Braden E. Hines

Micro-arcsecond metrology (MAM) testbed overview

One of the most critical technology requirements for the Space Interferometry Mission is that the difference in pathlength traveled by the starlight through each arm of the instrument be known with picometers of precision. SIM accomplishes this by using an internal laser metrology system to measure the optical path traveled by the starlight. The SIM technology program has previously demonstrated laser gauges with measurement accuracy below 10 picometers. The next challenge is to integrate one of these gauges into a full interferometer system and demonstrate that the system still operates at the required level. For SIM, the ultimate requirement is that the internal metrology system be able to give an accurate measure of the starlight internal path difference to about 150 picometers over its narrow-angle field, with a goal of 50 picometer accuracy. This accuracy must be maintained even as SIMs various active systems articulate the SIM optics and vary the SIM internal pathlengths. The Microarcsecond Metrology Testbed (MAM) is a full single-baseline interferometer coupled with a precision pseudostar, intended to demonstrate the level of agreement between starlight and metrology phase measurements needed to make microarcsecond-level measurements of stellar positions. MAM has been under development for several years and is now producing picometers-level consistency that translates into microarcseconds-level performance. This paper will present an overview of the MAM Testbed, together with recent results targeting the 150 picometer performance level required by SIM.

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.

Prototype high speed optical delay line for stellar interferometry

The long baselines of the next-generation ground-based optical stellar interferometers require optical delay lines which can maintain nm-level path-length accuracy while moving at high speeds. NASA-JPL is currently designing delay lines to meet these requirements. The design is an enhanced version of the Mark III delay line, with the following key features: hardened, large diameter wheels, rather than recirculating ball bearings, to reduce mechanical noise; a friction-drive cart which bears the cable-dragging forces, and drives the optics cart through a force connection only; a balanced PZT assembly to enable high-bandwidth path-length control; and a precision aligned flexural suspension for the optics assembly to minimize bearing noise feedthrough. The delay line is fully programmable in position and velocity, and the system is controlled with four cascaded software feedback loops. Preliminary performance is a jitter in any 5 ms window of less than 10 nm rms for delay rates of up to 28 mm/s; total jitter is less than 10 nm rms for delay rates up to 20 mm/s.

Micro-Precision Interferometer Testbed: end-to-end system integration of control structure interaction technologies

This paper describes the overall design and planned phased delivery of the ground-based Micro-Precision Interferometer (MPI) Testbed. The testbed is a half scale replica of a future space-based interferometer containing all the spacecraft subsystems necessary to perform an astrometric measurement. Appropriate sized reaction wheels will regulate the testbed attitude as well as provide a flight-like disturbance source. The optical system will consist of two complete Michelson interferometers. Successful interferometric measurements require controlling the positional stabilities of these optical elements to the nanometer level. The primary objective of the testbed is to perform a system integration of Control Structure Interaction (CSI) technologies necessary to demonstrate the end-to-end operation of a space- based interferometer, ultimately proving to flight mission planners that the necessary control technology exists to meet the challenging requirements of future space-based interferometry missions. These technologies form a multi-layered vibration attenuation architecture to achieve the necessary quiet environment. This three layered methodology blends disturbance isolation, structural quieting and active optical control techniques. The paper describes all the testbed subsystems in this end-to-end ground-based system as well as the present capabilities of the evolving testbed.

Common interferometer control systems architecture

Astronomical interferometry at the JPL has grown rapidly in the last two years. JPL is now engaged in a number of interferometry projects and is also developing a number of internal testbeds to support those projects. While each of these projects and testbeds has its own unique properties, they do share a lot of common features, and JPL is striving, through its interferometer technology program (ITP), to develop common components, software, and hardware that can be reused by multiple projects. The discipline where this commonality is probably most apparent is in the area of realtime control systems, specifically the software and electronics that drive the instrument control loops and sequence the subsystems. To this end, within the ITP, JPL has developed the realtime interferometer control systems testbed (RICST) as a facility where a common software and electronics core, essentially a control system for a generic interferometer, can be developed. The realtime control (RTC) team in the ITP program consists of about 20 full-time equivalent engineers, technicians, quality assurance personnel, architects, and managers. The remainder of this paper will describe the interferometry landscape at JPL, the RTC effort, an overview of the RICST testbed, and the generic interferometer control system architecture that has been developed.

Optical truss and retroreflector modeling for picometer laser metrology

Space-based astrometric interferometer concepts typically have a requirement for the measurement of the internal dimensions of the instrument to accuracies in the picometer range. While this level of resolution has already been achieved for certain special types of laser gauges, techniques for picometer-level accuracy need to be developed to enable all the various kinds of laser gauges needed for space-based interferometers. Systematic errors due to retroreflector imperfections become important as soon as the retroreflector is allowed to either translate in position or articulate in angle away from its nominal zero-point. Also, when combining several laser interferometers to form a three-dimensional laser gauge (a laser optical truss), systematic errors due to imperfect knowledge of the truss geometry are important as the retroreflector translates away from its nominal zero-point. In order to assess the astrometric performance of a proposed instrument, it is necessary to determine how the effects of an imperfect laser metrology system impact the astrometric accuracy. This paper show the development of an error propagation model from errors in the 1-D metrology measurements through the impact on the overall astrometric accuracy for OSI. Simulations are then presented based on this development which were used to define a multiplier which determines the 1-D metrology accuracy required to produce a given amount of fringe position error.

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.

SIM System Testbed III

This paper will present the current status of STB-3, our approach to feedforward stabilization, and designs and plans for phases 2 and 3.

Optical design issues for the Micro-Precision Interferometer Testbed for space-based interferometry

The Micro-Precision Interferometer Testbed is essentially a space-based Michelson interferometer suspended in a ground-based laboratory. The purpose of the testbed is to serve as a proving ground for technologies needed for future space-based missions requiring low- vibration environments. A layered control architecture, utilizing isolation, structural control, and active optical control technologies, allows the system to achieve its vibration attenuation goals. This paper focuses primarily on the optical design for the testbed and the systems-level tradeoffs between the optics and other systems due to the fact that the interferometer is on a large, lightly damped, flexible structure rather than on the ground. The testbed is designed to be a fully functioning interferometer spacecraft and makes use of flight-like hardware where possible, including an external star simulator, an attitude control system, fringe detection and tracking systems, delay lines, pointing control, laser metrology systems, and computers and electronic subsystems. The engineering decisions that led to the current optical configuration are presented and explained.

ASEPS-0 Testbed Interferometer control system

The ASEPS-O Testbed Interferometer control system design presents new challenges as compared to previous generation instruments. Increased instrument complexity due to narrow-angle astrometric observing techniques, longer baselines, and increased user expectations due to computer technology advances all contribute to the size of the effort required to bring the instrument on- line. This paper discusses the design objectives for the computer systems and software for the instrument and how the architecture selected was driven by the objectives and by the resources available; one of the major design objectives was to come up with an architecture that keeps software, network, and communications issues separate from scientific and subsystem implementation issues, making the most effective use of both scientifically- oriented and computer-oriented developers. The paper then presents the architecture that has been implemented in detail.