Diffraction losses in ground-based optical interferometers

Abstract

A B S T R A C TWe present a numerical analysis of free-space propagation of the beams inside a long-baseline optical/infrared interferometer. Unlike the models of beam propagation used in most previous studies, our analysis incorporates the effects of atmospheric seeing on the wavefronts entering the interferometer. We derive results for the changes in throughput, coherence loss and fringedetection signal-to-noise ratio arising from diffraction along the propagation path. Our results for conditions of moderate seeing show that although the flux throughput decreases with propagation distance for a given beam diameter, the fringe contrast increases at the same time. In this case it becomes possible for diffraction to increase the signal-to-noise ratio of the fringe measurements. Previous studies have only considered an arrangement where all the apertures in the beam-propagation system have the same diameter. If the light at the end of the propagation path is collected with a fixed size aperture, we find that in many cases the signal-to-noise ratio for fringe detection is maximized when the initial beam diameter is approximately 30 per cent smaller than the final collector diameter. We discuss the implications of our results in the context of future interferometer designs.

Optical interferometry in astronomy

“…In order to combine broad bandwidths, one must either transport the light through a vacuum or construct a dispersion compensator (Tango, 1990;ten Brummelaar, 1995), whereby wedges of glass are inserted into the beam to compensate for air's index of refraction; a combination of partial vacuum plus dispersion compensation is also possible. The size of the mirrors in this optics chain is also important for limiting the effect of diffraction (Horton et al, 2001), and often also sets the field-of-view of the interferometer. Lastly, because of the many reflections, high reflectivity of the relay optics must be maintained to maximize throughput and sensitivity; a side-benefit of evacuated relay optics is that the mirrors stay clean.…”

Augmented design for a fully automated alignment system at the Magdalena Ridge Observatory Interferometer

“…Record residual shear between starlight and UTLIS with BEASST as an offset for later measurements. 6. Using MOB as a proxy for starlight, use mirrors M11 and M12 to steer beams into the fringe tracker and science combiners 7.…”

“…Both of these effects can be mitigated by averaging over many exposures, but we seek to quantify any residual perturbations to tilt and shear measurements. We have developed a simulation of the intensity distribution arriving at the tilt and shear detector based upon diffraction code used in Horton et al 6 We generated a Kolgmogarov phase screen and masked it with the UT pupil, including a central obscuration and a trio of spider vanes. The resulting complex amplitude distribution is propagated over a distance of approximately 350m by Fresnel diffraction, 7 including several stages of beam compression.…”

Practical beam transport for PFI

“…3 to ∆z of 4 km (Table 1) and the longest possible science wavelength of 13 µm, we find that a beam size of D 0 z z0 λ λ0 = 1.8 m would be needed to keep the signal loss due to diffraction to 10% or less. If we restrict mid-IR operation to the L (3.5 µm) and M (5 µm) astronomical bands, the required beam size is reduced to 1.1 m. Even if we decide to tolerate higher losses (say 20%), and take advantage of the fact that diffraction can provide a helpful spatial filtering effect when atmospheric aberrations are present, 5 it is clear that the cost of the vacuum pipes and supports to accommodate such large beams would be prohibitively expensive (both in absolute terms and in comparison with the cost of the unit telescopes).…”