Sagan Postdoctoral Fellowship Recipients

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Michelson Postdoctoral Fellowship Recipients

2007 Postdoctoral Fellows

Daniel Fabrycky - Harvard-Smithsonian Center for Astrophysics

"Dynamical Detection And Migration Of Multiple-Body Planetary Systems"

With the discovery of planets orbiting other stars came numerous examples of multiple body systems: as of October 2006, 20 stars are known to host multiple planets and 31 planet-hosting stars are known to have at least one stellar companion. In our own planetary system, the planets only slightly perturb each other’s orbits because of their relatively small eccentricities and inclinations and their generous spacing. The planets of other stars, in contrast, generally have a strong dynamical coupling to one another. If one of the planets transits the host star, transit-timing measurements will exquisitely constrain the system’s parameters and may even permit the detection of additional planets. I will develop a theoretical and statistical framework to diagnose the dynamical state of planetary systems by such measurements. The long-term perturbation of a planet by a companion star can drive its orbit to high eccentricity; with the help of tidal dissipation the planet may migrate to become a hot Jupiter. I will make theoretical models of such evolution to constrain the histories of planets in binaries on the basis of their orbits observable by Navigator observatories SIM and TPF.

Michael Fitzgerald - Lawrence Livermore National Laboratory

"Architectures of Planetary Systems at High Contrast"

Circumstellar disks are ideal places to search for signs of planets and their formation. The grains in debris disks are generated through the attrition and evaporation of primitive planet-building material. Their scattered light is only seen at high contrast, and the advancement of coronagraphic techniques promises to greatly increase the number of resolved systems. I seek to maximize the sensitivity of diffraction-limited adaptive optics coronagraphy to both planets and debris disks by implementing a true roll deconvolution scheme. I will use the statistics and structure of speckles to facilitate the sifting of stellar point spread functions from circumstellar emission. I will carry out an observing campaign to resolve the disks, reveal their architectures, probe grain sizes and composition, and potentially detect young, self-luminous jupiters. Coupled with the advance in data processing, these observations are sensitive to indirect signatures of planets. Moreover, they trace the locations and evolution of primitive solid material. These advances in high-contrast technique will have direct application to instruments and missions currently in development. Understanding the diversity of debris disk architectures promises to uncover the mechanisms governing planet formation and disk evolution.

Andrea Isella - California Institute of Technology

"Planet Formation In Pre-Main Sequence Disks"

As a research project at Caltech/MSC I propose a multi-scale study of circumstellar disks based on Keck and CARMA interferometric observations of pre-main sequence stars. The aim of the project is to understand where and when planet formation take place and how this process modifies the structure of circumstellar disks. Infrared and millimeter observations will allow to characterize the disk structure at all distances from the central star, from the dust evaporation radius, at fraction of AU, until the disk outer radius, at hundreds of AUs. The data analysis, performed in the framework of the disk models developed during my Ph.D., will allow to determine physical properties of the circumstellar material (i.e., disk mass, gas and dust radial distribution and temperature, dust size and composition, gas cinematic), with the aim of investigating the dynamical perturbations driven by forming planets on the surrounding dust and gas. The proposed combination of Keck and VLTI observations will allow a fundamental improvement of the uv-plane coverage, bringing to a real breakthrough in our knowledge of pre-main sequence disks.

Hannah Jang-Condell - Goddard Space Flight Center

"Three-Dimensional Radiative Transfer in Disks with Planets"

I propose to apply three-dimensional radiative transfer modeling to various types of disk-planet interactions, determining observable consequences as well as implications for planet formation theory. One application is to determine observable signatures of both disk instability and core accretion, to help settle to debate over which mechanism gives rise to giant planets. A second application is to model inner holes and gaps in disks to interpret resolved images of disk structure as well as to determine how the holes and gaps might form. A third application is to determine how much slowing of Type I migration of planets occurs from temperature perturbations caused by shadowing in the disk. I also hope to improve the modeling of dust properties in my disk calculations and address the freezing out of volatiles in shadows created by planets. I will work with researchers at both University of Maryland and NASA-Goddard Space Flight Center in my work.

2005 Postdoctoral Fellows

Ruslan Belikov - Princeton University

The Terrestrial Planet Finder-Coronagraph (TPF-C) project requires a high contrast imaging system capable of attaining a contrast ratio of 10-10. Many coronagraph designs have been proposed as a solution to this formidable task. One of the designs currently being developed at Princeton University is 2-mirror pupil mapping, whose significant advantage is that it does not lose any light. This is an extremely attractive feature, because the light from extrasolar planets is very faint. However, some concerns were recently raised as to how well conventional wavefront sensing and correction methods will work with 2-mirror pupil mapping. If these concerns can be resolved, and a successful 2-mirror pupil mapping coronagraph demonstrated, there will be a significant improvement in data collection times for the TPF coronagraph mission, leading to both scientific and economic benefits. The proposed work consists of a theoretical and experimental study of 2-mirror pupil mapping coronagraphs, focusing on wavefront correction. The goal of the work is to demonstrate a prototype satisfying TPF contrast requirements at the end of the 2-year funding period.

Michael Ireland - California Institute of Technology

Optical Interferometric Polarimetry (OIP) offers a new and unique way to explore the environments in which exoplanets form. I will undertake a research program that will develop experimental OIP methods capable of characterizing scattered light from disks around young stars. The seeing that affects interferometric observations on earth affects all polarization states of light equally. This means that as a differential technique, OIP can be independent of atmospheric effects. I will develop these techniques both at the Palomar Testbed Interferometer (PTI) and at an aperture-masking system behind the PALAO adaptive optics system at the Hale 200 inch telescope. Using this aperture masking system I will also continue development of another high contrast method: closure-phase based techniques for detecting faint companions and asymmetric structure. Together, these techniques will enable unique characterization of dust around Young Stellar Objects (YSOs) such as Herbig Ae/Be stars, characterization of dust around Vega-like stars and a survey for close brown dwarf companions around young stars. OIP will open-up a new observable parameter space, and represents one of the last remaining frontiers of ground-based optical interferometry.

Amaya Moro-Martin - Princeton University

In a planetary system with a belt of planetesimals and interior giant planets, the trapping of dust in mean motion resonances with the planet, and the ejection of particles due to gravitational scattering, create structure in the dust disk. Because debris disk structure is sensitive to long period planets, complementing a parameter space not covered by other methods, we can learn about the diversity of planetary systems by studying these "dusty fingerprints". Dr. Moro-Martin proposes to use a self consistent combination of 3-D numerical tools for the simulation of debris disk structure, and a 3-D radiative transfer code for the calculation of their emergent SED and brightness density distribution at different wavelengths. These models will be useful for the interpretation of spatially resolved images e.g. by ALMA, and spatially unresolved spectrophotometry observations by Spitzer, in terms of planetary architectures. Physically realistic initial conditions will be determined by a planetesimal formation code that calculates the location of the dust-producing planetesimals and the perturbing planets. Additionally, she proposes to explore the effects of stochastic dust production, gas drag and mutual grain collisions on the shaping of the disk's structure.

2004 Postdoctoral Fellows

Jennifer Patience - California Institute of Technology

The Keck interferometer is the most powerful instrument currently available in the Navigator Program and is an important precursor to both SIM and TPF. With the unprecedented angular resolution and faint source sensitivity provided by the Keck interferometer, it is possible to study the circumstellar environments of young stars at spatial scales as small as 0.1-0.4 AU and to begin to address a key scientific goal of the Navigator Program: understanding the formation and evolution of planetary systems. For a project working with researchers at Caltech and the Michelson Science Center, I propose to observe a sample of young stars at a variety of stages including FUor-type objects with more active disks and weak-lined T Tauri stars and older stars with weaker disks. With the proposed interferomeric data, I will characterize the presence and distribution of circumstellar material in the inner disk as a function of age and will compare these direct measurements of the inner disks with theoretical models of disks and with the properties inferred from previous unresolved photometric observations.

Sean Brittain - National Optical Astronomical Observatories

Understanding the formation of planetary systems remains one of astronomy's most rewarding challenges. It represents a significant component of the search for our own cosmic origins, as well as the possibility of life on other worlds. Consequently, it figures prominently in the recommendations of the most recent NRC Decadal Survey for astronomy and astrophysics. Planet formation is tied to the evolution of gas and dust in disks around young stars. Thus, a comprehensive understanding of the physics and chemistry of young circumstellar disks is essential to a complete picture of star and planet formation, yet basic questions about this process remain unanswered. The proposed research program combines many of the advancements and accomplishments of the last few years and will build on previous work (see publication list) using high-resolution near infrared spectroscopy to peer into the inner 50 AU of disks around young stars. This program will address fundamental questions regarding circumstellar disk evolution and its potential for planet formation including: 1. Does the evolution of gas in the inner disk follow the evolution of dust? 2. Is there any evidence of gas/dust stratification in the disk? 3. What is the CO/H2, ratio in the inner disk around young stars? 4. What is the atmospheric composition and lifetime of known exoplanets?

Christopher Tycner - United States Naval Observatory

With the recent successful demonstration of the 6-way beam combination, and the soon to be implemented long delay-line extension, the Navy Prototype Optical Interferometer (NPOI) is opening a window of opportunity to study early-type main-sequence stars in ways not possible before. This is because the typically large distances to stars hotter than the Sun require interferometric observations with baselines longer than previously available to resolve their small apparent diameters. I propose to use the NPOI's reconfigurable array, with baselines of up to 437 m, to investigate the circumstellar disks of Be stars and measure the angular diameters of hot main-sequence stars. These observations, which will achieve angular resolution at the sub-milli-arcsecond level, will also be capable of detecting deviations from circular symmetry in rapidly rotating stars. The results of this observational study will have direct influence on the current state-of-the-art models of stellar photospheres and interiors, as well as on our understanding of the formation and variability mechanisms in the circumstellar environments of Be stars.

2003 Postdoctoral Fellows

Jason Aufdenberg - National Optical Astronomical Observatories

Recently commissioned interferometers on the earth (e.g. CHARA, VLTI, NPOI, Keck) and those planned for space (SIM, TPF) will be key tools for astrophysicists through the next decade and beyond. Recent technological developments make possible now, for the first time in 30 years, the direct measurement of stellar diameters and limb-intensity profiles of stars hotter than the sun. Using the Center for High Resolution Astronomy (CHARA) interferometric array, such measurements will be made and, by combining both experimental and theoretical expertise, the uncertainties in the direct effective temperatures of 33 stars will be significantly reduced. These data are crucial for testing state-of-the-art models of both stellar atmospheres and stellar interiors, and thereby our understanding of stellar and galactic evolution. The first limb-intensity profile measurements of 8 early-type giants and supergiants will double the number of stars for which limb-darkening has been directly measured and provide rare direct tests of model stellar atmosphere intensity predictions. Furthermore, independent mass-loss rates for B and A-type supergiants, critical for the calibration of these stars as independent distance indicators, will be determined via interferometry for the first time by comparing measurements with the predictions of expanding model atmospheres.

Pascal Borde - Harvard-Smithsonian Center for Astrophysics

In the context of NASA's Terrestrial Planet Finder, the Harvard-Smithsonian Center for Astrophysics, Princeton University, and Ball Aerospace have undertaken a program to demonstrate high-precision coronagraphs. I propose to join their efforts for a combined theoretical and experimental work to deepen our understanding and ability to design powerful planet-finding coronagraphs. On the theoretical side, I will optimize the shape and graded function of Lyot stops in image-plane masks, model the electromagnetic interaction between the light and the material constitutive of notch filter masks, and explore the possibility of combining shaped pupil with image-plane masks to further increase the instrumental dynamic range. On the experimental side, I will compare the performance of different coronagraphic designs, and work on the detection and correction of amplitude and phase errors using a deformable mirror. In addition, I will develop simple color and low-resolution diagnostics for the characterization of planets detected by visible-light coronagraphs.

Andrew Digby - American Museum of Natural History

With the existence of "exoplanets" only indirectly established, little is known about their formation, evolution, and physical characteristics. Knowledge of related circumstellars debris disks is similarly uncertain. Direct imaging of exoplanets is given the highest priority in the TPF Architecture Review. The work outlined in this proposal represents some of the initial steps required before TPF's goal of imaging Earth analogs is reached. In particular, the coronagraph being constructed at AMNH for the US Air Force's Advanced Electro-Optical System, a 941-actuator adaptive optics instrument on a 3.6-m telescope, will achieve unprecedented levels of contrast, enabling the first direct probe of companions and disks on solar-system scales and in the sub-brown dwarf mass range. I will make major contributions to the instrument testing, integration, observations and science of the AEOS Coronagraph ("The Lyot Project"), yielding significant advances in exoplanet, brown dwarf, and disk research, and in technologies such as coronagraphic occulting mask design, dual-stage adaptive optics and active alignment systems that are crucial to future planet-finding missions. The results of this work will be incorporated into museum exhibits and space shows seen by over 3 million visitors each year.

Ettore Pedretti - University of Michigan

The Center for High Angular Resolution Astronomy (CHARA) interferometric array is a state-of-the-art optical/infrared interferometer comprising of six one-meter telescopes, with baselines spanning hundreds of meters on a Y-shaped array. In collaboration with Georgia State University, the University of Michigan is developing an infrared imaging beam combiner capable of exploiting the full potential of the multiple baselines of the CHARA array. I propose to build a very-low noise infrared camera for this new combiner, using the experience I have gained at the Infrared Optical Telescope Array (IOTA), currently the lowest-noise infrared camera at an interferometric facility. This camera will use the Rockwell PICNIC detector and fast, very-low noise readout electronics based on Complex Programmable Logic Devices (CPLDs). It will be used in connection with a new fiber-fed image-plane beam combiner and a low-resolution spectrograph in development at the University of Michigan. The camera will deliver science data as dispersed fringes, which will be useful the keep the array in coherence. I will work with the IOTA and CHARA groups to obtain the first high-resolution images of a Young Stellar Object (YSO) which are beyond the angular resolution of the current ten-meter-class telescopes. This advance in instrumentation will be immediately transferable to new-generation interferometers, and will greatly enhance the understanding of the star and planet formation process.

Remi Soummer - Space Telescope Science Institute

I will focus on a theoretical, experimental, and observational study of high-contrast coronagraphic techniques aimed at the direct detection and characterization of exoplanets and the study of faint structure around nearby stars. I will implement a test-bed version of the apodized-pupil Lyot coronagraph, for which I have already developed a complete theory demonstrating that its rejection is dramatically improved by a factor of up to 10^5, for an extended search space. I will help commission the AEOS near-IR coronagraph being fabricated at AMNH. This instrument will deliver H-band on-sky Strehl ratios of about 90%, and is available as an in-lab optical bandpass testbed. I will use it to verify my simulations, as a step toward understanding space coronagraphy. I will extend my theoretical investigations of coronagraphy to explore my apodized Lyot coronagraphic design, and also continue addressing the chromatic problems, both in image scale as well as phase control, of phase-mask coronagraphs (I have already shown that a 20% bandwidth Roddier-style phase mask is possible without compromising dynamic range). I will participate in the NSF Center for Adaptive Optics Extreme AO coronagraphic effort aimed at an existing 8-10m telescopes, addressing the problem of atmospheric speckle noise.

2002 Postdoctoral Fellows

Brian Kern - California Institute of Technology

I have designed and begun to test a novel technique for rotation-shearing interferometry on single filled apertures (Palomar 5-m and Keck 10-m telescopes), incorporating a 180-degree rotation shear, simultaneous quadrature-phase measurement, and a high-speed, low-noise CCD array. This technique promises to overcome the limitations that have plagued previous rotation-shearing interferometers. I have begun testing this design in the laboratory. In addition to the scientific capabilities of this technique, it is appealing because it is very low-cost and requires relatively little investment of human capital and time to implement.

I propose to continue this investigation, proving the quadrature-phase measurements on astronomical data, and expanding the capabilities of the interferometer to allow differential interferometric observations of the broad emission line regions of active galactic nuclei (AGNs). The differential measurement converts a phase difference between the red and blue wings of the broad H-alpha emission line into an angular displacement. This would be the first direct measurement of the size of broad-line emission regions in AGNs, and one of the first interferometric measurements on any extragalactic object. The quadrature-phase interferometer is uniquely suited to the differential measurement, which could provide positive results on as small an aperture as the Palomar 5-m telescope.

Jajadev Ragagopal - University of Maryland

My current research has been in the area of modeling synthesis imaging with optical interferometers, studying effects of limited baseline coverage, confusion and noise issues. The other major area of interest has been gas and dust in the Galaxy and beyond. I believe that the Michelson fellowship would be a unique opportunity to work and expand on all these themes of research. After having worked on a variety of theoretical and modeling issues concerned with IR/optical/UV interferometry, my plans for the next stage of my career involve observing with and working on practical aspects of existing interferometers. A considerable part of my graduate school training was spent on implementing and observing with a ground-based single aperture optical interferometer. I believe that ground-based interferometry is currently poised to provide breakthroughs on a number of issues that have been beyond its reach till now. Star formation, structure and evolution of circumstellar disks and planetary systems are all areas with considerable overlap with my research interest in the ISM of our Galaxy and neighboring galaxies. Large apertures, and multiple baselines along with adaptive optics techniques in upcoming instruments like the Keck Interferometer, LBT and VLTI open up imaging and study of extended, moderate surface brightness sources.

Primarily motivated by these considerations, I am applying to take up the fellowship to work with Dr. William Danchi and his collaborators at NASA-GSFC. I describe next how the projects there fit in with my research interest and experience and what I envisage my contributions to be.

There are several exciting ongoing projects being pursued by Dr. Danchi and his collaborators. Of these, I wish to focus on two areas: 1) Multi- wavelength visibility observations of circumstellar material with the Keck interferometer and 2) Design and modeling of a mid-infrared imaging space- based interferometer.

Neda Safizadeh - California Institute of Technology

Currently, I am building a single-telescope Fourier Transform Spectrometer (FTS) to operate in the near infrared. Initially the FTS will have the capability of obtaining spectra with a resolution of 80. Later a resolution of ~50,000 will be used to observe Mira variables, Be stars and late-type supergiants.

I propose an extension to my current project that will involve building and utilizing a spatio-spectral FTS, first operating in the near IR with the goal of later pushing into the optical region. By obtaining double Fourier spectra with long baseline interferometry, the wavelength dependence of limb darkening and stellar diameters can be measured. Spectral features of stellar objects can be resolved and the atmospheric scale height inferred. Surface features associated with distinct spectral structure can be separately resolved and studied, e.g. convection plumes, magnetic fields in spots, and circumstellar material. These measurements are useful for studying the extended atmospheres and surface features of stars and for developing more accurate stellar atmospheric models.

2001 Postdoctoral Fellows

Philip Hinz - Steward Observatory, University of Arizona

I propose to undertake a comprehensive program at the University of Arizona to advance the search for extrasolar planetary systems through nulling interferometry, including a survey of nearby main-sequence stars with nulling at the 6.5 m Multiple Mirror Telescope (MMT), and developing a uniquely sensitive interferometric beam combiner for the Large Binocular Telescope (LBT). The MMT survey will be capable of detecting extrasolar dust down to 10-20 times the solar level, a factor of 100 times fainter than with current methods. The nulling instrument for the LBT will be designed to be capable of detecting extrasolar zodiacal dust down to near solar level. This work will be important in setting the stage, both scientifically and technically, for the Terrestrial Planet Finder (TPF) mission as currently envisioned by NASA.

Sam Ragland - Harvard-Smithsonian Center for Astrophysics

I propose to investigate about three dozen Mira stars with the state-of-art IOTA interferometer in order to (a) identify mode of pulsation in Mira stars, (b) detect and characterize photospheric asymmetries and stellar surface features. A byproduct programmatic benefit is that we expect to produce a significant contribution to astrophysics, and thereby improve the general acceptance of infrared interferometry by our astrophysical colleagues in the community.

2000 Postdoctoral Fellows

Jean-Philippe Berger - Harvard-Smithsonian Center for Astrophysics

The aim of my research work is to study star formation mechanisms with long baseline interferometers. Current interferometers have provided us with the first high angular observations of T Tauri and Herbig AeBe stars. The next step is to obtain real images of young stars and their circumstellar environment. My research work as a Michelson fellow will thus have both instrumental and science components.

I propose to promote a new generation of imaging instruments that should considerably simplify long baseline interferometer operations. It will open the way for real imaging capability with an array of optical and infrared telescopes. This instrumental part of my work is based upon the use of integrated optics technologies that allow us to integrate optical waveguides on a small chip similar to microelectronics.

From a practical point of view a close collaboration with the CfA IOTA team (in particular with R. Millan-Gabet, W. Traub and I. Porro) will allow me to carry on first "on sky" tests of an integrated optics 2-way and 3-way beam combiner and to learn more about the practice of integrated optics and its interfacing with classical bulk optics.

Following this exploration a second collaboration with the CHARA team (in collaboration with Theo ten Brummelaar, Steve Ridgway and Hall McAlister) will explore instrumental solutions for beam combination of a 6 telescope array which is a real technical challenge. This instrument require to develop dedicated integrated optics 6-way beam combiners and its associated instrumentation. With its six telescopes, CHARA, which is operated by the Georgia State University, will be one of the very first real imaging interferometers with enough sensitivity to study star formation.

If successful, these new instruments should allow to observe young stellar objects observations with much higher precision (or dynamic range). We expect that these new observations will allow tougher constraints on the environmental models, in particular on the physical mechanisms operating at the heart of the so-called accretion disks which are still poorly understood (accretion physics,accretion-ejection processes, planetary formation etc...)

Marc Kuchner - Harvard-Smithsonian Center for Astrophysics

I want to directly detect light from extrasolar planetary systems. The best way to do this without being overwhelmed by starlight appears to be nulling interferometry. During my tenure as Michelson fellow, I'll be continuing my work with Wes Traub at CfA and Eugene Serabyn at JPL on strategies for using the Keck Interferometer to study nearby stars in nulling mode to search for faint circumstellar emission from evolved planetary systems.

I'm also interested in applying interferometric techniques to study the likely sites of planet formation, disks around young stars. I'm hoping to work on high-resolution interferometric observations of disks around young stars with IOTA and the MMT, and to learn from Phil Hinz and Rafael Millan-Gabet. In particular, I want to use nulling to study the temperature structure of Herbig AeBe disks, and to use IOTA in concert with submillimeter interferometry to search for gaps in Herbig AeBe disks that might be related to the planet formation process.

I have some relevant theoretical interests. In general, the most visible component of planetary systems is not likely to be the planets themselves, but dust produced by collisions and outgassing of asteroids or comets. I am working on understanding the dynamics of belts of small bodies and the interaction between dust clouds and planets. I hope to continue this work with Scott Kenyon and Matthew Holman at CfA. The results of our simulations should help explain high-resolution observations of planetary systems.

Maciej Konacki - California Institute of Technology

High precision astrometry requires not only advanced technology but also elaborated methods of data analysis. The goal of my research is to develop techniques of planet detection for astrometric measurements obtained with the Space Interferometry Mission. It can be accomplished by solving several different problems. One of the most important is the derivation of orbital parameters. Such task is especially challenging for multiplanetary systems. Another one concerns the design of observing scenarios optimal from the planet detection point of view. These issues can be analyzed before the launch of SIM by means of numerical simulations that among others include a realistic SIM model. The end product of my research will be a number of theoretical and numerical tools that will help to prepare and conduct with SIM an efficient and productive planet search campaign.

1999 Postdoctoral Fellows

Rafael Millan-Gabet - Harvard-Smithsonian Center for Astrophysics

I will conduct my research in interferometry at the IOTA (Infrared Optical Telescope Array), both using the interferometer for astronomical observations of young and evolved stars and contributing to its instrumentation development.

On the scientific side, I study the circumstellar environment of young stars of intermediate mass, the Herbig AeBe stars, as well as (at the opposite evolutionary end) the pulsation properties of Mira variables. In the past year, I have also been involved in a program (with J. D. Monnier and collaborators) to combine IOTA/FLUOR and Keck aperture masking observations in order to better constrain models of dust enshrouded young and old stars. We also hope to make use of the Keck adaptive optics system to expand on the observations of the Herbig AeBe systems. Instrumental upgrades currently underway (described below) will also allow us to extend our investigations of young stars to the T Tauri class, as well as to other very embedded sources.

On the instrumental side, my principal responsibility concerns the development of our near-infrared detectors, based on NICMOS3 and PICNIC arrays. I am working on adapting our previous NICMOS3 design to PICNIC-based star tracker and upgrade science cameras. I also collaborate with M. Shure in his adaptation of our camera design for the CHARA array. At the IOTA, the new science camera will be used, in addition of our current mode of operation with two telescopes, for fringe detection from three simultaneous baselines, as work on our third telescope nears completion. Part of this work will done with J.P. Berger, who will bring the very interesting technology of integrated optics to the IOTA. Finally, I will be involved in an effort, led by J. D. Monnier, to build a prototype single mode fiber system for visible wavelengths, in order to be able to directly measure the sizes of Cepheid stars as they pulsate.