Jeffrey W. Yu

ASEPS-0 Testbed Interferometer

The ASEPS-O Testbed Interferometer is a long-baseline infrared interferometer optimized for high-accuracy narrow-angle astrometry. It is being constructed by JPL for NASA as a testbed for the future Keck Interferometer to demonstrate the technology for the astrometric detection of exoplanets from the ground. Recent theoretical and experimental work has shown that extremely high accuracy narrow-angle astrometry, at the level of tens of microarcseconds in an hour of integration time, can be achieved with a long-baseline interferometer measuring closely-spaced pairs of stars. A system with performance close to these limits could conduct a comprehensive search for Jupiter- and Saturn-mass planets around stars of all spectral types, and for short-period Uranus-mass planets around nearby M and K stars. The key features of an instrument which can achieve this accuracy are long baselines to minimize atmospheric and photon-noise errors, a dual-star feed to route the light from two separate stars to two beam combiners, cophased operation using an infrared fringe detector to increase sensitivity in order to locate reference stars near a bright target, and laser metrology to monitor systematic errors. The ASEPS-O Testbed Interferometer will incorporate these features, with a nominal baseline of 100 m, 50- cm siderostats, and 40-cm telescopes at the input to the dual- star feeds. The fringe detectors will operate at 2.2 micrometers , using NICMOS-III arrays in a fast-readout mode controlling high-speed laser-monitored delay lines. Development of the interferometer is in progress, with installation at Palomar Mountain planned to begin in 1994.

Hybrid ray trace and diffraction propagation code for analysis of optical systems

The Control Optics Modelling Package (COMP), is an optical modelling computer program capable of performing ray trace, differential ray trace and diffraction analyses for any optical design. COMP is particularly useful for optical systems that move, whether through interaction with dynamically or thermally varying structures, or optics that are actively controlled to perform particular tasks, such as steering mirrors or segmented mirrors.

Integrated structural and optical modeling of the orbiting stellar interferometer

The Integrated Modeling of Optical Systems (IMOS) Integration Workbench at JPL has been used to model the effects of structural perturbations on the optics in the proposed Orbiting Stellar Interferometer (OSI). OSI consists of 3 pairs of interferometers and delay lines attached to a 7.5 meter truss. They are interferometrically monitored from a separate boom by a laser metrology system. The spatially distributed nature of the science instrument calls for a high level of integration between the optics and support structure. Because OSI is designed to achieve micro-arcsecond astrometry, many of its alignment, stability, and knowledge tolerances are in the submicron regime. The spacecraft will be subject to vibrations caused by reaction wheels and on-board equipment, as well as thermal strain due to solar and terrestrial heating. These perturbations affect optical parameters such as optical path differences and beam co-parallelism which are critical to instrument performance. IMOS provides an environment that allows one to design and perturb the structure, attach optics to structural or non-structural nodes, trace rays, and analyze the impact of mechanical perturbations on optical performance. This tool makes it simple to change the structure and immediately see performance enhancement/degradation. We have employed IMOS to analyze the effect of reaction wheel disturbances on the optical path difference in both the science and metrology interferometers.

Separated-spacecraft interferometer concept for the New Millennium Program

A separated spacecraft optical interferometer mission concept proposed for NASAs New Millennium Program is described. The interferometer instrument is distributed over three small spacecraft: two spacecraft serve as collectors, directing starlight toward a third spacecraft which combines the light and performs the interferometric detection. As the primary objective is technology demonstration, the optics are modest size, with a 12-cm aperture. The interferometer baseline is variable from 100 m to 1 km, providing angular resolutions from 1 to 0.1 milliarcseconds. Laser metrology is used to measure relative motions of the three spacecraft. High-bandwidth corrections for stationkeeping errors are accomplished by feedforward to an optical delay line in the combiner spacecraft; low-bandwidth corrections are accomplished by spacecraft control with an electric propulsion or cold-gas system. Determination of rotation of the constellation as a whole uses a Kilometric Optical Gyro, which employs counter-propagating laser beams among the three spacecraft to measure rotation with high accuracy. The mission is deployed in a low-disturbance solar orbit to minimize the stationkeeping burden. As it is well beyond the coverage of the GPS constellation, deployment and coarse stationkeeping are monitored with a GPS-like system, with each spacecraft providing both transmit and receive ranging and attitude functions.

Acousto-optic tunable filter imaging spectrometer for NASA applications: system issues

An acousto-optic tunable filter imaging spectrometer system operating in the visible region (0.4-0.8 um) has been developed and demonstrated. A comparison between other types ofspectrometers and advantages to future NASA missions are discussed. Performance issues relating to the spectral and imaging capabilities of the spectrometer are discussed.

Palomar Testbed Interferometer

The Palomar Testbed Interferometer (PTI) is an infrared, phase-tracking interferometer in operation at Palomar Mountain since July 1995. It was funded by NASA for the purpose of developing techniques and methodologies for doing narrowangle astrometry for the purpose of detecting extrasolar planets. The instrument employs active fringe trackingin the infrared (2.0-2.4 μm) to monitor fringe phase. It is a dual-star interferometer; it is able to measure fringes on two separate stars simultaneously. An end-to-end heterodyne laser metrology system is used to monitor the optical path length of the starlight. Recently completed engineering upgrades have improved the initial instrument performance. These upgrades are:extended wavelength coverage, a single mode fiber for spatial filtering, vacuum pipes to relay the beams, accelerometers on the siderostat mirrors and a new baseline. Results of recent astrometry data indicate the instrument is approaching the astrometric limit as set by the atmosphere.

Picometer accuracy white light fringe modeling for SIM PlanetQuest spectral calibration development unit

The SIM PlanetQuest Mission will perform astrometry to one microarcsecond accuracy using optical interferometers requiring optical path delay difference (OPD) measurements accurate to tens of picometers. Success relies on very precise calibration. Spectral Calibration Development Unit (SCDU) has been built to demonstrate the capability of calibrating spectral dependency of the white light fringe OPD to accuracy better than 20pm. In this article, we present the spectral calibration modeling work for SCDU to achieve the SIM PlanetQuest Engineering Milestone 4. SCDU data analysis shows that the wave front aberrations cause the instrument phase dispersions to vary by tens of nanometers over the bandwidth of a CCD pixel making the previous model inadequate. We include the effect of the wave front aberrations in the white light fringe model and develop a procedure for calibrating the corresponding model parameters using long stroke fringe data based on Discrete Fourier Transform. We make the calibration procedure flight traceable by dividing the whole calibration into the instrument calibration and the source spectral calibration. End-to-end simulations are used to quantify both the systematic and random errors in spectral calibration. The efficacy of the calibration scheme is demonstrated using the SCDU experimental data.

Deep Space 3 metrology system

A metrology subsystem on board the Deep Space 3, a separated spacecraft interferometer mission, is used to determine stellar fringe delay jitter, delay rate, and initial delay. The subsystem implements two capabilities: linear metrology for optical pathlength determination and angular metrology needed to determine the configuration and orientation of the spacecraft constellation. Frequency modulated metrology concept is used to implement high-precision (5nm) interferometric linear measurements over large target ranges (1km). System is made angle sensitive by using an articulated flat mirror at the target.

LATOR: laser astrometric test of relativity

LATOR is a space-based experiment to accurately measure the gravitational deflectional deflection of light. The experiment uses two laser bearing spacecraft at the opposite side of the Sun and a very long baseline heterodyne interferometer to measure the angle at an accuracy of 0.2 uas. Combining this measurement with laser ranging from Earth to both spacecraft, gravitational deflection can be made with an accuracy 5000 times better than previously done and will allow measurements of the second order and frame dragging effects.

64 x 64 thresholding photodetector array for optical pattern recognition

A high performance 32 X 32 peak detector array is introduced. This detector consists of a 32 X 32 array of thresholding photo-transistor cells, manufactured with a standard MOSIS digital 2-micron CMOS process. A built-in thresholding function that is able to perform 1024 thresholding operations in parallel strongly distinguishes this chip from available CCD detectors. This high speed detector offers responses from one to 10 milliseconds that is much higher than the commercially available CCD detectors operating at a TV frame rate. The parallel multiple peaks thresholding detection capability makes it particularly suitable for optical correlator and optoelectronically implemented neural networks. The principle of operation, circuit design and the performance characteristics are described. Experimental demonstration of correlation peak detection is also provided. Recently, we have also designed and built an advanced version of a 64 X 64 thresholding photodetector array chip. Experimental investigation of using this chip for pattern recognition is ongoing.