James D. Phillips

The Milli-Arc-Second Structure Imager, MASSIM: A New Concept for a High Angular Resolution X-ray Telescope

MASSIM, the Milli-Arc-Second Structure Imager, is a mission that has been proposed for study within the context of NASAs Astrophysics Strategic Mission Concept Studies program. It uses a set of achromatic diffractive-refractive Fresnel lenses on an optics spacecraft to focus 5-11 keV X-rays onto detectors on a second spacecraft flying in formation 1000 km away. It will have a point-source sensitivity comparable with that of the current generation of major X-ray observatories (Chandra, XMM-Newton) but an angular resolution some three orders of magnitude better. MASSIM is optimized for the study of jets and other phenomena that occur in the immediate vicinity of black holes and neutron stars. It can also be used for studying other astrophysical phenomena on the milli-arc-second scale, such as those involving proto-stars, the surfaces and surroundings of nearby active stars and interacting winds. We describe the MASSIM mission concept, scientific objectives and the trade-offs within the X-ray optics design. The anticipated performance of the mission and possible future developments using the diffractive-refractive optics approach to imaging at X-ray and gamma-ray energies are discussed.

Full-sky Astrometric Mapping Explorer: an optical astrometric survey mission

The Full-sky Astrometric Mapping Explorer (FAME) is a MIDEX class Explorer mission designed to perform an all-sky, astrometric survey with unprecedented accuracy, determining the positions, parallaxes, proper motions, and photometry of 40 million stars. It will create a rigid, astrometric catalog of stars from an input catalog with 5 < mv < 15. For bright stars, 5 < mv < 9, FAMEs goal is to determine positions and parallaxes accurate to < 50 (mu) as, with proper motion errors < 50 (mu) as/year. For fainter stars, 9 < mv < 15, FAMEs goal is to determine positions and parallaxes accurate to < 500 (mu) as, with proper motion errors < 500 (mu) as/year. It will also collect photometric data on these 40 million stars in four Sloan DSS colors.

Higher-order-mode erbium-doped fiber amplifiers

Higher order modes for large-mode-area, high power fiber amplifiers are reviewed. Results from amplification of both CW and nanosecond pulsed signals in Er-doped HOM fibers with effective areas as large as 4000 μm2 are presented.

Beamsplitters for astronomical optical interferometry

We discuss an optical interferometers beamsplitter from the points of view of visibility loss and phase, critical to astrometry. A beamsplitter having no symmetries is described by 16 parameters, all functions of optical frequency; if all symmetries are present this number is reduced to four, only two of which are relevant to astronomical interferometry. We have developed a novel multi-layer design which covers fully a factor of three in wavelength while contributing a minimum of visibility loss, light loss, and systematic error.

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.

The Full-Sky Astrometric Mapping Explorer – Distances and Photometry of 40 Million Stars

The Full-sky Astrometric Mapping Explorer (FAME) is designed to perform an all-sky, astrometric survey with unprecedented accuracy. It will create a rigid astrometric catalog of 4 × 10 7 stars with 5 m V m V μ as, with proper motion errors μ as/yr. For fainter stars, 9 m V μ as, with proper motion errors μ as/yr. It will also collect photometric data on these 4 × 10 7 stars in four Sloan Digital Sky Survey colors. NASA selected FAME to be one of five MIDEX missions funded for a concept study. In October 1999, NASA selected FAME for launch in 2004 as the MIDEX-4 mission in its Explorer program.

Optical system for an astrometric survey from space

We present an optical design for a spaceborne instrument, of about half m aperture, to perform a combined astrometic and photometric survey via a scan similar to that of Hipparcos. A CCD detector array with time delayed integration will permit an astrometic mission accuracy better than 50 microarcseconds for stars brighter than 10th magnitude. 1 1/2 orders better than Hipparcos. The passband is nominally 0.4 to 0.9 microns. For the instrument to have both high measurements rate and high accuracy, the optical system just satisfy several requirements. It should have aberration well under diffraction, for high precision in centroiding and as a means of keeping unmolded shifts of the image centroids small. The system should have a wide field of view so that there is a large overlap of successive scans, have a large field of view for scientific throughput, and have low image distortion so that the stellar images moved at constant rate along columns of detector pixels. The design presented meets these requirements using aspheric surfaces that are manufacturable. We have demonstrated that the instrument will determine the temperature of an observed star without requiring a dispersive element or color filters. The design is thus free of transmissive elements, and protected from the systematic errors that they might have induced, e.g., due to thermal variation variation and to chromatic effects. This study was inspired by our previous consideration of scientific throughput. Our study of data reduction from a scanning astrometic survey mission demonstrated that there is a substantial gain in mission accuracy if the spacecraft precesses without discontinuities such as those that result from gas jet firings. Our study of methods of processing the spacecraft showed that smooth rotation would be possible using solar radiation pressure, but only if the spacecraft rotation rate were increased. Maintaining the integration time for each object would require an optical design of shorter focal length. Meanwhile, our study of mission accuracy as a function of focal length showed that another increase of accuracy would result from shorter focal length, via the greater number of lower-accuracy measurements. Therefore we performed this optical study to find a design with shorter focal length, having a proportionate increase in infield of view. We conceived and investigated a family of short focal length, wide-field designs, and developed a methodology to facilitate selection from among them. The new baseline design achieves diffraction-limited images over a 2.2 degree FOV with a 1.1 degree square central blockage, and has a 7.5 m focal length.