James A. Benson

Current status of the IOTA interferometer

The first two telescopes of the Infrared-Optical Telescope Array (IOTA) project are now in place and yielding data at the Smithsonian Institutions F. L. Whipple Observatory on Mt. Hopkins, near Tucson, Arizona. The IOTA collectors are 45 cm in diameter, and may be moved to various stations in an L-shaped configuration with a maximum baseline of 38 m. A third collector will be added as soon as funding permits. Each light-collector assembly consists of a siderostat feeding a stationary afocal Cassegrain telescope that produces a 10-X reduced parallel beam, which is in turn directed vertically downward by a piezo-driven active mirror that stabilizes the ultimate image position. The reduced beams enter an evacuated envelope and proceed to the corner of the array, where they are turned back along one arm for path compensation. The delay line, in one beam, consists of two parts: one dihedral reflector positioned in a slew-and-clamp mode to give the major part of the desired delay; and a second dihedral mounted on an air-bearing carriage to provide the variable delay that is needed. After delay, the beams exit from the vacuum and are directed by dichroic mirrors into the infrared beam-combination and detection system. The visible light passes on to another area, to the image-tracker detectors and the visible-light combination and detection system. The beams are combined in pupil-plane mode on beam splitters. The combined IR beams are conveyed to two cooled single-element InSb detectors. The combined visible-light beams are focussed by lenslet arrays onto multimode optical fibers that lead to the slit of a specially-designed prism spectrometer. For the visible mode, the delay line is run at several wavelengths on one side of the zero- path point, so that several cycles of interference occur across the spectrum. First results were obtained with the IR system, giving visibilities for several K and M stars, using 2.2 micrometers radiation on a N-S baseline of 21.2 m. From these measurements we obtained preliminary estimates of effective stellar diameters in the K band.

Fibered recombination unit for the Infrared-Optical Telescope Array

The Infrared Optical Telescope Array (IOTA) is an interferometric facility currently observing in the near infrared bands at the Smithsonian Institutions Fred Lawrence Whipple Observatory in Arizona. The 45 cm siderostats can be moved on an L-shaped track allowing discrete bases ranging between 5 and 38 m. The capability to combine beams with fiber optics in the K band (2 micrometers <EQ (lambda) <EQ 2.4 micrometers ) has been demonstrated on the Fiber Link Unit for Optical Recombination (FLUOR) at Kitt Peak National Observatory, in which two existing 0.8 m telescopes have been coherently coupled by means of optical fibers. FLUOR is now set as a focal instrument of IOTA. It uses single-mode fluoride glass waveguides and couplers as a substitute for mirrors and beamsplitters to perform beam transportation and recombination. Processing the light in single-mode waveguides offers the possibility to self-calibrate each interferogram against the loss of fringe visibility induced by atmospheric turbulence, thus improving the accuracy of the fringe visibility measurements. The FLUOR unit can be operated as a Mach-Zehnder interferometer to produce zero-baseline spectra used in double-Fourier interferometry to obtain the visibility as a function of wavelength. In the current status, a N-S baseline of 21.2 m is used to observe late-type starts and derive their angular diameters.

The fundamentals: angular diameter measurements of zero-crossing stars from the NPOI

We recently used archival and newly obtained data from the Navy Precision Optical Interferometer to measure the fundamental properties of 87 stars. The sample consisted of 5 dwarfs, 3 subgiants, 69 giants, 3 bright giants, and 7 supergiants, and spanned a wide range of spectral classes from B to M. We combined our angular diameters with photometric and distance information from the literature to determine each star’s physical radius, effective temperature, bolometric flux, luminosity, mass, and age. Several dozen of the stars have visibility curves sampled down to the first null, where the visibilities drop to zero. Here we present preliminary results showing limb-darkening fits for the five zero crossing stars that have the best coverage of the second lobe.

Adding baselines at the Navy Precision Optical Interferometer

The Navy Precision Optical Interferometer is an astronomical optical interferometer operating near Flagstaff, Arizona. A joint program between the United States Naval Observatory, the Naval Research Laboratory and Lowell Observatory, it has historically been involved in space imagery and astrometry. More recent work has pushed for the addition of more baselines. It is currently capable of co-phasing 6 elements, so the commissioning of additional baselines requires ease of use and reconfigurability. At the time of this publication, a seventh station has been added and the final commissioning work on an eighth and ninth station are being completed. These last two stations will increase the longest baseline to 435 meters. This paper discusses the work to date on adding these stations and provides details on increased capabilities.