The Space Interferometry Mission, scheduled for launch in 2008, is an optical stellar interferometer with a 10 meter baseline capable of micro-arcsecond accuracy astrometry. A mission-enabling technology development program conducted at JPL, has yielded the heterodyne interferometric displacement metrology gauges required for monitoring the geometry of optical components of the stellar interferometer, and for maintaining stable starlight fringes. The gauges have <20 picometer linearity, <10 micron absolute accuracy, are stable to <200 pm over the typical SIM observation periods (~1 hour), have the ability to track the motion of mirrors over several meters. We discuss the technology that led to this level of performance: lowcross- talk, low thermal coefficient optics and electronics, active optical alignment, a dual wavelength laser source, and a continuously averaging, high data rate phase measurement technique. These technologies have wide applicability and are already being used outside of the SIM project, such as by the James Webb Space telescope (JWST) and Terrestrial Planet Finder (TPF) missions.
Projects such as the Space Interferometry Mission (SIM)  and Terrestrial Planet Finder (TPF)  rely heavily on sub-nanometer accuracy metrology systems to define their optical paths and geometries. The James Web Space Telescope (JWST) is using this metrology in a cryogenic dilatometer for characterizing material properties (thermal expansion, creep) of optical materials. For all these projects, a key issue has been the reliability and stability of the electronics that convert displacement metrology signals into real-time distance determinations. A particular concern is the behavior of the electronics in situations where laser heterodyne signals are weak or noisy and subject to abrupt Doppler shifts due to vibrations or the slewing of motorized optics. A second concern is the long-term (hours to days) stability of the distance measurements under conditions of drifting laser power and ambient temperature.<p> </p>This paper describes heterodyne displacement metrology gauge signal processing methods that achieve satisfactory robustness against low signal strength and spurious signals, and good long-term stability. We have a proven displacement-measuring approach that is useful not only to space-optical projects at JPL, but also to the wider field of distance measurements.
The Space Interferometry Mission, scheduled for launch in 2008, is an optical stellar interferometer with a 10 meter baseline capable of micro-arcsecond accuracy astrometry. A mission-enabling technology development program is underway at JPL, including the design and test of heterodyne interferometer metrology gauges to monitor the separation of optical components of the stellar interferometer. The gauges are required to have a resolution of 15 picometers and to track the motion of mirrors over several meters. We report laboratory progress in meeting these goals.
Space telescope designs driven by science goals such as the infrared observation of high-redshift galaxies and the
infrared observation of objects that would otherwise be obscured by dust in the visible push the operating temperatures
of the optics to cryogenic temperatures. Typical temperatures, 30 to 100 K are a challenging regime for actuators, but
little information is available on the low-temperature performance of Piezo-electric actuator products currently on the
market. Work is underway to measure actuator stroke and CTE, at low temperatures for typical PZTs, such as those
available "off-the-shelf" from P.I. and Thorlabs.
The development of stellar coronagraphs for exoplanet detection requires apodized occulting masks to effectively remove the light from the central star while allowing planet light to propagate past. One possible implementation, a gray-scale mask, includes the placement of micron-scale neutral density light absorbing patterns using High Energy Beam Sensitive (HEBS) glass. A second implementation, binary masks, uses micron-scale diffractive/reflective patterns.
Coronagraph performance will be influenced by wavefront phase shifts introduced by the masks, hence accurate characterization of the fundamental optical properties, namely optical density (OD), phase advance/delay and optical constants of the material is needed for occulter design, development and modeling.
In this paper we describe an interferometric apparatus that measures wavefront phase advance/delay through grey-scale and binary masks as functions of wavelength and optical density, which is also measured. Results for HEBS gray-scale masks will be presented along with ellipsometric measurements of optical constants.
Three samples of Schott Zerodur were recently measured using Jet Propulsion Laboratory's Cryogenic Dilatometer Facility. The initial purpose of these tests was to provide precision CTE measurements to help correlate thermomechanical models with the actual performance of NASA's Space Interferometry Mission (SIM) TOM-1C testbed. A total of six Zerodur test samples, as well as the SIM testbed mirror were machined from the same block of glass. Thermal strain as a function of time, sample temperature, and cooling rate were measured over a temperature range of 270K to 310K. Presented in this paper is a discussion of the sample configuration, test facilities, test method, data analysis, test results, and future plans.
Linear thermal expansion measurements of nine samples of Lead Magnesium Niobate (PMN) electroceramic material were recently performed in support of NASA's Terrestrial Planet Finder Coronagraph (TPF-C) mission. The TPF-C mission is a visible light coronagraph designed to look at roughly 50 stars pre-selected as good candidates for possessing earth-like planets. Upon detection of an earth-like planet, TPF-C will analyze the visible-light signature of the planet's atmosphere for specific spectroscopic indicators that life may exist there. With this focus, the project's primary interest in PMN material is for use as a solid-state actuator for deformable mirrors or compensating optics. The nine test samples were machined from three distinct boules of PMN ceramic manufactured by Xinetics Inc. Thermal expansion measurements were performed in 2005 at NASA Jet Propulsion Laboratory (JPL) in their Cryogenic Dilatometer Facility. All measurements were performed in vacuum with sample temperature actively controlled over the range of 270K to 310K. Expansion and contraction of the test samples with temperature was measured using a JPL-developed interferometric system capable of sub-nanometer accuracy. Presented in this paper is a discussion of the sample configuration, test facilities, test method, data analysis, test results, and future plans.
As part of the James Webb Space Telescope (JWST) materials working group, a novel cryogenic dilatometer was designed and built at NASA Jet Propulsion Laboratory to help address stringent coefficient of thermal expansion (CTE) knowledge requirements. Previously reported results and error analysis have estimated a CTE measurement accuracy for ULE of 1.7 ppb/K with a 20K thermal load and 0.1 ppb/K with a 280K thermal load. Presented here is a further discussion of the cryogenic dilatometer system and a description of recent work including system modifications and investigations.
The James Webb Space Telescope (JWST) will be a 6-meter diameter segmented reflector that will be launched at room temperature and passively cooled to about 40 Kelvin at the L2 point. Because of the large thermal load, understanding the thermophysical properties of the mirror, secondary optics, and supporting structure materials is crucial to the design of an instrument that will provide diffraction limited performance at 2 microns. Once deployed, JWST will perform continuous science without wave front re-calibrations for durations ranging from one day to a month. Hence understanding of how small temperature fluctuations will impact the nanometric stability of the optical system through thermal expansion is required. As a result, the JWST materials testing team has designed and built a novel cryogenic dilatometer capable of coefficient of thermal expansion (CTE) measurements of ULE accurate to ~ 1.6 and 0.1 ppb/K for a nominal CTE = 30 ppb/K and 20 and 280 K thermal loads, respectively. The dilatometer will be used to measure the CTE of samples from JWST primary mirror prototypes, local CTE variations from multiple locations on a prototype mirror, CTE variations from batch to batch of the same material, and thermal and mechanical creep measurements from room temperature down to 30 K.
Visible interferometry at µarc-second accuracy requires measurement of the interferometric baseline length and orientation at picometer accuracy. The optical metrology instruments required for these interferometers must achieve accuracy on order of 1 to 10 picometers. This paper discusses the progress in the development of optical interferometers for use in distance measurement gauges with systematic errors below 100 picometers. The design is discussed as well as test methods and test results.
The Micro-Arcsecond Metrology (MAM) team at the Jet Propulsion Laboratory has developed a precision phasemeter for the Space Interferometry Mission (SIM). The current version of the phasemeter is well-suited for picometer accuracy distance measurements and tracks at speeds up to 50 cm/sec, when coupled to SIM's 1.3 micron wavelength heterodyne laser metrology gauges. Since the phasemeter is implemented with industry standard FPGA chips, other accuracy/speed trade-off points can be programmed for applications such as metrology for earth-based long-baseline astronomical interferometry (planet finding), and industrial applications such as translation stage and machine tool positioning. The phasemeter is a standard VME module, supports 6 metrology gauges, a 128 MHz clock, has programmable hardware averaging, and maximum range of 2<SUP>32</SUP> cycles (2000 meters at 1.3 microns).
The micro-arcsecond metrology testbed (MAM) is a high- precision long baseline interferometer inside a vibration- isolated vacuum tank. The instrument consists of an artificial star, a laser metrology system, and a single- baseline interferometer with a 1.8m baseline and a 5cm clear aperture. MAM's purpose is to demonstrate that the astrometric error budget specified for the Space Interferometry Mission can be met.