
Multi-100 node thermal model
of SIM-(lite) in solar orbit
executing an orange peel. Plot is
temperature on the ABC bench.
100
0
10
20
30
40
50
Time, hrs
Inside Testbed Vac Tank
temperature measurement
The MAM optics in the MAM vacuum chamber was reconfigured
and the testbed called SCDU. But the thermal properties of the chamber
were 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.
60
70
•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
Terrestrial Planet search
Single epoch precision 1mas
Systematic error floor
~ 40 nanoarcsec
MAM testbed March 2006
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.)