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Four telescopes are better than one

Astronomers have combined light from four widely separated telescopes to image the surface features of a Sun-like star for the first time.
23 August 2007, SPIE Newsroom. DOI: 10.1117/2.1200708.0828

Images made using ground-based telescopes are blurred by the turbulent motions of the atmosphere and also by diffraction. While one can eliminate most effects of turbulence by using either a space telescope or adaptive optics, there is no known way to eliminate diffraction. Generally speaking, the only way to minimize diffraction and to thereby improve the telescope's angular resolution is to make the telescope as large as possible.

Modern engineering approaches (and budgets) permit optical telescopes with mirrors no larger than approximately 10m (a 30m telescope is currently in the design stage1,2), which provides an angular resolution of approximately 1/20 of an arcsecond when observing in the infrared. This is woefully inadequate for imaging even the closest and largest stars (except for the Sun, of course). The technique of stellar optical interferometry overcomes the limitations of a single telescope by combining light from an array of several small telescopes, thus producing an imaging power commensurate with the extent of the array rather than with the size of a single element.

Optical interferometers have been combining light from several telescopes since the 1970s.3 Images have even been made of binary stars4 and a few red giants.5 Imaging a Sun-like star, however, required two major recent advances. The first was the construction by Georgia State University of the six-telescope CHARA (Center for High Angular Resolution Astronomy) Array6 on Mt. Wilson, CA, boasting the longest baselines in the world (maximum 330m). The second major advance was the development of a powerful beam combiner, an instrument that can combine light from all these telescopes simultaneously. Here we describe the first results from such an instrument: the Michigan Infrared Combiner (MIRC).

Figure 1. The top schematic shows the optical flow of the MIRC combiner, illustrating how telescope light injected into fiber optics is manipulated to form an interference pattern at the slit of an infrared spectrograph. The bottom picture shows the implementation of the combiner optics using commercial V-groove and lenslet technologies.

MIRC was designed and built by my team at the University of Michigan (UM), in particular Ettore Pedretti and Nathalie Thureau. MIRC collects light from each CHARA telescope by focusing each beam into a single-mode fiber. These fibers are then arranged into a silicon V-groove array to form a line (see Figure 1). The light is allowed to exit the fibers again and each beam is re-collimated using an array of microlenses: consider that the 1m-diameter beams of stellar light have now been compressed into beams only 250μm across! The tiny beams of light are then focused together onto the slit of a spectrograph, allowing the interference pattern, or fringes, to be measured as a function of wavelength. MIRC obtained first fringes in October 2005, and the first science observations were made in 2006.7

Figure 2. The left panel shows our model for the fast-spinning star Altair. The right panel shows our image made using the MIRC combiner on the CHARA Interferometer. Image credit: Ming Zhao, University of Michigan.

On 31 August and 1 September 2006, our team combined four CHARA telescopes using MIRC to observe the nearby hot and massive star Altair, previously known to be a rapidly rotating star spinning so fast that it's elongated along the equator.8 By using MIRC on the CHARA Interferometer, an actual image of the star was made9 and this image appears here in Figure 2. One can see that the upper-right (northwest) part of the star is much brighter than the rest. This corresponds to the pole of the spinning star. This image is direct confirmation of an effect known as gravity darkening. The equator, which is located farther from the stellar core in a rapid rotator, is cooler than the rest of the star.

Detailed modeling of Altair by UM graduate student Ming Zhao found that the MIRC image was much darker along the equator than predicted by current hot star models. The most likely explanation is that the surface of Altair has differential rotation, whereby the equator spins faster than the rest of the star. Or perhaps the equatorial regions might have convective flows due to the relatively low temperature. Upcoming observations should determine which is the right explanation.

Imaging a star other than Sun is more than just a technical milestone. The most powerful and influential stars in the galaxy are massive and have more in common with Altair and others like it than with the Sun. These massive stars are hard to study since they are far away and our detailed observations of the relatively low-mass Sun are not applicable. Stellar optical interferometry allows new detailed studies of these important objects, providing crucial information for an accurate picture of how our galaxy changes over cosmological timescales.

This project would not have been possible without helpful cooperation from companies such as Luminos, High-wave, OZ Optics, and SUSS Micro-optics.

John Monnier
Astronomy Department
University of Michigan
Ann Arbor, MI 

John Monnier is an assistant professor of astronomy at the University of Michigan and was the chairman of the 2006 symposium Advances in Stellar Interferometry, part of the SPIE conference Astronomical Telescopes and Instrumentation.