Improved Three-Telescope Measurements at IOTA (March 2002)

After our initial demonstration data (see news of February 2002 below), the quality of our raw three-telescope data has been much improved thanks to the efforts of John Monnier and Ettore Pedretti of the IOTA team to better understand the polarization properties of the IONIC beam combiner. Shown below are interferograms corresponding to each telescope pair obtained on the star Alf UMa (HD 95689, V=1.8, H=-0.6). It can be seen that the signal-to-noise ratio on this bright star is very high, and that the contrast in all three fringes is also very high (compare with our very first data below, particularly on the A+B fringe).

Observing has continued since our first fringes, with the dual goals of understanding all aspects of our instrumental performance and obtaining first scientific results. Below are shown plots of calibrated visibility amplitudes (top plot, for all three simultaneous baselines) and closure phases (bottom plot, for the triangle of three telescopes) prepared by John Monnier, which further illustrate our ability to measure and calibrate these quantities with our new setup. The data is for the well known binary Capella (HD 34029, V=0.08, H=-1.8) and show the distinctive variation and phase jump expected for a binary source. Comparison with model calculations based on the well known orbit of this binary (not shown in the plots below) indicate good agreement with our measurements.

Rafael Millan-Gabet
April 12 2002

First Three-Telescope Starlight Fringes at the IOTA (February 2002)

First starlight fringes were observed on three simultaneous baselines at the IOTA (Infrared Telescope Optical Array) on 23 February 01:30 a.m. local time. The IOTA is a long baseline interferometer operated by the Smithsonian Astrophysical Observatory, a member of the Harvard-Smithsonian Center for Astrophysics. This crucial milestone culminates work undertaken over the last three years to install the IOTA's third telescope and its beam-line, and also demonstrates our new interferometer control system, new focal plane camera, and new three-way beam combiner. Coherent combination of starlight from more than two separated telescopes is a feat that has only been accomplished at two other long-baseline interferometers worldwide (COAST in the UK and NPOI in the US).

The plot above shows interference fringes obtained in H-band (1.65 microns) on the star Alf Lyn (HD80493, H-magnitude = -0.5). The configuration of the array was that of a triangle with two perpendicular baselines of length 15 meters (between telescopes [A,C] and [B,C]) and a diagonal baseline oriented North-South of length 21 meters (between telescopes [A,B]). The panels show example raw interferograms detected simultaneously on the three baselines (the complementary outputs from our pair-wise combiner for each telescope pair have been subtracted to reduce photometric fluctuations). No attempt was made in these data to optimize the flux input. Also, no attempt was made to balance the light contribution from each telescope, or the polarization state at the combiner, both of which impact the measured fringe contrast.

In this experiment, we used an integrated optics (IO) beam combiner developed and integrated at the IOTA by our collaborators at the Laboratoire d'Astrophysique de l'Observatoire de Grenoble (LAOG, France). IO components implement optical functions such as beam division and recombination in glass or glass plus silicon chips of only a few millimeters in size, and therefore are regarded by many as the solution to the problems of size and stability facing the next generation of ground based and space interferometers containing a large number of apertures. These observations are the first ever to use an IO component to combine the light from more than two telescopes. The technique was first demonstrated on the sky also at the IOTA using a two-beam combiner in November 2000.

The plot above shows the closure phase measurements from the first recorded three-telescope fringe observations. The IOTA uses a pair-wise combination scheme, which naturally produces (constant) non-zero closure phases even for point sources. The measurement variance is dominated by varying fringe packet alignment in the initial observations, which did not utilize active packet tracking.

The happy team! From left to right: Mike Brewer (UMass-Amherst), Rafael Millan-Gabet (SAO), Marc Lacasse (SAO), and Ettore Pedretti (SAO/LISE). Needless to say, missing from this celebratory picture are the rest of the heroic IOTA team, all of which contributed crucially to this important milestone: SAO Team, UMass-Amherst Team, LAOG Team. Below is a somewhat more complete photo, from our earlier celebration in December 2000 of first light through our third telescope, counter-clockwise from center: John Monnier (SAO), Nat Carleton (SAO), Pete Schloerb (UMass-Amherst), Rafael Millan-Gabet, Marc Lacasse, Ettore Pedretti, and Mike Brewer (Wes Traub - SAO - taking the picture).

To learn more:

IOTA Home Page

The IONIC project at LAOG

Rafael Millan-Gabet
February 24 2002