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Image reconstruction from optical interferometric data

Combining coherently integrated multiwavelength observations with differential phases of the incident light waves enables imaging of astronomical sources using standard radio-interferometry methods.
16 December 2008, SPIE Newsroom. DOI: 10.1117/2.1200811.1312

Generating images of astronomical sources is a significant challenge for optical interferometry, although this is routinely done for similar observations using radio telescopes. One needs to measure both the ‘fringe-visibility’ amplitude and phase of the incident light waves. However, while the atmosphere usually changes on long timescales at radio wavelengths, in the optical the fluctuations are on the order of a few milliseconds, thus resulting in corrupted fringe-visibility phases.

One particular image-reconstruction approach that has recently benefited from significant improvement1, 2 uses the square of the visibility amplitude and the so-called closure phases. A closure phase is the sum of the visibility phases around a triangle of interferometer baselines, resulting in the cancellation of atmospheric-turbulence effects. This technique has reliably reproduced test images,3 but it also leads to the loss of a fraction of the phase information and can reduce the signal-to-noise ratio of the final images. Therefore, one of the major challenges is the development of techniques that recover as much phase information as possible.

Figure 1. Images of the Hα hydrogen emission of the eclipsing binary star β Lyrae at orbital phases of 0.24 (left) and 0.78 (right). The movement of the Hα emission relative to the continuum photocenter (green dots) is evident. The elongation of the Hα images is caused by the shape of the image-reconstruction beam. The green bar corresponds to 2 milliarcseconds.

We have been working on the generation of such techniques for use with the Navy Prototype Optical Interferometer (NPOI). An important recent achievement is the implementation of an integration method4,5 that first aligns the phases of the 2ms interferometric observations and subsequently adds the data frames coherently over much longer timescales. This technique increases the observational signal-to-noise ratio and retains the maximum phase information.

We have also improved two methods aimed at correcting the observed phases for atmospheric effects. We first developed a differential phase technique6 to recover the phase information from a spectral channel containing hydrogen Balmer-line emission from stars with hot circumstellar disks. This approach uses the a priori knowledge that the stellar photospheres are circularly symmetric, so that their intrinsic phases are zero. This allows us to determine the atmospheric contribution to the observed phases in the continuum channels near the Hα emission line. For the Hα channel, where the structure of the circumstellar disk may produce a nonzero intrinsic phase, we interpolate and subtract the atmospheric contribution. The phase-corrected data can then be imaged using standard radio-interferometry techniques. In Figure 1 two interferometric images of the eclipsing binary star β Lyrae show the change in displacement of the Hα from the continuum photocenter between two orbital phases.

The second technique used to correct the observed phases for the effects of the atmosphere is the ‘phase self-calibration’ method,7 which is widely used in radio interferometry. It uses closure-phase information to remove the atmospheric effects and recover the intrinsic source phases. This approach was successfully used to image the binary and triple systems8 in Figure 2.

Figure 2. Images of the binary system κ Ursa Majoris (left) and the triple system η Virginis (right) based on the phase self-calibration method. The green bars correspond to 0.02 arcseconds. The residual interference pattern contributes less than 3% of the peak level.

Unlike other imaging techniques that use only observables not corrupted by the atmosphere (i.e., squared visibilities and closure phases), our newly developed methods have the advantage of using all available information. Since we do not lose information by combining multiple measurements, as in the closure-phase approach, we should be able to obtain higher-precision measurements and higher-dynamic-range images.

The NPOI is a collaboration between the Naval Research Laboratory and the US Naval Observatory in association with Lowell Observatory. It is funded by the Office of Naval Research and the Oceanographer of the Navy.

Henrique R. Schmitt
Interferometrics, Inc.
Herndon, VA
Remote Sensing Division
Naval Research Laboratory
Washington, DC
Thomas A. Pauls, J. Thomas Armstrong, Robert B. Hindsley
Remote Sensing Division
Naval Research Laboratory
Washington, DC 
David Mozurkewich
Seabrook Engineering
Seabrook, MD 
Anders M. Jorgensen
Electrical Engineering Department
New Mexico Institute of Mining and Technology
Socorro, NM 
Christopher Tycner
Department of Physics
Central Michigan University
Mt. Pleasant, MI 
Robert T. Zavala, James A. Benson, Donald J. Hutter
US Naval Observatory
Flagstaff Station
Flagstaff, AZ