Yekta Gursel

LIGO: The Laser Interferometer Gravitational-Wave Observatory

The goal of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project is to detect and study astrophysical gravitational waves and use data from them for research in physics and astronomy. LIGO will support studies concerning the nature and nonlinear dynamics of gravity, the structures of black holes, and the equation of state of nuclear matter. It will also measure the masses, birth rates, collisions, and distributions of black holes and neutron stars in the universe and probe the cores of supernovae and the very early universe. The technology for LIGO has been developed during the past 20 years. Construction will begin in 1992, and under the present schedule, LIGOs gravitational-wave searches will begin in 1998.

The Palomar Testbed Interferometer

The Palomar Testbed Interferometer (PTI) is a long-baseline infrared interferometer located at Palomar Observatory, California. It was built as a testbed for interferometric techniques applicable to the Keck Interferometer. First fringes were obtained in 1995 July. PTI implements a dual-star architecture, tracking two stars simultaneously for phase referencing and narrow-angle astrometry. The three fixed 40 cm apertures can be combined pairwise to provide baselines to 110 m. The interferometer actively tracks the white-light fringe using an array detector at 2.2 μm and active delay lines with a range of ±38 m. Laser metrology of the delay lines allows for servo control, and laser metrology of the complete optical path enables narrow-angle astrometric measurements. The instrument is highly automated, using a multiprocessing computer system for instrument control and sequencing.

Laser metrology gauges for OSI

Heterodyne interferometers have been commercially available for many years. In addition, many versions have been built at JPL for various projects. This activity is aimed at improving the accuracy of such interferometers from the 1 - 30 nanometer level to the picometer level for use in the proposed OSI and SONATA missions as metrology gauges. In the null-gauge configuration, we obtained a precision of 0.6 picometers at time scales of 2,500 seconds. In the relative-gauge configuration, we obtained an accuracy of 3.5 picometers rms in vacuum at time scales of few minutes. As absolute gauge with an accuracy of 10 microns over a distance of 10 meters in under construction.

Multipole moments for stationary systems: The equivalence of the Geroch-Hansen formulation and the Thorne formulation

It is proved that the multipole moments of a stationary, asymptotically flat system in general relativity theory as defined by Thorne are identical, aside from normalization, to those defined by Geroch and Hansen: Here is Thornes mass moment of orderl, is the Geroch-Hansen mass moment, is Thornes current moment of orderl, and is Hansens current moment. The mathematical techniques of Thorne are combined with those of Geroch and Hansen to prove several new theorems about multipole moments, and to give new proofs to some of the old theorems.AbstractIt is proved that the multipole moments of a stationary, asymptotically flat system in general relativity theory as defined by Thorne are identical, aside from normalization, to those defined by Geroch and Hansen: Here is Thornes mass moment of orderl, is the Geroch-Hansen mass moment, is Thornes current moment of orderl, and is Hansens current moment. The mathematical techniques of Thorne are combined with those of Geroch and Hansen to prove several new theorems about multipole moments, and to give new proofs to some of the old theorems.

Metrology for spatial interferometry

The accuracy of the relative metrology gauge developed for the proposed OSI and SONATA missions is improved to subpicometer level. An accuracy of 0.15 picometers is obtained in vacuum at time scales of a few minutes. A surface metrology gauge with an initial accuracy of (lambda) /1000 is under construction. Photometry accurate to better than 1 part in 103 for the surface metrology gauge is demonstrated using a commercial grade, 8-bit, uncooled CCD camera and a commercial grade frame grabber at time scales of 10 seconds with a resolution of 320 by 240 pixels.

Performance of a precision high-density deformable mirror for extremely high contrast imaging astronomy from space

Active wavefront correction of a space telescope provides a technology path for extremely high contrast imaging astronomy at levels well beyond the capabilities of current telescope systems. A precision deformable mirror technology intended specifically for wavefront correction in a visible/near-infrared space telescope has been developed at Xinetics and extensively tested at JPL over the past several years. Active wavefront phase correction has been demonstrated to 1 Angstrom rms over the spatial frequency range accessible to a mirror with an array of actuators on a 1 mm pitch. It is based on a modular electroceramic design that is scalable to 1000s of actuator elements coupled to the surface of a thin mirror facesheet. It is controlled by a low-power multiplexed driver system. Demonstrated surface figure control, high actuator density, and low power dissipation are described. Performance specifications are discussed in the context of the Eclipse point design for a coronagraphic space telescope.

Dual-stage passive vibration isolation for optical interferometer missions

Future space-based optical instruments such as the Space Interferometer Mission have vibration-induced error allocations at the levels of a few nano-meters and milli-arc-seconds. A dual stage passive isolation approach has been proposed using isolation first at the vibration-inducing reaction wheels, and a second isolation layer between the bus portion of the space vehicle (the backpack) and the optical payload. The development of the backpack isolator is described, with unit transmissibility results for individual isolator struts. The dual stage isolation approach is demonstrated on a dynamically feature-rich, 7-meter structural testbed (STB3). A new passive suspension that mitigates ground vibrations above 0.4 Hz has been integrated into the testbed. A series of OPD performance predictions have been made using measured transfer functions. These indicate that the 5-nm dynamic OPD allocation is within reach using the dual isolator approach. Demonstrating these low response levels in a noisy air environment has proven to be difficult. We are sequentially executing a plan to mitigate acoustic transmission between backpack and flight structure, as well as developing techniques to mitigate effects of background acoustic noise.

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.

Metrology for spatial interferometry II

Very high resolution spatial interferometry requires picometer level 1D metrology, surface metrology and 3D metrology. Micron level accuracy is required for absolute metrology systems for spacecraft like the proposed Orbiting Stellar Interferometer carrying high resolution spatial interferometers. A surface metrology system with a repeatability of less than 0.1 nm over an aperture of several inches in vacuum has been demonstrated. An absolute calibration system for this gauge is in development. An absolute metrology system with an accuracy of 10 microns over a distance of 10 meters is also under construction. This system uses a 1319 nm, solid-state, infrared laser locked to an Ultra-Low-Expansion glass cavity to an accuracy exceeding 1 part in 1010. The length of the cavity is controlled by a thermal vacuum oven. 1 millidegree Centigrade root-mean-squared (rms) cavity temperature stability with the oven in vacuum has been achieved for time scales of days. The digital laser servo is capable of following the length of the cavity with an Allen deviation of few hundred Hertz for time scales of a day. Two lasers locked to the same cavity are used to supply a simultaneous cavity length measurement as well as the absolute distance measurement. The absolute distance measuring part of the gauge is under construction. An auto alignment system is being developed for our linear relative metrology gauge which had achieved an accuracy of 0.1 picometers. This gauge will be used to construct a 3D metrology gauge with an accuracy of less than 10 pm rms for time scales of minutes initially.

Picometer-accuracy, laser-metrology gauge for Keck interferometer differential-phase subsystem

Keck Interferometer differential-phase planet-detection system requires a picometer accuracy, large (2 μm to 4 μm) amplitude optical path-length modulator that can operate at fairly high frequencies (250 Hz, 750 Hz, and 1250 Hz, a partial, triangular wave motion). We have developed a gauge which monitors the amplitude of the motion of the path-length modulator and which is capable of reaching a sensitivity of at least 3 pm per sqrt(Hz) within a band width of 1 Hz at 250 Hz, 750 Hz, and 1250 Hz. Two of these gauges are built. The gauges are compared to each other while monitoring a common optical path-length modulator to determine their accuracy. In this paper, the gauge construction details, the results of the gauge accuracy tests as well as the final path-length modulator performance details are presented.