The Primary Mirror Control System (M1CS) of the Thirty Meter Telescope (TMT) incorporates 1476 precision electromechanical soft actuators that are used to perform closed-loop control of the 492 primary mirror segments in piston, tip and tilt. This paper describes the evolution of the M1CS actuator design from the early concept through several prototype rounds towards a design suitable for production at-scale. It offers insight into how TMT has and continues to meet the unique challenges and opportunities associated with manufacturing across an international partnership, in particular the need for high quality technical oversight and documentation at all stages of the process from prototype development, modeling, drawing production and subsequent assembly, test and verification. Key design decisions, refined through prototyping and testing to ensure optimum performance, reliability and serviceability are highlighted. Insight is given into the activities undertaken by TMT partners and vendors in India, especially as part of the vendor selection and vendor qualification that was undertaken as part of TMT’s Production Qualification Phase (PQP) process.
The Nancy Grace Roman Space Telescope, NASA's next flagship mission in astrophysics, is due for launch in May 2027 with an onboard Coronagraph Instrument (CGI) which will serve as a technology demonstrator for exoplanet direct imaging. The Roman Coronagraph will be capable of detecting and characterizing exoplanets and circumstellar disks in visible light at an unprecedented contrast level of ~108 or better at small separations. The instrument is equipped with six precision alignment mechanisms (PAMs) which enable ultra-stable, sub-micrometer positioning of optical elements such as coronagraphic masks, optical filters and polarizers. In order to achieve contrast level, which are 2 to 3 orders of magnitude better than state-of-the-art visible or near-infrared coronagraphs, the mechanisms need to be stable at sub-microradian levels during a typically 10 hour long science observation. We report here about the development of these mechanisms and present their performance test results from the qualification/flight acceptance test program. All PAM flight models were delivered in the year 2022 and integrated into the CGI flight instrument. Meanwhile CGI has successfully completed all testing at JPL and was shipped to NASA GSFC in May 2024 for final integration into the Roman spacecraft.
The Nancy Grace Roman Space Telescope is NASA’s flagship astrophysics mission planned for launch in 2026. The Coronagraph Instrument (CGI) on Roman will demonstrate the technology for direct imaging and spectroscopy of exoplanets around nearby stars. It will work with the 2.4-meter diameter telescope to achieve starlight suppression and point source detection limits that are 2–3 orders of magnitude deeper than previous space-based and groundbased coronagraphs by using active wavefront control with deformable mirrors. CGI has passed its Critical Design Review (CDR) in April of 2021, and System Integration Review (SIR) in June of 2022. We describe the status of CGI’s development and plans for the upcoming integration and testing phase.
NASA’s next flagship mission, the Nancy Grace Roman Space Telescope, is a 2.4-meter observatory set to launch no later than May 2027. Roman features two instruments: the Wide Field Imager and the Coronagraph Instrument. The Roman Coronagraph is a Technology Demonstration that will push the current capabilities of direct imaging to smaller contrast ratios (∼10−9) and inner-working angles (3 λ/D). In order to achieve this high precision, Roman Coronagraph data must be calibrated to remove as many potential sources of error as possible. Here we present a detailed overview of the current plans for the Nancy Grace Roman Space Telescope Coronagraph Instrument Observation Calibration Plan, including identifying potential sources of error and how they will be mitigated via on-sky calibrations.
NASA’s Nancy Grace Roman Space Telescope (formerly known as WFIRST) is a flagship astrophysics mission planned for launch in 2025. The coronagraph instrument (CGI) on Roman will demonstrate the technology for direct imaging and spectroscopy of exoplanets around nearby stars. It will work with the 2.4-meter diameter telescope to achieve starlight suppression that is 2-3 orders of magnitude deeper than previous space-based and ground-based coronagraphs by using active wavefront control in space with deformable mirrors. CGI has passed its Preliminary Design Review (PDR) in September 2019 and is working toward the instrument Critical Design Review (CDR) in the spring of 2021. We describe the CGI engineering design going into CDR and the operational concept planned for CGI observations.
Primary mirror segment shape correction via Warping Harness (WH) control adjustment is key to obtaining the required image performance of the Thirty Meter Telescope (TMT). We analyzed two separate experimental activities to better predict the segment WH performance. First, we took measurements of WH influence functions and Singular Value Decomposition (SVD) modes on a prototype TMT segment and compared these to model predictions. Second, we applied the TMT control algorithm on-sky at the Keck Observatory during their segment exchange and warping activities. We then used these measurements to improve our WH control simulations to include the observed effects. Altogether, the prototype segment measurements, on-sky TMT control algorithm measurements, and detailed simulation helped to better predict segment correction performance for TMT.
We present an estimate of the optical performance of the Thirty Meter Telescope (TMT) after execution of the full telescope alignment plan. The TMT alignment is performed by the Global Metrology System (GMS) and the Alignment and Phasing System (APS). The GMS first measures the locations of the telescope optics and instruments as a function of elevation angle. These initial measurements will be used to adjust the optics positions and build initial elevation look-up tables. Then the telescope is aligned using starlight as the input for the APS at multiple elevation angles. APS measurements are used to refine the telescope alignment to build elevation and temperature dependent look-up tables. Due to the number of degrees of freedom in the telescope (over 10,000), the ability of the primary mirror to correct aberrations on other optics, the tight optical performance requirements and the multiple instrument locations, it is challenging to develop, test and validate these alignment procedures. In this paper, we consider several GMS and APS operational scenarios. We apply the alignment procedures to the model-generated TMT, which consists of various quasi-static errors such as polishing errors, passive supports errors, thermal and gravity deformations and installation position errors. Using an integrated optical model and Monte-Carlo framework, we evaluate the TMT's aligned states using optical performance metrics at multiple instrument and field of view locations. The optical performance metrics include the Normalized Point Source Sensitivity (PSSN), RMS wavefront error before and after Adaptive Optics (AO) correction, pupil position change, and plate scale distortion.
The Keck telescope segments were manufactured by stressed mirror polishing of large circular pieces of Zerodur that were then cut into hexagons and finished by Ion Beam Figuring (IBF). It has long been believed that this process results in segments with little or no edge effects. As a result, this same general approach is planned for segment manufacturing for the Thirty Meter Telescope (TMT) and the European Extremely Large Telescope (E-ELT). However, recent measurements at the Keck telescope suggest that at least some of the Keck segments have significant aberrations within 60 mm of the edge. These aberrations impact the telescope phasing and the overall telescope image quality. We present interferometric measurements of multiple Keck segments, characterizing the surface errors near the edges over spatial periods from ~5 cm down to ~1 mm. We show that the largest phasing and image quality effects are due to plateaus of unremoved material, left behind after IBF as a result of obscuration by the IBF supports. Apart from these plateaus, the edge quality is relatively good, though not as good as in the segment interiors. Some residual phasing and image quality effects remain, and these are not currently understood.
Ground-baseed long baselinne interferomeeters have lonng been limiteed in sensitiviity by the shoort integration periods imposed by atmospheric tuurbulence. Thee first observaation fainter thhan this limit wwas performedd on January 222, 2011 when the Keck Interferommeter observedd a K=11.5 taarget, about onne magnitude fainter than iits K=10.3 limmit. This observation wwas made posssible by the Duual Field Phase Referencing instrument of the ASTRA pproject: simultaaneously measuring thhe real-time efffects of the atmmosphere on a nearby bright guide star, andd correcting foor it on the fainnt target, integration tiime longer thaan the turbulennce time scale are made possible. As a preelude to this ddemonstration, we first present the implementatioon of Dual FField Phase RReferencing onn the interferoometer. We tthen detail itss on-sky performance focusing on tthe accuracy oof the turbulennce correction, and on the reesulting fringe contrast stabiility. We conclude witth a presentatioon of early resuults obtained wwith Laser Guidde Star AO andd the interferommeter.
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 modes. The
instrument improvements and status of the Keck Interferometer since the 2010 SPIE meeting are summarized. We
discuss the current capabilities of the KI, operational improvements, and the science from the KI during the past two
years. We will conclude with a brief note on the closure of the KI facility. Details of dual field phase referencing
developments and nulling science results are presented elsewhere at this conference.
We report here on some of the major astronomical observations obtained by the Keck Interferometer Nuller (KIN), the
high dynamic range instrument recombining the Keck Telescopes at wavelengths of 8 to 13 microns. A few science
targets were observed during the commissioning phase (2004-2007). These early observations aimed at demonstrating
the KIN’s ability to spatially resolve and characterize circumstellar dust emission around a variety of targets, ranging
from evolved stars to young debris disks. Science operations started then in 2008 with the more demanding KIN exozodi key science programs, augmented by observations of YSOs and hot debris disks between 2009 and 2011. The last
KIN observations were gathered in 2011B, and the interpretation of some of the results depicted here is still preliminary
(exo-zodi survey) or pending (complicated behavior observed in YSOs). We discuss in particular the initial results of the
KIN’s exo-zodi observations, which targeted a total of 40 nearby main sequence single stars. We look for trends in this
sample, searching for possible correlations between the measured KIN excesses and basic stellar properties such as
spectral type or the presence of dust inferred from separate observations.
The Thirty Meter Telescope primary mirror is composed of 492 segments that are controlled to high precision in the presence of wind and vibration disturbances, despite the interaction with structural dynamics. The higher bandwidth and larger number of segments compared with the Keck telescopes requires greater attention to modeling to ensure success. We focus here on the development and validation of a suite of quasi-static and dynamic modeling tools required to support the design process, including robustness verification, performance estimation, and requirements flowdown. Models are used to predict the dynamic response due to wind and vibration disturbances, estimate achievable bandwidth in the presence of control-structure-interaction (CSI) and uncertainty in the interaction matrix, and simulate and analyze control algorithms and strategies, e.g. for control of focus-mode, and sensor calibration. Representative results illustrate TMT performance scaling with parameters, but the emphasis is on the modeling framework itself.
KEYWORDS: Actuators, Mirrors, Turbulence, Servomechanisms, Adaptive optics, Telescopes, Data modeling, Systems modeling, Wind energy, Control systems design
The principal dynamic disturbances acting on a telescope segmented primary mirror are unsteady wind pressure
(turbulence) and narrowband vibration from rotating equipment. Understanding these disturbances is essential
for the design of the segment support assembly (SSA), segment actuators, and primary mirror control system
(M1CS). The wind disturbance is relatively low frequency, and is partially compensated by M1CS; the response
depends on the control bandwidth and the quasi-static stiffness of the actuator and SSA. Equipment vibration is
at frequencies higher than the M1CS bandwidth; the response depends on segment damping, and the proximity
of segment support resonances to dominant vibration tones. We present here both disturbance models and
parametric response. Wind modeling is informed by CFD and based on propagation of a von Karman pressure
screen. The vibration model is informed by analysis of accelerometer and adaptive optics data from Keck. This
information is extrapolated to TMT and applied to the telescope structural model to understand the response
dependence on actuator design parameters in particular. Whether the vibration response or the wind response
is larger depends on these design choices; "soft" (e.g. voice-coil) actuators provide better vibration reduction
but require high servo bandwidth for wind rejection, while "hard" (e.g. piezo-electric) actuators provide good
wind rejection but require damping to avoid excessive vibration transmission to the primary mirror segments.
The results for both nominal and worst-case disturbances and design parameters are incorporated into the TMT
actuator performance assessment.
In order to validate various assumptions about the operating environment of the Thirty Meter Telescope (TMT),
to validate the modeling packages being used to guide the design work for the TMT and to directly investigate
the expected operation of several subsystems we have embarked on an extensive campaign of environmental
measurements at the Keck telescopes. We have measured and characterized the vibration environment around
the observatory floor and at certain locations on the telescope over a range of operating conditions. Similarly the
acoustic environment around the telescope and primary mirror has been characterized for frequencies above 2 Hz.
The internal and external wind and temperature fields are being measured using combined sonic anemometer
and PRT sensors. We are measuring the telescope position error and drive torque signals in order to investigate
the wind induced telescope motions. A scintillometer mounted on the telescope is measuring the optical
turbulence inside the telescope tube. This experimental work is supplemented by an extensive analysis of telescope
and engineering sensor log files and measurements, primarily those of accelerometers located on the main
telescope optics, primary mirror segment edge sensor error signals (residuals), telescope structure temperature
measurements and the telescope status information.
Finite element models (FEMs) are being used extensively in the design of the Thirty Meter Telescope (TMT). One such
use is in the design and analysis of the Primary Segment Assembly (PSA). Each PSA supports one primary mirror
segment on the mirror cell, as well as three actuators, which are used to control three degrees of freedom - tip, tilt, and
piston - of the mirror segment. The dynamic response of the PSA is important for two reasons: it affects the response
of the mirror to fluctuating wind forces, and high-Q modes limit the bandwidth of the control loops which drive the
actuators, and impact vibration transmissivity, thereby degrading image quality. We have completed a series of tests on
a prototype PSA, in which the dynamic response was tested. We report on the test methods used to measure the dynamic
response of the PSA alone and with candidate actuators installed, and we present comparisons between the measured
response and FEM predictions. There is good agreement between FEM predictions and measured response over the
frequency range within which the dynamic response is critical to control system design.
The Thirty Meter Telescope has 492 primary mirror segments, each incorporated into a Primary Segment Assembly
(PSA), each of which in turn has three actuators that control piston, tip, and tilt, for a total of 1476 actuators. Each
actuator has a servo loop that controls small motions (nanometers) and large motions (millimeters). Candidate actuators
were designed and tested that fall into the categories of "hard" and "soft," depending on the offload spring stiffness
relative to the PSA structural stiffness. Dynamics models for each type of actuator are presented, which respectively use
piezo-electric transducers and voice coils. Servo design and analysis are presented that include assessments of stability,
performance, robustness, and control structure interaction. The analysis is presented for a single PSA on a rigid base, and
then using Zernike approximations the analysis is repeated for 492 mirror segments on a flexible mirror cell. Servo
requirements include low-frequency stiffness, needed for wind rejection; reduced control structure interaction, specified
by a bound on the sensitivity function; and mid-frequency damping, needed to reduce vibration transmission. The last of
these requirements, vibration reduction, was found to be an important distinguishing characteristic for actuator selection.
Hard actuators have little inherent damping, which is improved using PZT shunt circuits and force feedback, but still
these improvements were found to result in less damping than is provided by the soft actuator. Results of the servo
analysis were used for an actuator down-select study.
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 γ. 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.
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.
The use of a rotating-baseline nulling interferometer for exoplanet detection was proposed several decades ago, but the
technique has not yet been fully demonstrated in practice. Here we consider the faint companion and exozodiacal disk
detection capabilities of rotating-baseline nulling interferometers, such as are envisioned for space-based infrared
nullers, but operating instead within the aperture of large single telescopes. In particular, a nulling interferometer on a
large aperture corrected by a next-generation extreme adaptive optics system can provide deep interferometric contrasts,
and also reach smaller angles (sub λ/D) than classical coronagraphs. Such rotating nullers also provide validation for an
eventual space-based rotating-baseline nulling interferometer. As practical examples, we describe ongoing experiments
with rotating nullers at Palomar and Keck, and consider briefly the case of the Thirty Meter Telescope.
In this paper, we present the results of three different studies of the Fomalhaut debris disk with infrared interferometry.
First, VLTI/AMBER measurements are used to determine the position angle of the slightly oblate
rapidly rotating photosphere by means of differential phase measurements across the Br-gamma photospheric
line. This measurement allows us to confirm that the debris disk is located in the equatorial plane of its host
star. Second, we use VLTI/VINCI to search for resolved near-infrared emission around the stellar photosphere,
which would correspond to the presence of large amounts of hot dust grains located between the sublimation
radius and the habitable zone. Our observations reveal a small excess of 0.88%±0.12% in K band relative to the
photospheric flux. Finally, we use the Keck Interferometer Nuller in order to derive additional constraints on the
nature of the resolved infrared emission. Our observations suggest a marginal detection of a circumstellar excess
at 10 μm, which we use together with the VINCI detection to model the circumstellar emission. Preliminary results from this modeling effort are discussed.
KEYWORDS: Stars, Interferometers, Calibration, K band, L band, Interferometry, Nulling interferometry, Visibility, Active galactic nuclei, Data modeling
The addition of new observational capabilities and continued sensitivity improvements have allowed observations
with the Keck Interferometer to encompass new areas of astrophysics and expanded significantly the available sample size in areas which had been the focus of previous work. The technical details of the instrument techniques (including nulling, L-band and increased spectral resolution) are covered in other contributions to this conference. Here, we will highlight the astrophsyics enabled by these instruments including: a summary of the NASA Exo-zodical Dust Survey Key Project, observations across a range of dust temperatures with K and L-band measurements and faint target studies of active galactic nuclei and young stellar disks.
The Keck Interferometer combines the two 10 m Keck telescopes as a long baseline interferometer. It is funded by
NASA as a joint development among the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the NASA
Exoplanet Science Institute. In February 2008, the 10 um nulling mode began a 32 night observing program with three
key science teams to perform a survey of nearby stars for exozodiacal dust. This program has recently concluded, and
has been followed by nuller observing on a variety of science topics through the standard proposal process. We provide a
review and update of the nuller implementation, and describe the data reduction process, including the calibration
approach. We then review the technical performance of the instrument based on the full key science data set, including
sensitivity and systematic errors. We also provide some summary data on atmospheric effects applicable to the cophasing approach.
The ASTrometric and phase-Referenced Astronomy (ASTRA) project will provide phase referencing and astrometric
observations at the Keck Interferometer, leading to enhanced sensitivity and the ability to monitor
orbits at an accuracy level of 30-100 microarcseconds. Here we discuss recent scientific results from ASTRA,
and describe new scientific programs that will begin in 2010-2011. We begin with results from the "self phase
referencing" (SPR) mode of ASTRA, which uses continuum light to correct atmospheric phase variations and
produce a phase-stabilized channel for spectroscopy. We have observed a number of protoplanetary disks using
SPR and a grism providing a spectral dispersion of ~ 2000. In our data we spatially resolve emission from dust
as well as gas. Hydrogen line emission is spectrally resolved, allowing differential phase measurements across the
emission line that constrain the relative centroids of different velocity components at the 10 microarcsecond level.
In the upcoming year, we will begin dual-field phase referencing (DFPR) measurements of the Galactic Center
and a number of exoplanet systems. These observations will, in part, serve as precursors to astrometric monitoring
of stellar orbits in the Galactic Center and stellar wobbles of exoplanet host stars. We describe the design
of several scientific investigations capitalizing on the upcoming phase-referencing and astrometric capabilities of ASTRA.
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.
We report observations of the nova RS Ophiuchi using the Keck Interferometer Nuller (KIN) taken approximately
3.8 days following the most recent outburst that occurred on 2006 February 12. The KIN operates in N-band
from 8 to 12.5 μm in a nulling mode. In this mode the stellar light is suppressed by a destructive fringe, effectively
enhancing the contrast of the circumstellar material located near the star. In a second, constructive-fringe mode,
the instrument detects primarily the light from the central, bright source. These are the outer and inner spatial
regimes, respectively. We will describe the capabilities of the KIN, including these unique modes, and outline
how they were key in our discovery that dust was created between nova events. We also show how these first
results from the KIN are consistent with Spitzer data. The KIN data show evidence of enhanced neutral atomic
hydrogen emission and atomic metals including silicon located in the inner spatial regime (< 4 AU from theWD)
relative to the outer regime. There are also nebular emission lines and evidence of hot silicate dust in the outer
spatial region, centered at approximately ~ 17 AU from the WD, that are not found in the inner regime. The
KIN and Spitzer data suggest that these emissions were excited in the outer spatial regime before the blast wave
reached these regions. We describe the present results in terms of a new model for dust creation in recurrent
novae 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. These data show the power
and potential of the nulling technique which has been developed for the detection of Earth-like planets around
nearby stars for the Terrestrial Planet Finder Mission and Darwin missions.
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.
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.
The Keck interferometer has its V2 science mode open to its astronomical community and the Nuller science mode is
maturing in its development. In order to push on and improve the limits of the instrument a program of analyzing the
characteristics of its beamlines has begun. The purpose of this endeavor is to understand beamtrain characteristics for
assessing and improving overall system performance. In this paper we present some of the initial results from
measurements as well as preliminary analyses of polarization and wavefront quality. Polarization measurements were
made on the internal beamtrains for two orthogonal telescope azimuth positions. The wavefront test is a static beamline
measurement using a Shack-Hartmann sensor to sample the wavefront quality of the beamtrain. Also results from a
dynamic beamtrain monitoring scheme is presented that involves measurements from the angle tracking system during
on-sky operation.
The Keck Angle Tracker (KAT) is a key subsystem in the NASA-funded Keck Interferometer at the Keck Observatory on the summit of Mauna Kea in Hawaii. KAT, which has been in operation since the achievement of first fringes in March 2001, senses the tilt of the stellar wavefront for each of the beams from the interferometer telescopes and provides tilt error signals to fast tip/tilt mirrors for high-bandwidth, wavefront tilt correction. In addition, KAT passes low-bandwidth, desaturation offsets to the adaptive optics system of the Keck telescopes to correct for slow pointing drifts. We present an overview of the instrument design and recent performance of KAT in support of the V2 science and nulling observing modes of the Keck Interferometer.
Water vapor is the dominant source of randomly-changing atmospheric dispersion on timescales of seconds to minutes in the near- and mid-infrared. The dispersion changes are sufficient to limit the performance of the Keck Nuller unless steps are taken to measure and compensate for them. Here we present the first measurements of water vapor differential column fluctuations with the mid-infrared Keck Nuller and its near-infrared fringe tracker, taken in October 2005, and discuss theoretical and practical aspects of our dispersion feedforward implementation. The data show much larger fluctuations than were seen in median Mauna Kea conditions measured at radio wavelengths, and probably account for the generally poor performance of the Nuller during the observing run. The measurements in the two bands show strong correlations, indicating that the planned feedforward of the near-infrared value to stabilize the dispersion in the mid-infrared will substantially reduce the residual dispersion fluctuations seen by the Nuller.
The Keck Interferometer Nuller is designed to detect faint off-axis mid-infrared light a few tens to a few hundreds of milliarcseconds from a bright central star. The starlight is suppressed by destructive combination along the long (85 m) baseline, which produces a fringe spacing of 25 mas at a wavelength of 10 μm, with the central null crossing the position of the star. The strong, variable mid-infrared background is subtracted using interferometric phase chopping along the short (5 m) baseline. This paper presents an overview of the observing and data reduction strategies used to produce a calibrated measurement of the off-axis light. During the observations, the instrument cycles rapidly through several calibration and measurement steps, in order to monitor and stabilize the phases of the fringes produced by the various baselines, and to derive the fringe intensity at the constructive peak and destructive null along the long baseline. The data analysis involves removing biases and coherently demodulating the short-baseline fringe with the long-baseline fringe tuned to alternate between constructive and destructive phases, combining the results of many measurements to improve the sensitivity, and estimating the part of the null leakage signal which is associated with the finite angular size of the central star. Comparison of the results of null measurements on science target and calibrator stars permits the instrumental leakage - the "system null leakage" - to be removed and the off-axis light to be measured.
The Keck Interferometer Nuller (KIN) is now largely in place at the Keck Observatory, and functionalities and
performance are increasing with time. The main goal of the KIN is to examine nearby stars for the presence of exozodiacal
emission, but other sources of circumstellar emission, such as disks around young stars, and hot exoplanets are
also potential targets. To observe with the KIN in nulling mode, knowledge of the intrinsic source spectrum is essential,
because of the wide variety of wavelengths involved in the various control loops - the AO system operates at visible
wavelengths, the pointing loops use the J-band, the high-speed fringe tracker operates in the K-band, and the nulling
observations take place in the N-band. Thus, brightness constraints apply at all of these wavelengths. In addition, source
structure plays a role at both K-band and N-band, through the visibility. In this talk, the operation of the KIN is first
briefly described, and then the sensitivity and performance of the KIN is summarized, with the aim of presenting an
overview of the parameter space accessible to the nuller. Finally, some of the initial observations obtained with the KIN
are described.
We describe the results of laboratory experiments, using a mock-up stellar interferometer equipped with specialized hardware, undertaken to measure differential-phase to considerable precision (0.1 mrad) over an octave of bandwidth in the infrared. Differential-phase is a precision technique that can detect subtle temporal changes in the relative (color-dependent) photocenter of an astronomical target - making it useful for direct detection of some hot-Jupiter planets from the ground. The set up described herein was built as part of the Keck Interferometer project.
The Keck Interferometer links the two 10m Keck Telescopes located atop Mauna Kea in Hawaii. It was the first 10m
class, fully AO equipped interferometer to enter operation. Further, it is the first large interferometer to implement a
nuller, whereby the on axis light from a bright point source (e.g. a star) can be removed interferometrically, allowing
study of light from nearby, low contrast sources (e.g. exo-zodiacal dust).
This paper describes the control system we have implemented to enable operation of the Keck interferometer nuller. We
give a general overview of the control system, plus details of how control differs from the already implemented and
operational, standard visibility science mode of the interferometer. The nuller is challenging in its requirements for
control because of the necessary control precision and the complexity of the number of points of control. We have
implemented some novel control methods to meet these requirements and we describe those here.
KEYWORDS: Servomechanisms, Nulling interferometry, Control systems, K band, Interferometers, Signal to noise ratio, Mirrors, Data corrections, Detection and tracking algorithms, Secondary tip-tilt mirrors
The real-time control system for the Keck Interferometer Nuller provides the N-band fringe tracking capabilities of the instrument, as well as correcting for atmospheric dispersion in the system. There are three closed-loop servos for controlling the N-band path, as well as two K-band servos which provide open-loop control. A system of synchronized "gates" allows all N-band fringe trackers to operate simultaneously, making it possible to interleave servo corrections with data collection. Several methods of improving servo performance and maintenance of control schemes are discussed.
In this paper we report on progress at the Keck Interferometer since the 2004 SPIE meeting with an emphasis on the operations improvements for visibility science.
A new observing mode for the Palomar Testbed Interferometer was developed in 2002-2003 which enables differential astrometry at the level of 20 micro-arcseconds (μas) for binary systems with separations of several hundred milli-arcseconds (mas). This phase-referenced mode is the basis of the Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES), a search for giant planets orbiting either the primary or secondary star in fifty binary systems. We present the first science results from the PHASES search. The properties of the stars comprising binary systems are determined to high precision. The mutual inclinations of several hierarchical triple star systems have been determined. We will present upper limits constraining the the existence of giant planets
in a few of the target systems.
The nulling mode of the Keck Interferometer is being commissioned at the Mauna Kea summit. The nuller combines the two Keck telescope apertures in a split-pupil mode to cancel the on-axis starlight and coherently detect the residual signal. The nuller, working at 10 um, is tightly integrated with the other interferometer subsystems including the fringe and angle trackers, the delay lines and laser metrology, and the real-time control system. Since first 10 um light in August 2004, the system integration is proceeding with increasing functionality and performance, leading to demonstration of a 100:1 on-sky null in 2005. That level of performance has now been extended to observations with longer coherent integration times. An overview of the overall system is presented, with emphasis on the observing sequence, phasing system, and differences with respect to the V2 system, along with a presentation of some recent engineering data.
The Keck Interferometer Nuller (KIN) will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. A successful pre-ship review was held for the KIN in June 2004, after which the KIN was shipped to the Keck Observatory. The integration of the KIN's many sub-systems on the summit of Mauna Kea, and initial on-sky testing of the system, has occupied the better part of the past year. This paper describes the KIN system-level configuration, from both the hardware and control points of view, as well as the current state of integration of the system and the measurement approach to be used. During the most recent on-sky engineering runs in May and July 2005, all of the sub-systems necessary to measure a narrowband null were installed and operational, and the full nulling measurement cycle was carried out on a star for the first time.
Mid-infrared (8-13μm) nulling is a key observing mode planned for the NASA-funded Keck Interferometer at the Keck Observatory on the summit of Mauna Kea in Hawaii. By destructively interfering and thereby canceling the on-axis light from nearby stars, this observing mode will enable the characterization of the faint emission from exo-zodiacal dust surrounding these stellar systems. We report here the null leakage error budget and pre-ship results obtained in the laboratory after integration of the nulling beam combiner with its mid-infrared camera and key components of the Keck Interferometer. The mid-infrared nuller utilizes a dual-polarization, modified Mach-Zehnder (MMZ) beam combiner in conjunction with an atmospheric dispersion corrector to achieve broadband achromatic nulling.
The first high-dynamic-range interferometric mode planned to come on line at the Keck Observatory is mid-infrared nulling. This observational mode, which is based on the cancellation of the on-axis starlight arriving at the twin Keck telescopes, will be used to examine nearby stellar systems for the presence of circumstellar exozodiacal emission. This paper describes the system level layout of the Keck Interferometer Nuller (KIN), as well as the final performance levels demonstrated in the laboratory integration and test phase at the Jet Propulsion Laboratory prior to shipment of the nuller hardware to the Keck Observatory in mid-June 2004. On-sky testing and observation with the mid-infrared nuller are slated to begin in August 2004.
The visibility science mode of the Keck Interferometer fully transitioned into operations with the successful completion of its operational readiness review in April 2004. The goal of this paper is to describe this science mode and the operations structure that supports it.
One of the science goals of NASA's Navigator program is ground-based narrow-angle astrometry for extra-solar planet detection, which could be done as part of the proposed Outrigger Telescopes Project. The narrow-angle measurement process, which would use the outrigger telescopes, starts with the determination of the conventional interferometer astrometric baseline, determined from wide-angle astrometry of Hipparcos stars. A baseline monitor system would be employed at each outrigger telescope. This system monitors the pivot point of each telescope - the end point of the astrometric baseline - to measure telescope imperfections that would cause the baseline to vary with telescope rotation. The baseline monitor includes azimuth and elevation cameras that monitor runout along the azimuth and elevation axes of the telescopes. In conjunction with the baseline monitor system, a pivot monitor camera in the dual-star module is used to register the laser metrology corner-cube reflector to the telescope pivot, tying the narrow-angle baseline, which applies to the narrow-angle astrometric measurement, to the wide-angle baseline. In this paper we present the proposed designs for the baseline monitor and pivot-point camera.
Keck Interferometer is a NASA-funded project to combine the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 μm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot-Jupiters" by their direct emission. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Recent activity has included formal visibility mode commissioning, as well as science observations, and we briefly review some of the significant technical aspects and updates to the system. We have also completed laboratory development of the nuller. The nuller uses two modified Mach-Zehnder input nullers, a Michelson cross combiner, and a 10 μm array camera to produce background-limited null measurements. To provide required temporal stability for the nuller, the system incorporates end-to-end laser metrology with phase referencing from two 2.2 μm fringe trackers. The nuller recently completed its pre-ship review and is being installed on the summit. After nuller integration and test, the differential phase mode will be deployed, which will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control.
We summarize the implementation of the Differential Phase (DP) mode at the Keck Interferometer. Multicolor phase measurements are potentially a powerful astrophysical probe -- and can allow direct detection of Roaster-type exoplanets from the ground. The Keck Interferometer will measure differential phases between H-K, and K-L bands to levels of 3 mrad or better. At JPL, we are engaged in development and testing of instrumentation that will enable these extremely sensitive measurements. First on-sky observations are expected to start in middle 2005. In this article we describe DP and other related techniques, provide an outline of the instrument and present results from preliminary laboratory experiments.
The Keck Interferometer includes an autoalignment system consisting of pop-up targets located at strategic locations along the beam trains of each arm of the instrument along with a sensor and control system. We briefly describe the hardware of the system and then proceed to a description of the two operational modes of the system. These are: 1) to provide an initial alignment of the coude paths in each arm, and 2) to recover coude alignments between changes of the static delay sled positions. For the initial alignment mode, we review the system performance requirements along with the software used for image acquisition and centroiding. For coudé alignment recovery, we describe beam-train surveys through the static delay (Long Delay Line) and criteria for a successful recovery of a coudé alignment. Finally, we describe the results of testing of the autoalignment system.
The NASA Outrigger Telescope Project is a ground-based component of NASA's Navigator Program. The proposed project would utilize four to six 1.8-meter telescopes with co-rotating domes configured as an interferometer. One of the project’s scientific goals is the detection of exoplanets, which would be accomplished with long baseline narrow-angle astrometry. This astrometry mode would be able to detect Uranus mass planets up to 60 light years away. The requirements of narrow-angle astrometry, both technically and operationally, levy requirements on the telescopes and enclosures, including, for example, wavefront quality, pivot stability, and slew speed. This paper will describe these requirements and how they were achieved in the design. It will also discuss the testing and verification of these requirements. Actual telescope performance as tested at EOS Technologies is presented elsewhere in these proceedings.
We discuss recent work from the Palomar Testbed Interferometer (PTI), including science results and system improvements. In the past two years PTI has been used to observe a wide range of scientifically interesting sources, including binaries, Cepheids and Miras. In addition PTI has been used to observe departures from spherical symmetry in several stars. Recent system improvements incude a new low read-noise camera based on a HAWAII infrared array, routine opteration in two baselines, and operation in the J band. Future developments include an upgrade to three-aperture combination and closure phase measurements, and double-Fourier interferometry.
The Keck Interferometer combines the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 μm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot Jupiters" by their direct emission. First fringes with the interferometer were obtained in March 2001 using the two Kecks with their adaptive optics systems. Subsequent engineering work has been focused toward the visibility mode in the areas of system validation, and improving sensitivity, increasing automation, and adding functionality in preparation for nulling and differential phase. Recently four shared-risk teams were selected by NASA to participate in early science observations, and initial shared-risk science observations have begun.
The control of longitudinal dispersion, which determines the position of the null fringe as a function of wavelength, is central to the problem of producing deep broadband interferometric nulls. The dispersion is the sum of terms due to environmental factors such as the dry-air and water-vapor atmospheric seeing, the unbalanced air column due to the unequal delay-line paths between the telescopes the combiner, and to the distance from the central fringe. The difference between an achromatic nuller and a normal constructive combiner operating at its first (chromatic) null can be thought of as an added longitudinal dispersion, which for the case of the Keck Interferometer is smaller than the environmental terms. We demonstrate that the sum of these effects can be adequately compensated by an appropriate thickness of ZnSe combined with an additional achromatic pathlength. The Keck Interferometer nulling combiners take advantage of this result. They are intrinsically constructive combiners made to produce achromatic nulls by inserting a ZnSe dispersion corrector into each of the four input beams. We describe the null fringe stabilization control algorithm and present calculations of the required loop bandwidth and precision. A potentially important advantage of the present approach is that the system will be able to function as either a destructive or constructive combiner, depending on the value of a single control-loop parameter (the target fringe phase).
KEYWORDS: Interferometers, Cameras, Stars, Signal to noise ratio, Control systems, Servomechanisms, Electronics, Staring arrays, Adaptive optics, K band
The fringe detection and tracking system of the Keck Interferometer, Fatcat, has been operational ever since first fringes at Keck, albeit not in full capacity. At present it supports single baseline (Keck-Keck) operations only. We briefly discuss the instrument design from a hardware and controls standpoint. We also show some recent data from the instrument and summarize some performance limits.
A key thrust of NASA's Origins program is the development of
astronomical interferometers. Pursuing this goal in a cost-effective and expedient manner from the ground has led NASA to develop the Keck Interferometer, which saw first fringes between the twin 10m Keck telescopes in March of 2001. In order to enhance the imaging potential of this facility, and to add astrometric capabilities for the detection of giant planets about nearby stars, four 1.8 m 'outrigger' telescopes may be added to the interferometer. Robust performance of the multi-aperture instrument will require precise alignment of the large number of optical elements found in the six optical beamtrains spread about the observatory site. The requirement for timely and reliable alignments dictated the development of an automatic alignment system for the Keck Interferometer. The autoaligner consists of swing-arm actuators that insert light-emitting diodes on the optical axis at the location of each optical element, which are viewed by a simple fixed-focus CCD camera at the end of the beamtrain. Sub-pixel centroiding is performed upon the slightly out-of-focus target spots using images provided by a frame grabber, providing steering information to the two-axis actuated optical elements. Resulting mirror-to-mirror alignments are good to within 2 arcseconds, and trimming the alignment of a full beamtrain is designed to take place between observations, within a telescope repointing time. The interactions of the autoaligner with the interferometer delay lines and coude trains are discussed in detail. The overall design of the interferometer's autoaligner system is presented, examining the design philosophy, system sequencing, optical element actuation, and subsystem co-alignment, within the context of satisfying performance requirements and cost constraints.
The W.M. Keck Observatory is conducting a focused effort to identify and mitigate facility vibrations that significantly affect optimal optical performance. This effort should improve the performance of both Keck adaptive optics systems, the laser guide star, the AO instruments, and the interferometer, and will benefit future high precision instruments.
We present our strategy for mitigating vibrations in a large ground-based telescope. Our approach is to establish reasonable confidence in identifying the facility vibration sources that most significantly deteriorate optical performance. For the interferometer we completed vibration surveys that correlate vibrations on the interferometer beam path with direct vibration measurements on the telescope structure and facility. We developed a metric to evaluate the effect of vibrations on the entire interferometer beamline. From our surveys, we prioritized facility components to be addressed, and developed approaches to mitigate key vibrations contributions. Initial results show large local improvements, and global improvements to our vibration environment.
Following a previous successful study, we present new and more complete interferometric observations of FU Orionis. The combination of both IOTA (Infrared and Optical Telescope Array, Mt. Hopkins, AZ) and PTI (Palomar Testbed Interferometer, Palomar Observatory, CA) interferometers allowed an increase in (u, v) coverage and H and K bands measurements. We confirm the presence of a resolved structure around FU Ori that can be interpreted in terms of accretion disk. However, we find significant differences between our results and standard accretion disks models. In particular the temperature power law is best explained if two different radial regimes are used. Moreover, a clear visibility oscillation trend at 110 m is well fitted with a binary (or hot spot) model. This may have important implications on accretion disk models for such objects.
The Palomar Testbed Interferometer is a long-baseline interferometer that uses both phase and group-delay measurements for narrow-angle astrometry. The group-delay measurements are performed using 5 spectral channels across the band from 2.0 to 2.4 micrometers . Group delay is estimated from phasors (sine and cosine of fringe phase) calculated for each spectral channel using pathlength modulation of one wavelength. Normally the group delay is estimated to be the delay corresponding to the peak of the power spectrum of these complex phasors. The Fourier transform does not however yield a least-squares estimate of the delay. Nevertheless, the precision of phase estimation can be achieved in a group-delay estimate using a least-squares approach. We describe the least-squares group-delay estimator that has been implemented at PTI and illustrate its performance as applied to narrow-angle astrometry.
The Palomar Testbed Interferometer is a long-baseline near- infrared interferometer operating at Palomar Observatory, CA. The interferometer has a maximum baseline of 110 m, 40- cm collecting apertures, and active fringe tracking. It also incorporates a dual-star architecture to enable cophasing and narrow-angle astrometry. We will discuss recent system improvements and engineering results. These include upgrades to allow for longer coherent integration times, H band operation, and cophasing using delay line feedforward. Recent engineering tests of astrometry in dual-star mode have shown a night-to-night repeatability of 100 (mu) as on a bright test target. Several new observation planning tools have been developed, and data reduction tools have been automated to allow fully pipelined nightly reductions and archiving.
The Keck Interferometer is a NASA-funded joint development between JPL and the W. M. Keck Observatory. The interferometer will combine the two 10-m Keck telescopes with four 1.8-m outrigger telescopes in several observing modes. These include: nulling interferometry at 10 micrometers to measure the quantity of exozodiacal emission around nearby stars; near-infrared differential-phase measurements to detect `hot Jupiters' by their direct emission; narrow-angle astrometry to search for exoplanets by their astrometric signature; and near-infrared imaging to address a variety of imaging science. Active development of the instrument subsystems and associated infrastructure is underway at JPL and CARA.
We present the motivation and development of the novel `differential phase' technique being developed for the Keck Interferometer with the goal of detecting faint companions near a bright source. The differential phase technique uses simultaneous phase measurements at several infrared wavelengths to detect the astrophysical signature produced by a chromatic, asymmetric brightness distribution. We discuss the origin of the differential phase signature and present results of test observations taken at the Palomar Testbed Interferometer. One important test result is the larger than expected effect of water vapor turbulence on these multi-wavelength observations due to the infrared dispersion of water. In order to reach the design goal of 0.1 milliradians, the phase noise caused by both temperature and water vapor fluctuations in the atmosphere must be corrected, and we discuss several ways to achieve this.
The Palomar Testbed Interferometer (PTI) is an infrared, phase-tracking interferometer in operation at Palomar Mountain since July 1995. It was funded by NASA for the purpose of developing techniques and methodologies for doing narrowangle astrometry for the purpose of detecting extrasolar planets. The instrument employs active fringe trackingin the infrared (2.0-2.4 μm) to monitor fringe phase. It is a dual-star interferometer; it is able to measure fringes on two separate stars simultaneously. An end-to-end heterodyne laser metrology system is used to monitor the optical path length of the starlight. Recently completed engineering upgrades have improved the initial instrument performance. These upgrades are:extended wavelength coverage, a single mode fiber for spatial filtering, vacuum pipes to relay the beams, accelerometers on the siderostat mirrors and a new baseline. Results of recent astrometry data indicate the instrument is approaching the astrometric limit as set by the atmosphere.
A key thrust of NASA's Origins program is the search for and detection of planetary systems about other stars. Pursuing this goal in a cost-effective and expedient manner from the ground has led NASA to begin work on the Keck Interferometer, which will add 4 1.8m 'outrigger' telescopes at the Keck Observatory on Mauna Kea. In addition to the imaging science to be performed by the Keck 10m telescopes with the outriggers, another one of the principal capabilities of the instrument will be the ability for the outriggers to conduct relative astrometry at the 25 microarcsecond level per root hour. Astrometry of this accuracy will enable the array to detect planetary systems composed of Uranus-mass or larger bodies orbiting at 5 AU solar mass stars at a distance of 20 pc; over 300 stars are to be surveyed by the outriggers annually. The astrometric capabilities of the Keck array can also be utilized other astrophysical investigations, such as characterization of spectroscopic binary orbits, and the measurement of the center-of-light shift of MACHO microlensing events, which will allow for a model-independent determinations of lens masses.
KEYWORDS: Calibration, Visibility, Stars, Interferometers, Data modeling, Visualization, Spectroscopy, Information technology, K band, Computing systems
The Palomar Testbed Interferometer is a long-baseline, near-infrared astronomical interferometer capable of visibility measurement and narrow-angle differential astrometry. In this submission we consider the problem of fringe amplitude calibration as applied to the study of single and binary star systems with PTI. Methodologies for selecting appropriate calibration objects, and performing the calibrations to produce consistent multi-night datasets are considered. Applications of such calibrated datasets to binary orbit determination and stellar diameter measurements will be presented.
JPL and CARA are building a multi-element, IR interferometer for NASA to be situated at the twin Keck Observatories on Mauna Kea, Hawaii. Initially, the 10-m diameter Keck telescopes will be augmented with four fixed-location 2-m class outrigger telescopes resulting in 15 non-redundant baselines, the longest being approximately equals 110 m or nearly 5 X 107 ((lambda) /2.2micrometers )-1 wavelengths. Fast adaptive optics and tip-tilt corrections will be used to phase up the Keck and outrigger apertures, respectively. The entire array will be co-phased by observing a relatively bright target on the photon rich Keck-Keck (K-K) and Keck- outrigger (K-O) baselines. When fully phased, the projected fringe phaser sensitivity for unresolved targets will be K- 22.0, 20.0 and 17.9 on the K-K, K-O and O-O baselines, respectively. Synthetic imaging capability will be available in the 1.6-10.0 micrometers atmospheric transmission bands at angular resolutions of 4.0 milli-arcseconds. In this article, we briefly outline the adopted methodology, imaging hardware, projected sensitivities and summarize the scientific potential of the instrument as an imaging interferometer.
The Keck Interferometer is being developed by JPL and CARA as one of the ground-based components of NASA's Origins Program. The interferometer will combine the two 10-m Keck telescopes with four proposed 1.8-m outrigger telescopes located at the periphery of the Keck site on Mauna Kea. Incorporation of adaptive optics on the Keck telescopes with cophasing using an isoplanatic reference provides high sensitivity. Back-end instrumentation will include two-way combiners for cophasing and single-baseline measurements, a nulling combiner for high-dynamic range measurements, and a multi-way imaging combiner. Science objectives include the characterization of zodiacal dust around other stars, detection of hot Jupiters and brown dwarfs through multi- color differential-phase measurements, astrometric searches for planets down to Uranus-mass, and a wide range of IR imaging.
A separated spacecraft optical interferometer mission concept proposed for NASA's New Millennium Program is described. The interferometer instrument is distributed over three small spacecraft: two spacecraft serve as collectors, directing starlight toward a third spacecraft which combines the light and performs the interferometric detection. As the primary objective is technology demonstration, the optics are modest size, with a 12-cm aperture. The interferometer baseline is variable from 100 m to 1 km, providing angular resolutions from 1 to 0.1 milliarcseconds. Laser metrology is used to measure relative motions of the three spacecraft. High-bandwidth corrections for stationkeeping errors are accomplished by feedforward to an optical delay line in the combiner spacecraft; low-bandwidth corrections are accomplished by spacecraft control with an electric propulsion or cold-gas system. Determination of rotation of the constellation as a whole uses a Kilometric Optical Gyro, which employs counter-propagating laser beams among the three spacecraft to measure rotation with high accuracy. The mission is deployed in a low-disturbance solar orbit to minimize the stationkeeping burden. As it is well beyond the coverage of the GPS constellation, deployment and coarse stationkeeping are monitored with a GPS-like system, with each spacecraft providing both transmit and receive ranging and attitude functions.
The Stellar Interferometer Technology Experiment (SITE) is a near-term precursor mission for spaceborne optical interferometry. Proposed by the MIT Space Engineering Research Center and NASA's Jet Propulsion Laboratory, SITE is a two-aperture stellar interferometer located in the payload bay of the Space Shuttle. It has a baseline of four meters, operates with a detection bandwidth of 300 nanometers in the visible spectrum, and consists of three optical benches kinematically mounted inside a precision truss structure. The objective of SITE is to demonstrate system-level functionality of a space-based stellar interferometer through the use of enabling and enhancing Controlled Structures Technologies such as vibration isolation and suppression. Moreover, SITE will validate, in the space environment, technologies such as optical delay lines, laser metrology systems, fringe detectors, active fringe trackers, and high- bandwidth pointing control systems which are critical for realizing future space-based astrometric and imaging interferometers.
The Mark III Interferometer on Mt. Wilson, a long-baseline optical interferometer, was in daily operation for more that seven years. During that time it achieved milliarcsecond angular resolution for binary star astronomy, with submilliarcsecond accuracy. For the first time many spectroscopic binaries have been resolved, including binaries in which the companion cannot be detected with spectroscopy. The high angular resolution means that the traditional gap between visual and spectroscopic binaries has been decreased by more than an order of magnitude. In order to confirm the performance of the Mark III Interferometer, this paper uses the results of astronomical observations, and compares the Mark III Interferometer with other high-resolution techniques, including astrometry, lunar occultation, photometry, speckle, and spectroscopy. Comparisons for a variety of binary stars among these techniques indicate that long baseline optical interferometry proves a reliable, fully automatic, daily accessible astronomical capability for achieving high resolution, high accuracy, high dynamic range, and high photometric measurement precision for the study of binary stars.
The ASEPS-O Testbed Interferometer is a long-baseline infrared interferometer optimized for high-accuracy narrow-angle astrometry. It is being constructed by JPL for NASA as a testbed for the future Keck Interferometer to demonstrate the technology for the astrometric detection of exoplanets from the ground. Recent theoretical and experimental work has shown that extremely high accuracy narrow-angle astrometry, at the level of tens of microarcseconds in an hour of integration time, can be achieved with a long-baseline interferometer measuring closely-spaced pairs of stars. A system with performance close to these limits could conduct a comprehensive search for Jupiter- and Saturn-mass planets around stars of all spectral types, and for short-period Uranus-mass planets around nearby M and K stars. The key features of an instrument which can achieve this accuracy are long baselines to minimize atmospheric and photon-noise errors, a dual-star feed to route the light from two separate stars to two beam combiners, cophased operation using an infrared fringe detector to increase sensitivity in order to locate reference stars near a bright target, and laser metrology to monitor systematic errors. The ASEPS-O Testbed Interferometer will incorporate these features, with a nominal baseline of 100 m, 50- cm siderostats, and 40-cm telescopes at the input to the dual- star feeds. The fringe detectors will operate at 2.2 micrometers , using NICMOS-III arrays in a fast-readout mode controlling high-speed laser-monitored delay lines. Development of the interferometer is in progress, with installation at Palomar Mountain planned to begin in 1994.
SONATA--Small OSI (Orbiting Stellar Interferometer) for Narrow angle Astrometry with Two Apertures--is a concept for a space based interferometer capable of detecting extra-solar planets. The instrument is an extension of the TOPS-0 interferometer testbed concept which is a ground-based dual feed interferometer, and the space-based OSI concept, which is being studied for the Astrophysics Division of NASA. The SONATA instrument uses a quadruple- feed interferometer which will be capable of measuring fringes on four stars simultaneously within a 10 arcminute field of view. The starlight is collected by two 0.4 meter telescopes separated by 7 meters. The use of common collecting optics results in cancellation of a large number of systematic errors found in multiple baseline designs. The targeted astrometric accuracy for SONATA is 0.5 uas. In the photon-noise limit, this performance can be achieved on 14th magnitude objects by integrating for 4 hours. This level of accuracy will enable detection of nearby Earth type planets. The SONATA design uses a non-deploying structure and will be launched on an Atlas II/Centaur for insertion into a 900 km Sun-synchronous orbit.
The long baselines of the next-generation ground-based optical stellar interferometers require optical delay lines which can maintain nm-level path-length accuracy while moving at high speeds. NASA-JPL is currently designing delay lines to meet these requirements. The design is an enhanced version of the Mark III delay line, with the following key features: hardened, large diameter wheels, rather than recirculating ball bearings, to reduce mechanical noise; a friction-drive cart which bears the cable-dragging forces, and drives the optics cart through a force connection only; a balanced PZT assembly to enable high-bandwidth path-length control; and a precision aligned flexural suspension for the optics assembly to minimize bearing noise feedthrough. The delay line is fully programmable in position and velocity, and the system is controlled with four cascaded software feedback loops. Preliminary performance is a jitter in any 5 ms window of less than 10 nm rms for delay rates of up to 28 mm/s; total jitter is less than 10 nm rms for delay rates up to 20 mm/s.
The moon offers particular advantages for interferometry, including a vacuum environment, a large stable base on which to assemble multi-kilometer baselines, and a cold nighttime temperature to allow for passive cooling of optics for high IR sensitivity. A baseline design for a Lunar Optical Interferometer (LOI) which exploits these features is presented. The instrument operates in the visible to mid-IL region, and is designed for both astrometry and synthesis imaging. The design uses a Y-shaped array of 12 siderostats, with maximum arm lengths of about 1 km. The inner siderostats are monitored in three dimensions from a central laser metrology structure to allow for high precision astrometry. The outer siderostats, used primarily for synthesis imaging, exploit the availability of bright reference stars in order to determine the instrument geometry. The path delay function is partitioned into coarse and fine components, the former accomplished with switched banks of range mirrors monitored with an absolute laser metrology system, and the latter with a short cat's eye delay line. The back end of the instrument is modular, allowing for beam combiners for astrometry, visible and IR synthesis imaging, and direct planet detection. With 1 m apertures, the instrument will have a point-source imaging sensitivity of about 29 mag; with the laser metrology system, astrometry at the microarcsecond level will be possible.
Optical interferometry has been shown to be a viable method for making high-precision astrometric measurements, with the capability for unprecedented accuracy in both narrow- and wide-angle regimes.' As astrometric resolution increases, however, the contribution of certain systematic internal errors of the instrument itself to the measured optical delay can become significant compared to the contributions due to angular position variations on the scale of 5-10 miliiarcseconds. In particular, the baseline of the interferometer can no longer be regarded as a fixed quantity at scales below about 1 micron. On the Mark III interferofneter at Mt. Wilson2, the major moving parts, aside from the optical delay lines, are the siderostats which mark the endpoints of the baseline. Non-ideal motions of the siderostat mirrors can cause changes in the instrument's baseline at the 1 micron level. To compensate for this motion, a prototype laser metrology system in the form of an optical tripod has been installed to measure changes in the instrument's baseline and supply corrections to the optical delay and delay offset data. This system has revealed a number of issues that are crucial to the development of future systems. In particular, future space-based systems3 will require laser metrology systems with accuracies of the order of 0.01-0. 1 nanometers, and the implications of the Mt. Wilson results on future systems are discussed, and current thinking on future designs is presented.
For the first time, four spectroscopic binaries have been directly resolved with the Mark III Stellar Interferometer. Observations in 1988 and 1989 were analyzed, and visual orbits for four binaries have been determined. The semimajor axes for Beta Tri, Alpha Equ, Alpha And and Beta Ari are approximately 0.008 arcsec, 0.012 arcsec, 0.024 arcsec and 0.037 arcsec, respectively. The magnitude differences between two components are 0.5, 0.7, 1.8 and 2.6 mag, respectively. All of the orbital elements for Alpha And and Beta Ari were determined from interferometric data only, and agree well with spectroscopic observations. Predictions of relative position between the two components for these binaries are consistent with the measurements to less than 0.001 arcsec. Combined with data from spectroscopy, masses and distance for the double-lined spectroscopic binary Beta Ari are derived, and the results indicate that both components of Beta Ari agree well with the empirical mass-luminosity relation.
Phase measurements made with the Mark III interferometer at Mt. Wilson using baselines up to 32 m show excellent agreement with the standard Kolmogorov theory, and give no evidence of an atmospheric outer scale smaller than 1 km. Thus, very long baseline interferometers (100's of m) can expect rms path length fluctuations to continue to grow nearly linearly with baseline length. With a wideband fringe tracker, atmospheric dispersion will cause significant reductions in fringe visibility for large instantaneous path length errors. A solution to this problem using an active dispersion tracker is presented. In addition, the problem of diffraction of the propagated beam, and the constraints it presents on the choice of beam diameter are discussed.
Reliable stellar angular diameters can now be determined using the Mark III Optical Interferometer located on Mt. Wilson, California. The Mark III is a Michelson Interferometer capable of measuring the interferometric fringe visibility for stars using interferometer baselines varying from 3 to 31.5 meters in length. Angular diameters measured with the Mark III Optical Interferometer are presented for 12 stars at wavelengths of 450 and 800 nm.
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