Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450

IntroductionThe Berkeley Infrared Spatial Interferometer (ISI) is locatedat Mount Wilson Observatory near Pasadena, CA, a siteknown for good “seeing” conditions. The ISI saw first light in1988. It is presently a 3 telescope array, operating in mid-infrared (~10 micron). The 1.6 m diameter telescopes aremounted in semi-trailers, which allows them to be movedinto various configurations according to observationalrequirements. The ISI has been used to study manycharacteristics of stars, including their diameters, thedistribution and composition of material around them, andhow they change with time. In order to study theseproperties, extremely high angular resolution is requiredsince stars and the material surrounding them are so faraway that they appear to be nearly point sources to mosttelescopes.

Stellar Modeling and Closure PhaseA single plot of visibility versus spatial frequency is only aone dimensional representation of a stellar object. Manyvisibility measurements over a series of baselineorientations must be combined in order to build up a twodimensional image. Until recently, ISI stellar modelingassumed the imaged stars and their surrounding dustwere symmetric. This is because the Fourier phaseinformation required to detect asymmetries wascorrupted by atmospheric fluctuations, which introducerandom optical path length changes between thetelescopes and the star. With the addition of a thirdtelescope in 2003, however, useful information aboutasymmetrical structure can be determined by the use of"closure phase." Atmospheric effects are cancelled bysumming the phases of each of the 3 fringes from eachof the 3 baselines ( "closure" coming from the need touse a closed triangle of telescopes to derive the summedphase).

Stellar Evolution and The InterstellarMediumThe Interstellar Medium (ISM) not only contributes materialat the birth and early development of stars and planets, butalso receives material throughout the evolution and latestages of their existence. The ISI is especially well suitedto study late-type stars since they are typically bright in themid-infrared, and high-resolution measurements areneeded to study the material being cast off by these starsinto the ISM. Most of these stars are on the so-calledAsymptotic Giant Branch (AGB) in their life-cycles--as willbe our Sun in a few billion years. Much can be learned onshorter time scales, however, as seen by ISI observationsthat show significant changes in dust shell structure andposition over just a few years. The circumstellar out-flowthat creates such dynamic dust shells is not wellunderstood, nor is the history and evolution of mass lossfrom AGB stars.

InterferometryInterferometry is an important observational technique inwhich light from multiple telescopes is combined to givemuch higher resolution than can be obtained using any oneof the telescopes alone. The resolution of a telescope isdetermined by its diameter and operating wavelength; for aninterferometer, however, the separation ("baseline") betweenthe telescopes determines the resolution. Thus, with itsmaximum 75 m baselines, the ISI has a resolutioncomparable to a single telescope that is ~75 m in diameter,larger than any conventional telescope. Furthermore, theatmosphere is not uniform over the entire beam of largediameter telescopes so their resolution is limited byatmospheric “seeing”. Stellar interferometry was firstdeveloped successfully by Albert Michelson in the 1920's,coincidentally also at Mt. Wilson.

Heterodyne Detection Unlike all other existing optical/IR interferometers, the ISI

employs a very novel, indirect method of interferometry:heterodyne detection, followed by radio frequency (RF)correlation. The heterodyne detection uses a carbon-dioxidelaser as an IR local oscillator (LO) to mix with the incoming IRstarlight in each telescope, producing an RF signal withphase and amplitude corresponding to those of the originalstarlight. This RF signal can then be transported by coaxialcables rather than elaborate systems of mirrors and lightpipes and can be amplified, filtered, measured, etc. byrelatively common RF components. Since correlationrequires the relative phase between the telescopes to bepreserved, each of the 3 telescope LOs is phase-locked to amaster laser. Also, each of the RF signals is delayed by theproper amount to make the travel time from the star to acentral correlator equal, even as the star moves across thesky.

Visibility MeasurementsMuch like a double-slit interferometer generating patternsof visible light and dark "fringes", the ISI correlatorproduces electrical fringes that can be recorded andanalyzed. The term "visibility" is used to describe thecontrast in intensity between the peaks and troughs ofthe fringe pattern, measured as Imax and Imin respectively.The visibility of a stellar object depends on its size andshape, its position in the sky during the observation, andthe length and orientation of the baseline. Maximumvisibility (= 1) occurs when the object is completelyunresolved, such as a point-source star at a relativelyshort baseline. In this case, light enters each telescopepair as a plane wave across the baseline, thus producingRF signals that match well in phase when broughttogether and correlated. Larger, more extended sourcescan be thought of as a number of point sources whoseseparate plane waves are not in phase and thus produceRF signals that correlate less well. Minimum visibility (=0) occurs for longer baselines when the RF signals are180 degrees out of phase and do not correlate at all.Many details of source size and structure can bedetermined from plots of visibility versus spatialfrequency (which are equivalent to plotting Fouriercomponents of the image).

The Berkeley Infrared Spatial InterferometerA Heterodyne Stellar Interferometer For The Mid-Infrared

R.L. Griffith, A.A. Chandler, K. Tatebe, D.D.S. Hale, E.H. Wishnow, W. Fitelson, C.H. Townes

ISI 4 – 8 – 12 meter linear-baseline configuration.

Laser and starlight path with correlation

A spherically symmetric model of a star and surrounding dust.Radiation intensities are computed assuming radiative transfer.

Asymmetrical model NML Tauri

Master laser oscillator optics table

Visibility measurements for NML Tauri

Telescope trailer design ( Pfund design)

Visibility formula Alternative Visibility formula

Future Goals and DirectionsThe 3 telescopes of the ISI were moved in 2006 from alinear, short baseline configuration to a triangular one ofabout 35 m sides. This layout allows visibility and closurephase measurements to be taken at an optimal resolutionfor studying AGB star diameters and their asymmetries.The complexities of dust shells might be partiallyexplained by these asymmetries, which may result fromhot spots on the star surface or from physicaldeformations of the photosphere. While the precisedynamics responsible for these asymmetries are not yetknown, the ISI hopes to continue to add to ourunderstanding.

4-8-12 mBaseline 2003

34-35-39 mBaseline 2006

Baseline configurations

Telescope 1

Telescope 2

Closure phase NML Tau

Telescope 3