Robert William Babcock

Newcomb Astrometric Satellite

Newcomb is a design concept for an astrometric optical interferometer satellite with a nominal single measurement accuracy of 100 microarcseconds. In a 30-month mission life, it will make scientifically interesting measurements of O stars, RR Lyrae and Cepheid distances, probe dark matter in our Galaxy via parallax measurements of K giants in the disk, establish a reference grid with internal consistency better than 50 microarcseconds, and lay the groundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during the next century.

Internal laser metrology for POINTS

We present the designs for laser distance gauges to be used in the POINTS instrument, and preliminary performance data. For the target 5 micro-arcsecond astrometric accuracy, we must hold or monitor some critical internal dimensions of the POINTS instrument with 2 picometer (pm, 10-12 m) accuracy for a few hours. The POINTS architecture makes good use of these gauges, minimizing the number and range of dimensions that must change during operation, and maximizing the similarity of the starlight and metrology measured paths. Gauge designs have been developed for both optical-path-differencing (Michelson) and point-to-point measurements (Fabry-Perot). The Michelson fringes have been measured in a differential (comparison) test; the root-two-point variance (analogous to the Allan variance) in the difference of two measurements over essentially identical 1-meter paths was about 2 pm for averaging times between 40 seconds and 6 hours. A second design for the point-to-point measurements incorporates cornercube retro-reflectors in a resonant cavity. We discuss the new problems anticipated in this design, including the problem of maintaining laser alignment in these point-to-point gauges over the +/- 3 degree range of instrument articulation.

POINTS: an astrometric spacecraft with multifarious applications

POINTS is a dual astrometric optical interferometer with nominal baseline length of 2 m and measurement accuracy of 5 microarcsecs for targets separated by about 90 degrees on the sky. If selected as the ASEPS-1 mission, it could perform a definite search for extra-solar planetary systems, either finding and characterizing a large number of them or showing that they are far less numerous than now believed. If selected as AIM, it could be a powerful new multidisciplinary research tool, opening new areas of astrophysical research and changing the nature of the questions being asked in some old areas. Based on a preliminary indication of the observational needs of the two missions, we find that a single POINTS mission lasting ten years would meet the science objectives of both ASEPS-1 and AIM. POINTS, which is small, agile, and mechanically simple, would be the first of a new class of powerful instruments in space and would prove the technology for the larger members of the class that are expected to follow. The instrument is designed around a metrology system that measures both the critical distances internal to the starlight interferometers and the angle between them. Rapid measurement leads to closure on the sky and the ability to detect and correct time-dependent measurement biases.

POINTS: the instrument and its mission

POINTS comprises a pair of independent Michelson stellar interferometers and a laser metrology system that measures both the critical starlight paths and the angle between the two baselines. The nominal design has baselines of 2 m, subapertures of 35 cm, and a single- measurement accuracy of 5 microarcseconds for targets separated by approximately equals 90 degree(s). In a five-year mission, POINTS could yield, e.g., a 1% Cepheid distance scale, galactic mass distribution, knowledge of cluster dynamics, and stellar masses and luminosities. In a ten-year mission, POINTS could perform a deep search for other planetary systems, using only 20% of the available observing time. POINTS does global astrometry, i.e., it measures widely separated targets, which yields closure calibration, numerous bright reference stars, and absolute parallax. The instrument has only three moving-part mechanisms, and only one of these must move with sub-milliradian accuracy. On each side of the interferometer, there are only three (interferometrically critical) optical surfaces preceding the beamsplitter or its fold flat. POINTS is small, agile, and mechanically simple. It would prove much of the technology for future imaging interferometers.

Newcomb: a scientific interferometry mission at low cost

Newcomb is a design concept for an astrometric optical interferometer with nominal single-measurement accuracy of 100 microseconds of arc ((mu) as). In a 30-month mission life, it will make scientifically interesting measurements of O-star, RR Lyrae, and Cepheid distances, probe the dark matter in our Galaxy via parallax measurements of K giants in the disk, establish a reference grid with internal consistency better than 50 (mu) as, and lay groundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during the next century. With an extended mission life, Newcomb could do a useful search for other planetary systems.

Newcomb: a small astrometric interferometer

Newcomb is a design concept for a low-cost astrometric optical interferometer with nominal single-measurement accuracy of 100 microseconds of arc ((mu) as). In a 30 month mission, it will make scientifically interesting measurements of O-star, RR Lyrae, and Cepheid distances, probe the dark matter in our Galaxy via parallax measurements of K giants in the disk, establish a reference grid with internal consistency better than 50 microsecond(s) , and lay groundwork for the larger optical interferometers that are expected to produce a profusion of scientific results during the next century. With an extended mission life, Newcomb could do a useful preliminary search for other planetary systems.

POINTS: the first small step

POINTS, an astrometric optical interferometer with a nominal measurement accuracy of 5 microarcseconds for the angle between a pair of stars separated by about 90 deg. is presently under consideration by two divisions of NASA-OSS. Based on a preliminary indication of the observational needs of the two missions, we find that a single POINTS mission will meet the science objectives of both TOPS-1 and AIM. The instrument detects a dispersed fringe (channelled spectrum) and therefore can tolerate large pointing errors. In operation, the difficult problem of measuring the angular separation of widely spaced star pairs is reduced to two less difficult problems: that of measuring the angle between the two interferometers and that of measuring interferometrically the small offset of each star from the corresponding interferometer axis. The question of systematic error is the central theme of the instrument architecture and the data-analysis methods. Stable materials, precise thermal control, and continuous precise metrology are fundamental to the design of the instrument. A preliminary version of the required picometer laser metrology has been demonstrated in the laboratory. Post-measurement detection and correction of time-dependent bias are the essential elements in data analysis. In that post-measurement analysis, individual star-pair separations are combined to determine both the relative positions of all observed stars and several instrument parameters including overall time-dependent measurement bias. The resulting stellar separation estimates are both global and bias-free at the level of the uncertainty in the reduced (i.e., combined and analyzed) measurements.