Modern Optical Interferometry

The first two-telescope optical interferometer¹⁶ was constructed in 1974 by A. Labeyrie using light beams from small telescopes at the Nice observatory. This was followed by the construction of the Mark 1 prototype interferometer, which used modern optical detectors and mechanical control systems to measure and track phase variations produced by fluctuations in the Earth's atmosphere¹⁷. These fluctuations are produced by turbulence and local density variations in the atmosphere. A light ray from a distant source refracted by an angle and displaced laterally by a distance a is shown in Figure 15. Experimental measurements¹⁸ have shown that both and the optical path length fluctuate significantly in as little as five milliseconds. Fluctuations in a are generally very much smaller than the interferometer baseline, and do not significantly affect its performance. For interferometric purposes the atmosphere can be successfully modelled as a phase screen, such as that shown in Figure 16. Variations in _A and _B can be eliminated by tracking the light source with computer controlled tilt mirrors. Changes in the optical path lengths l_A and l_B can be reduced with computer controlled delay lines, but as with very long baseline radio interferometry, fluctuations in the phase of the incident light cannot be completely eliminated. There are two principle approaches to imaging under these conditions:

1. Phase-referenced imaging -- utilise measurements of a bright "reference" star very close to your target and accurately measure the phase difference between interference fringes formed with light from this reference star with those from the target
2. Closure phase imaging -- use the fringe signal from the target or from a faint reference star nearby to get the optical paths approximately correct for three or more telescopes, and then measure Jennison's closure phase for the science target

Method 1 has the benefit that it can be used on fainter science targets, but unfortunately the need for a very bright reference star extremely close to the target of interest is very limiting, so it is expected that only a small number of astronomical sources could be observed in this way. Method 2 is applicable to a wider range of sources as a fainter reference star can be used (even the science target itself), and the reference star can be a greater distance from the science target. The principle disadvantage is that much longer observations are required using this approach in order to get results of the same quality as method 1.

The limited applicability of method 1 due to the need for bright nearby reference stars meant that the first imaging interferometry experiments used the approach described in method 2. In 1984 a team from Cambridge¹⁹ succeeded in making the first astronomical measurements of Jennison's closure phase at optical wavelengths by placing aperture masks over an 88 inch telescope on Mauna Kea and recording the fringe patterns electronically. This soon led to the production of crude images²⁰ using the aperture synthesis techniques described on the previous page.

Figure 15 - Refraction of light in the atmosphere

Figure 16 - For interferometric purposes the atmosphere can be satisfactorily modelled as an oscillating phase screen

In 1988 construction of the Cambridge Optical Aperture Synthesis Telescope (COAST)^21,22 began. The layout of this instrument is shown in Figure 17. Light beams from four 400mm tip-tilt corrected telescope mirrors are combined and fringe visibilities are measured in the visible and infrared. A fifth telescope has now been added, and the light from this telescope can be switched in place of the light from telescope 1 in Figure 17. The light from each telescope enters the main building at point H. In the figure, the beams from telescopes 1-4 are reflected from computer controlled mobile mirrors labelled A-D respectively. These mirrors compensate for the differences in optical path length from the source. The beams are then either reflected by dichroic^* reflectors placed at point E into the infrared beam combining apparatus or by dichroics placed at point F into the visual (red light) beam combining apparatus. The dichroics allow blue light to pass into the autoguiding system, which controls tilt mirrors within each telescope.

^*these dichroics reflect red and infrared light but are transparent to blue light

Figure 17 - Layout of the COAST array

Mirrors A-D scan backwards and forwards about their ideal position so that interference between the beams produces oscillating intensities at the detectors. The position of these mirrors is tracked to an accuracy of a few nanometres using laser interferometers. This allows phase changes in the complex visibility to be measured, giving calculated closure phases to about 2% accuracy. In the beam combiners the beam from each telescope is split into four using an arrangement of beam splitters²² which enables accurate measurement of fringe visibility on all six baselines.

The first images²³ produced from visibility and closure phase measurements made with COAST are shown in Figures 18 and 19. These have a resolution of approximately twenty milliarcseconds, and a dynamic range of about fifty. The sensitivity of the COAST array presently allows observation of objects as faint as 7th magnitude. The sensitivity and near-infrared capability of the COAST array makes it particularly suited to the observation of surface structure on red giant, supergiant and long period variable stars. Local atmospheric effects limit the transverse coherence of light to Fried lengths²⁴ of fifty to two hundred millimetres. This limits the usable aperture of the telescopes to between one hundred and four hundred millimetres, depending on the wavelength used and the local weather conditions. Changes in the optical path length through the atmosphere due to atmospheric turbulence are significant on time scales of five to ten milliseconds, limiting integration times (analogous to camera shutter speed) to millisecond time scales.

A fringe tracking capability is currently being installed at COAST allowing optical path differences to be actively corrected during observations. The geometric arrangement of the COAST array allows the optical path differences on the longest baselines at COAST to be calculated from measurements made on short baselines (baseline bootstrapping). This is essential for observations of highly resolved sources.

� Cavendish Astrophysics Group	IMAGES KINDLY PROVIDED BY THE CAVENDISH ASTROPHYSICS GROUP, CAMBRIDGE
Figure 18, September 13 1995	Figure 19, September 28 1995

Capella in 1995

Images of Capella from measurements taken on September 13 and September 28 1995. The images clearly show the rotation of this binary star over the fifteen-day period. J. A. Anderson determined the orbital motions of the Capella binary system in 1920 using a Michelson interferometer5, the first interferometric measurement of an object outside the solar system. In 1995 it became the first object to be imaged with a separate element interferometer.

The Navy Prototype Optical Interferometer (NPOI) astrometric and imaging arrays on Anderson Mesa, Arizona have recently produced a number of images²⁵. The NPOI array currently uses siderostats with small apertures, but these will soon be upgraded using a beam compression system. Both of the arrays are capable of very high precision visibility amplitude and closure phase measurements both at thirty-two wavelength bands in the visible spectrum and also at infra-red wavelengths. Observations are currently limited to sources brighter than 4th magnitude, largely due to the small apertures and because of the narrow spectral channels used. The high spectral resolution, long baselines and high measurement precision of the NPOI arrays make them particularly suitable for surface structure and stellar diameter measurements of bright stars and for accurate orbital determination of binary systems. The geometry of the NPOI array is ideal for baseline bootstrapping, and should allow very high resolution imaging of complex resolved sources.

Most of the imaging interferometer arrays under construction are aiming for 1-10 mas resolution with much higher sensitivity than COAST and NPOI. Two examples are the Keck and VLTI interferometers. Both of these are based around groups of very large conventional optical telescopes. Both arrays have relatively poor coverage of the u-v plane and limited capabilities for baseline bootstrapping. Initially they will only be suited to observations of compact sources, and will not be capable of generating accurate maps of complex sources. The introduction of a phase referencing system at the VLTI should eventually allow imaging of a limited number of complex sources - those which happen to reside close to a star bright enough for the phase-referenced imaging system. The introduction of a closure phase measurement instrument (AMBER) and the development of tracking of the fringe envelope (group delay tracking) will later allow the VLTI to undertake observations using both fainter reference stars and reference stars which are further away in the sky from the science target using the closure phase imaging approach. This will allow observations of a larger range of astronomical targets than can be observed using phase-referenced imaging. The main telescopes on both the VLTI and Keck sites will also be used for conventional astronomy, and it is expected that only a small fraction of the observing time will be available for interferometric imaging.

One of the most promising ground based very high resolution imaging arrays currently under construction is the CHARA array. This instrument consists of six fixed telescopes with separations of up to 350m, allowing imaging of simple sources with 0.2 mas resolution. An alternative UK designed array has also been proposed which would have ten movable telescopes with baselines up to 400m. The greater number of telescopes would allow more complex sources to be imaged. Two of the primary goals of this instrument would be near infra-red imaging of matter around extra-galactic black holes (AGN broad line regions) and imaging of dust disks around newly formed stars. This array is currently being studied by the Magdelena Ridge Observatory Consortium in collaboration with Cambridge University. A list of other optical and infra-red interferometry projects can be found at the OLBIN links webpage.

NASA's Space Interferometry Mission may have some imaging capability at optical wavelengths. This mission will allow direct measurement of visibility phase for optical sources.

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