VI.B Bracewell Interferometry - Magellan

As a specific example of a potential exozodiacal interferometer, Figure 10a displays the antenna pattern of the 6.5-m Magellan I telescope and Figure 10b the fringe pattern formed by the 60-m separation of Magellan I and II. The single-telescope "beam gain" is rotationally symmetric, being just the diffraction pattern of a centrally obscured circular aperture. The two-telescope "fringe gain" is a linear pattern of interference fringes projected onto the sky, where the fringe orientation is perpendicular to the projected baseline between the telescopes and additionally averaged in azimuth. The product of these two gains can be directly multiplied point-by-point across a model exozodiacal map to obtain the signal in a single detector element. The fringe phase is adjusted to give a central null on the star, blocking most of the star's glare.

Figure 10: Magellan telescope diffraction and fringe pattern sensitivities versus distance from a central star viewed from 10 pc. a) Single telescope diffraction. b) Interferometer fringes from both Magellan telescopes combined, employing central Bracewell nulling.

Note that the single-telescope diffraction-limited beam extends out to about 200 mas, or about 2 AU radius at 10 pc, which is sufficient to accept essentially all of the model dust emission at 10 µm. Also note that (in star-nulling mode) the inner interference fringe has a transmission peak at about 10 mas or 0.1 AU, which is an excellent match to the 50% power points of the dust emission noted above. The product of the IRAS model dust emission at 10 pc and the antenna pattern of the Magellan interferometer results in reduction of total IRAS model power observed at 10 µm from about 0.20 mJy to about 0.06 mJy due to the interferometer gain pattern.

Finally, note that stellar leakage through the central null of an ideal sinusoidal interference pattern varies as the square of the ratio of the projected star diameter to fringe period. Wavefront perturbations determine a Strehl ratio (on-axis relative power), the departure from unity of which governs how much additional star light leaks into the central diffraction-limited mode. Here the combined leakages are a factor of a few greater than the dust emission signal, showing that the initial contrast ratio of star/dust ~ 10,000 can be reduced to a more manageable value in the range 1-10. A 3 10 µm flux given above for a 1-zodi cloud at 10 pc is possible in about 2 hours assuming smooth telescope mirrors, a cooled interferometer, and noise dominated by fluctuations in telescope thermal photon emission (Traub et al. 1996).