[Show abstract][Hide abstract] ABSTRACT: The Keck Interferometer (KI) combined the two 10m W. M. Keck Observatory telescopes on Mauna Kea, Hawaii, as a long-baseline near- and mid-infrared interferometer. Funded by NASA, it operated from 2001 until 2012. KI used adaptive optics on the two Keck telescopes to correct the individual wavefronts, as well as active fringe tracking in all modes for path-length control, including the implementation of cophasing to provide long coherent integration times. KI implemented high sensitivity fringe-visibility measurements at H (1.6m), K (2.2m), and L (3.8m) bands, and nulling measurements at N band (10m), which were used to address a broad range of science topics. Supporting these capabilities was an extensive interferometer infrastructure and unique instrumentation, including some additional functionality added as part of the NSF-funded ASTRA program. This paper provides an overview of the instrument architecture and some of the key design and implementation decisions, as well as a description of all of the key elements and their configuration at the end of the project. The objective is to provide a view of KI as an integrated system, and to provide adequate technical detail to assess the implementation. Included is a discussion of the operational aspects of the system, as well as of the achieved system performance. Finally, details on V-2 calibration in the presence of detector nonlinearities as applied in the data pipeline are provided.
Publications of the Astronomical Society of the Pacific 10/2013; 125(932):1226-1264. DOI:10.1086/673475 · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: ASTRA (ASTrometric and phase-Referencing Astronomy) is an upgrade to the existing Keck Interferometer which aims at providing new self-phase referencing (high spectral resolution observation of YSOs), dual-field phase referencing (sensitive AGN observations), and astrometric (known exoplanetary systems characterization and galactic center general relativity in strong field regime) capabilities. With the first high spectral resolution mode now offered to the community, this contribution focuses on the progress of the dual field and astrometric modes.
Proceedings of SPIE - The International Society for Optical Engineering 08/2012; · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Ground-based long baseline interferometers have long been limited in
sensitivity by the short integration periods imposed by atmospheric turbulence.
The first observation fainter than this limit was performed on January 22, 2011
when the Keck Interferometer observed a K=11.5 target, about one magnitude
fainter than its K=10.3 limit. This observation was made possible by the Dual
Field Phase Referencing instrument of the ASTRA project: simultaneously
measuring the real-time effects of the atmosphere on a nearby bright guide
star, and correcting for it on the faint target, integration time longer than
the turbulence time scale are made possible. As a prelude to this
demonstration, we first present the implementation of Dual Field Phase
Referencing on the interferometer. We then detail its on-sky performance
focusing on the accuracy of the turbulence correction, and on the resulting
fringe contrast stability. We conclude with a presentation of early results
obtained with Laser Guide Star AO and the interferometer.
Proceedings of SPIE - The International Society for Optical Engineering 07/2012; 8445. DOI:10.1117/12.926404 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: As part of the astrometric and phase-referenced astronomy (ASTRA)
project, three new science modes are being developed for the Keck
Interferometer that extend the science capabilities of this instrument
to include higher spectral resolution, fainter magnitudes, and
astrometry. We report on the successful implementation of the first of
these science modes, the self-phase-referencing mode, which provides a
K-band (λ=2.2 μm) spectral resolution of R˜1000 on
targets as faint as 7.8 mag with spatial resolution as fine as
λ/B = 5 mas in the K band, with the 85 m interferometer baseline.
This level of spectral resolution would not have been possible without a
phase-referencing implementation extending the integration time limit
imposed by atmospheric turbulence. For narrow spectral features, we
demonstrate a precision of ±0.01 on the differential
V2(λ), and ±1.7 mrad on the differential
Φ(λ), equivalent to a differential astrometry precision of
±1.45 μas. This new Keck Interferometer instrument is
typically used to study the geometry and location of narrow spectral
features at high angular resolution, referenced to a continuum. By
simultaneously providing spectral and spatial information, the geometry
of velocity fields (e.g., rotating disks, inflows, outflows, etc.)
larger than 150 km s-1 can also be explored.
Publications of the Astronomical Society of the Pacific 01/2012; 124(911):51-61. DOI:10.1086/664075 · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the successful science verification phase of a new observing
mode at the Keck interferometer, which provides a line-spread function width
and sampling of 150km/s at K'-band, at a current limiting magnitude of K'~7mag
with spatial resolution of lam/2B ~2.7mas and a measured differential phase
stability of unprecedented precision (3mrad at K=5mag, which represents 3uas on
sky or a centroiding precision of 10^-3). The scientific potential of this mode
is demonstrated by the presented observations of the circumstellar disk of the
evolved Be-star 48Lib. In addition to indirect methods such as multi-wavelength
spectroscopy and polaritmetry, the here described spectro-interferometric
astrometry provides a new tool to directly constrain the radial density
structure in the disk. We resolve for the first time several Pfund emission
lines, in addition to BrGam, in a single interferometric spectrum, and with
adequate spatial and spectral resolution and precision to analyze the radial
disk structure in 48Lib. The data suggest that the continuum and Pf-emission
originates in significantly more compact regions, inside of the BrGam emission
zone. Thus, spectro-interferometric astrometry opens the opportunity to
directly connect the different observed line profiles of BrGam and Pfund in the
total and correlated flux to different disk radii. The gravitational potential
of a rotationally flattened Be star is expected to induce a one-armed density
perturbation in the circumstellar disk. Such a slowly rotating disk oscillation
has been used to explain the well known periodic V/R spectral profile
variability in these stars, as well as the observed V/R cycle phase shifts
between different disk emission lines. The differential line properties and
linear constraints set by our data lend support to the existence of a
radius-dependent disk density perturbation.
The Astrophysical Journal 08/2010; 721(1):802. DOI:10.1088/0004-637X/721/1/802 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Keck Interferometer (KI) combines the two 10m diameter Keck telescopes providing milliarcsecond angular resolution. KI has unique observing capabilities such as sensitive K-band V2, L-band V2 and N-band nulling operations. The instrument status of the Keck Interferometer since the last SPIE meeting in 2008 is summarized. We discuss the performance of new visibility observing capabilities including L-band and self-phase referencing modes. A simultaneous dual-beam-combiner mode in the K and L-band has been demonstrated, nearly doubling operational efficiency for bright targets. Operational improvements including simplified reliable operations with reduced personnel resources are highlighted. We conclude with a brief review of the current and future developmental activities of KI. Details of ASTRA developments, nulling performance and science results are presented elsewhere at this conference.
Proceedings of SPIE - The International Society for Optical Engineering 07/2010; 8445. DOI:10.1117/12.925462 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, the Keck interferometer was upgraded to do self-phase-referencing (SPR) assisted K-band spectroscopy at R ~ 2000. This means, combining a spectral resolution of 150 km/s with an angular resolution of 2.7 mas, while maintaining high sensitiviy. This SPR mode operates two fringe trackers in parallel, and explores several infrastructural requirements for off-axis phase-referencing, as currently being implemented as the KI-ASTRA project. The technology of self-phasereferencing opens the way to reach very high spectral resolution in near-infrared interferometry. We present the scientific capabilities of the KI-SPR mode in detail, at the example of observations of the Be-star 48 Lib. Several spectral lines of the cirumstellar disk are resolved. We describe the first detection of Pfund-lines in an interferometric spectrum of a Be star, in addition to Br ÃÂ³. The differential phase signal can be used to (i) distinguish circum-stellar line emission from the star, (ii) to directly measure line asymmetries tracing an asymetric gas density distribution, (iii) to reach a differential, astrometric precision beyond single-telescope limits sufficient for studying the radial disk structure. Our data support the existence of a radius-dependent disk density perturbation, typically used to explain slow variations of Be-disk hydrogen line profiles.
Proceedings of SPIE - The International Society for Optical Engineering 07/2010; DOI:10.1117/12.856922 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The sensitivity and astrometry upgrade ASTRA of the Keck Interferometer is introduced. After a brief overview of the underlying interferometric principles, the technology and concepts of the upgrade are presented. The interferometric dual-field technology of ASTRA will provide the KI with the means to observe two objects simultaneously, and measure the distance between them with a precision eventually better than 100 μas. This astrometric functionality of ASTRA will add a unique observing tool to fields of astrophysical research as diverse as exo-planetary kinematics, binary astrometry, and the investigation of stars accelerated by the massive black hole in the center of the Milky Way as discussed in this contribution.
New Astronomy Reviews 11/2009; DOI:10.1016/j.newar.2010.07.009 · 6.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We describe the Keck Interferometer nuller theory of operation, data reduction, and on-sky performance, particularly as it applies to the nuller exozodiacal dust key science program that was carried out between 2008 February and 2009 January. We review the nuller implementation, including the detailed phasor processing involved in implementing the null-peak mode used for science data and the sequencing used for science observing. We then describe the Level 1 reduction to convert the instrument telemetry streams to raw null leakages, and the Level 2 reduction to provide calibrated null leakages. The Level 1 reduction uses conservative, primarily linear processing, implemented consistently for science and calibrator stars. The Level 2 processing is more flexible, and uses diameters for the calibrator stars measured contemporaneously with the interferometer’s K-band cophasing system in order to provide the requisite accuracy. Using the key science data set of 462 total scans, we assess the instrument performance for sensitivity and systematic error. At 2.0 Jy we achieve a photometrically-limited null leakage uncertainty of 0.25% rms per 10 minutes of integration time in our broadband channel. From analysis of the Level 2 reductions, we estimate a systematic noise floor for bright stars of ~0.2% rms null leakage uncertainty per observing cluster in the broadband channel. A similar analysis is performed for the narrowband channels. We also provide additional information needed for science reduction, including details on the instrument beam pattern and the basic astrophysical response of the system, and references to the data reduction and modeling tools.
Publications of the Astronomical Society of the Pacific 10/2009; 121(884). DOI:10.1086/606063 · 3.50 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report interferometric observations of the semiregular variable star RS CrB, a red giant with strong silicate emission features. The data were among the first long-baseline mid-infrared stellar fringes obtained between the Keck telescopes, using parts of the new nulling beam combiner. The light was dispersed by a low-resolution spectrometer, allowing simultaneous measurement of the source visibility and intensity spectra from 8 to 12 μm. The interferometric observations allow a nonambiguous determination of the dust shell spatial scale and relative flux contribution. Using a simple spherically symmetric model, in which a geometrically thin shell surrounds the stellar photosphere, we find that ~30% to ~70% of the overall mid-infrared flux—depending on the wavelength—originates from 7-8 stellar radii. The derived shell opacity profile shows a broad peak around 11 μm (τ 0.06), characteristic of Mg-rich silicate dust particles.
The Astrophysical Journal 12/2008; 634(2):L169. DOI:10.1086/498864 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report observations of the nova RS Ophiuchi (RS Oph) using the Keck Interferometer Nuller (KIN), approximately 3.8 days following the most recent outburst that occurred on 2006 February 12. These observations represent the first scientific results from the KIN, which operates in N band from 8 to 12.5 μm in a nulling mode. The nulling technique is the sparse aperture equivalent of the conventional coronagraphic technique used in filled aperture telescopes. In this mode the stellar light itself is suppressed by a destructive fringe, effectively enhancing the contrast of the circumstellar material located near the star. By fitting the unique KIN data, we have obtained an angular size of the mid-infrared continuum emitting material of 6.2, 4.0, or 5.4 mas for a disk profile, Gaussian profile (FWHM), and shell profile, respectively. The data show evidence of enhanced neutral atomic hydrogen emission and atomic metals including silicon located in the inner spatial regime near the white dwarf (WD) relative to the outer regime. There are also nebular emission lines and evidence of hot silicate dust in the outer spatial region, centered at ~17 AU from the WD, that are not found in the inner regime. Our evidence suggests that these features have been excited by the nova flash in the outer spatial regime before the blast wave reached these regions. These identifications support a model in which the dust appears to be present between outbursts and is not created during the outburst event. We further discuss the present results in terms of a unifying model of the system that includes an increase in density in the plane of the orbit of the two stars created by a spiral shock wave caused by the motion of the stars through the cool wind of the red giant star.
The Astrophysical Journal 12/2008; 677(2):1253. DOI:10.1086/529422 · 5.99 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Keck Interferometer combines the two 10 m Keck telescopes as a long baseline interferometer, funded by NASA, as a joint development among the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Since 2004, it has offered an H-and K-band fringe visibility mode through the Keck TAC process. Recently this mode has been upgraded with the addition of a grism for higher spectral resolution. The 10 um nulling mode, for which first nulling data were collected in 2005, completed the bulk of its engineering development in 2007. At the end of 2007, three teams were chosen in response to a nuller key science call to perform a survey of nearby stars for exozodiacal dust. This key science observation program began in Feb. 2008. Under NSF funding, Keck Observatory is leading development of ASTRA, a project to add dual-star capability for high sensitivity observations and dual-star astrometry. We review recent activity at the Keck Interferometer, with an emphasis on the nuller development.
Proceedings of SPIE - The International Society for Optical Engineering 07/2008; DOI:10.1117/12.789476 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Keck Interferometer combines the two 10m diameter Keck telescopes for near-infrared fringe visibility, and mid-infrared nulling observations. We report on recent progress with an emphasis on new visibility observing capabilities, operations improvements for visibility and nulling, and on recent visibility science. New visibility observing capabilities include a grism spectrometer for higher spectral resolution. Recent improvements include a new AO output dichroic for increased infrared light throughput, and the installation of new wave-front controllers on both Keck telescopes. We also report on recent visibility results in several areas including (1) young stars and their circumstellar disks, (2) pre-main sequence star masses, and (3) Circumstellar environment of evolved stars. Details on nuller instrument and nuller science results, and the ASTRA phase referencing and astrometry upgrade, are presented in more detail elsewhere in this conference.
Proceedings of SPIE - The International Society for Optical Engineering 01/2008; DOI:10.1117/12.788070 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we report on progress at the Keck Interferometer since the
2004 SPIE meeting with an emphasis on the operations improvements for
Proceedings of SPIE - The International Society for Optical Engineering 06/2006; 6268:21. DOI:10.1117/12.672069 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Keck Interferometer Nuller (KIN) is one of the major scientific and technical precursors to the Terrestrial Planet Finder Interferometer (TPF-I) mission. KIN's primary objective is to measure the level of exo-zodiacal mid-infrared emission around nearby main sequence stars, which requires deep broad-band nulling of astronomical sources of a few Janskys at 10 microns. A number of new capabilitites are needed in order to reach that goal with the Keck telescopes: mid-infrared coherent recombination, interferometric operation in “split pupil” mode, N-band optical path stabilization using K-band fringe tracking and internal metrology, and eventually, active atmospheric dispersion correction. We report here on the progress made implementing these new functionalities, and discuss the initial levels of extinction achieved on the sky.
Proceedings of the International Astronomical Union 09/2005; 1:227 - 232. DOI:10.1017/S1743921306009367