The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for an imaging and nulling interferometer for the near infrared to mid-infrared spectral region (3-8 microns). FKSI is a scientific and technological pathfinder to TPF/DARWIN as well as SPIRIT, SPECS, and SAFIR. It will also be a high angular resolution system complementary to JWST. There are four key scientific issues the FKSI mission is designed to address. First, we plan to characterize the atmospheres of the known extra-solar giant planets. Second, we will explore the morphology of debris disks to look for resonant structures to find and characterize extrasolar planets. Third, we will observe young stellar systems to understand their evolution and planet forming potential, and study circumstellar material around a variety of stellar types to better understand their evolutionary state. Finally, we plan to measure detailed structures inside active galactic nuclei. We report results of simulation studies of the imaging capabilities of the FKSI with various configurations of two to five telescopes including the effects of thermal noise and local and exozodiacal dust emission. We also report preliminary results from our symmetric Mach-Zehnder nulling testbed.
The Fourier-Kelvin Stellar Interferometer (FKSI) is a mission concept for a nulling interferometer for the near-to-mid-infrared spectral region (3-8µm). FKSI is conceived as a scientific and technological precursor to TPF. The scientific emphasis of the mission is on the evolution of protostellar systems, from just after the collapse of the precursor molecular cloud core, through the formation of the disk surrounding the protostar, the formation of planets in the disk, and eventual dispersal of the disk material. FKSI will answer key questions about extrasolar planets:
Σ What are the characteristics of the known extrasolar giant planets?
Σ What are the characteristics of the extrasolar zodiacal clouds around nearby stars?
Σ Are there giant planets around classes of stars other than those already studied?
We present preliminary results of a detailed design study of the FKSI. Using a nulling interferometer configuration, the optical system consists of two 0.5m telescopes on a 12.5m boom feeding a Mach-Zender beam combiner with a fiber wavefront error reducer to produce a 0.01% null of the central starlight. With this system, planets around nearby stars can be detected and characterized using a combination of spectral and spatial resolution.
A laboratory testbed to demonstrate and characterize control systems needed for interferometric nulling has been under development for two years at Ball Aerospace. Our testbed uses visible light and ambient
temperature operation in air, unlike the mid-IR, cryogenic vacuum operation that will be used for Terrestrial Planet Finder. We have developed automated, closed-loop control in delay and tip-tilt.
Successful planet searches with a Terrestrial Planet Finder (TPF) coronagraph will require a highly uniform wavefront, in both phase and amplitude. Requirements for phase and amplitude uniformity are derived. A Deformable Mirror (DM) can achieve broadband correction of phase errors across the full observing band. Correction of amplitude errors with a DM is also possible, but only over half the
image plane, and only for a limited bandwidth. For a 500-600 nm observing band, use of a DM can relax the reflectivity uniformity requirements on TPF mirrors by a factor of 10-15, and these
relaxed requirements appear to be within the current capability for mirror coatings.
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.
KEYWORDS: Telescopes, Space telescopes, Stars, Space operations, Interferometers, Sensors, Control systems, Mirrors, Nulling interferometry, Cryogenics
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 $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.
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.
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.
Mid-infrared (10 micron band) interferometry 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 can be performed with an interferometer. A key capability of such an instrument is the achievement of a deep (~1E-06), stable, broadband (~1 octave) interferometric null, with dependence on sky angle of quartic or broader. This performance sets requirements on amplitude, delay, polarization, and pointing (wavefront tilt) matching between different apertures of the interferometer. The wavefront quality must be ~1/1000 of a wavelength rms, probably requiring a high performance spatial filter. An additional technology challenge is to reject scattered sunlight and thermal emission from each telescope at the beam combiner optics and detector. This stray radiation will arrive at small angles to the starlight beams, making suppression difficult.
The current status and suggested development path of these technologies will be discussed.
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.
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.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.