Roger P. Linfield

The New Millennium Formation Flying Optical Interferometer

Spaceborne opt i ca l inte~erometry has been identified as a critical technology for many of NASA’s 21* centu~ science visions. Included in this m“sion are interferometers that can probe the orighs of stars and can ultimately study Earthlike planets around nearby stars. To accomplish this feat, separation of an interferometer’s collecting apertures by large baselines are required from hundreds of meters up to thousands of kilometers. Thus the large separations require multiple spacecraft jlying in a formation. Furthermore, optical pathlengths over these distances must be controlled to the nanometer level. This level of control demands precision spacecraft controls, active optics, metrology, and starlight detection technologies, To date, some of these technologies have been demonstrated only in ground applications with baselines of order of a hundred meters; space operation will require a significant capability enhancement. This paper describes the New Millennium formation flying optical interferometer concept and associated technologz”es. The mission is designed to provide a technology demonstration for multiple spacecraft precision formation flying and very long baseline optical interferometry. The interferometer would be distributed over three spacecraft: two spacecraft would serve as collectors, directing ——— ——— ————-— ———---— ——— ——— ——--— —— —--— Copyright@ 1997 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Governmertt has a royalty-free license to exercise all rights under the copyright claimed herein for governmental purposes. All other rights are reserved by the copyright owner. starlight toward a third spacecraft which would combine the light and perform the in terferometric detection. The interferometer baselines would be variable, allowing baselines of 100 m to 1 km in an equilateral formation, providing angular resolutions from 1 to 0.1 milliarcsec.

Science capabilities of the DS3 mission

The DS3 mission will launch a space optical interferometer into heliocentric orbit, for observation of 50-100 sources on baselines up to 1000-2000 m. The detection threshold will be visual magnitude 12-13, and the angular resolution in the 500-900 nm passband will be approximately ≈100 microseconds. Interesting science targets which could be imaged include: Cyg X-1, Wolf-Rayet stars, and FU Ori stars. With a modest improvement in sensitivity, the structure of the broad line emission regions of a few bright AGNs could be measured.

Interferometer instrument design for New Millennium Deep Space 3

Deep Space 3 will fly a stellar optical interferometer on three separate spacecraft in heliocentric orbits: one spacecraft for the Michelson beam combining optics, and two spacecraft for each of the starlight apertures. The spacecraft will formation fly to relative spacecraft distances from 100 meters to 1 kilometer, enabling an instrument resolution of 1 to 0.1 milliarcsecond. At each baseline length and orientation - up to 100 points in the synthetic aperture plane for a given astrophysical target - the instrument will measure source visibility amplitude form which the source brightness distribution can be determined. An infrared metrology system performs both linear and angular metrology between spacecraft and is sued to estimate delay jitter, interferometer delay and delay rate. Pointing and control mechanisms use the metrology error signals to stabilize delay jitter and to null delay and delay rate to enable detection and tracking of a white light fringe on a photon-counting detector. Once stabilized, fringes can be dispersed on a CCD in up to 80 spectral channels to attain high-accuracy measurements of visibility amplitude as a function of wavelength.

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.

Error analysis of a compound nulling interferometer

Nulling interferometry at mid-infrared wavelengths holds promise for finding and characterizing Earth-like planets that orbit nearby stars. By strongly suppressing light from a nearby star, the instrument becomes sensitive enough for direct detection of planets orbiting that star. A compound nulling interferometer (combining light from more than 2 telescopes) is needed for these searches, in order to achieve adequate light suppression across the full disk of the star. We present an error analysis of quasi-static and chopping variants of a four element nulling interferometer, including the dependence on amplitude, delay, baseline length, and telescope pointing errors.

Phenomenology of extra-solar planets in reflected starlight and system level requirements for detection and characterization

During our NASA sponsored study of candidate architectures for the Terrestrial Planet Finder mission we estimated the values of observable properties that would be accessible to an instrument intended to detect starlight reflected by a planet in the habitable zone of the system. These properties include architecture and wavelength independent geometrical properties such as angular separation between the star and planet, and timescales associated with orbital motion. Properties that do depend on the detection technique and wavelength include the brightness of the planet, its contrast relative to the star, and variability associated with diurnal and seasonal phenomena. The search space for a reflected light TPF is the range of these parameters calculated for a sample of 200 main sequence stars whose stellar properties make them potential targets. A scientific investigation such as that described by the TPF Science Working Group then leads to requirements on the sensitivity of the system, angular resolution, suppression of starlight and operational efficiency. We will describe our star sample, the search space of planetary observables and apparent system requirements.

Optical requirements for a Terrestrial Planet Finder optical coronagraph primary mirror

One possible implementation of an optical coronagraphic approach to finding exo-solar planets incorporates a large, monolithic primary mirror (PM) that is approximately 4 meters by 10 meters in size. The optical requirements on a mirror that is part of a suppression system to achieve at least 1010 rejection are extremely challenging, and a series of pathfinder demonstrations and testbeds are warranted. We examine the optical manufacturing and tolerancing requirements on the mirror itself as a function of spatial frequency where in certain regimes we desire better than 1/1000th of a wave surface accuracy. An atypical requirement is also imposed on the optical coatings where the uniformity of reflectance is desired to be a few parts in 10,000. In addition, we present an optical design for a sub-scale coronagraphic testbed as an essential step in examining the system sensitivities.

Technology requirements and development path for coronagraphic planet detection

Visible light coronagraphy from space is a promising technique for extrasolar planet detection and characterization. However, technology development in several areas is needed before a search for terrestrial planets is feasible with a coronagraph. The most challenging technologies appear to be: construction of 10 m scale, precision lightweight optics; millikelvin level control of temperature changes; and achieving reflectivity uniformity of 10-4 across all mirrors. Additional technical challenges include: wavefront sensing to the sub-angstrom level; precise, stable deformable mirrors; and construction of coronagraphic masks with accurate shape or transmission profile. The current status and suggested development path of these technologies will be discussed.

Cold interferometric nulling demonstration in space (CINDIS)

The Cold Interferometric Nulling Demonstration in Space (CINDIS) is a modest-cost technology demonstration mission, in support of interferometer architectures for Terrestrial Planet Finder (TPF). It is designed to provide as complete as possible a demonstration of the key technologies needed for a TPF interferometer at low risk, for a cost less than

Technology requirements and development path for planet detection by mid-IR interferometry

300M. CINDIS foregoes scientific objectives at the outset, enabling significant cost savings that allow us to demonstrate important features of a TPF interferometer, such as high-contrast nulling interferometry at 10 μm wavelength, vibration control strategies, instrument pointing and path control, stray light control, and possibly 4-aperture compound nulling. This concept was developed in response to the NASA Extra-Solar Planets Advanced Concepts NRA (NRA-01-OSS-04); this paper presents the results of the first phase of the study.