Numerical simulations of pinhole and single-mode fibre spatial filters for optical interferometers

Monthly Notices of the Royal Astronomical Society, Oct 2001

We use a numerical simulation to investigate the effectiveness of pinhole spatial filters for optical/IR interferometers and to compare them with single-mode optical fibre spatial filters and interferometers without spatial filters. We show that fringe visibility measurements in interferometers containing spatial filters are much less affected by changing seeing conditions than equivalent measurements without spatial filters. This reduces visibility calibration uncertainties, and hence can reduce the need for frequent observations of separate astronomical sources for calibration of visibility measurements. We also show that spatial filters can increase the signal-to-noise ratios (SNRs) of visibility measurements and that pinhole filters give SNRs within 17 per cent of the values obtained with single-mode fibres for aperture diameters up to 3r0. Given the simplicity of the use of pinhole filters we suggest that it represents a competitive, if not optimal, technique for spatial filtering in many current and next generation interferometers.

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Numerical simulations of pinhole and single-mode fibre spatial filters for optical interferometers

Numerical simulations of pinhole and single-mode fibre spatial filters for optical interferometers J. W. Keen 0 P D. F. Buscher 0 P. J. Warner 0 0 Astrophysics Group, Cavendish Laboratory , Madingley Road, Cambridge, CB3 0HE We use a numerical simulation to investigate the effectiveness of pinhole spatial filters for optical/IR interferometers and to compare them with single-mode optical fibre spatial filters and interferometers without spatial filters. We show that fringe visibility measurements in interferometers containing spatial filters are much less affected by changing seeing conditions than equivalent measurements without spatial filters. This reduces visibility calibration uncertainties, and hence can reduce the need for frequent observations of separate astronomical sources for calibration of visibility measurements. We also show that spatial filters can increase the signal-to-noise ratios (SNRs) of visibility measurements and that pinhole filters give SNRs within 17 per cent of the values obtained with single-mode fibres for aperture diameters up to 3r0. Given the simplicity of the use of pinhole filters we suggest that it represents a competitive, if not optimal, technique for spatial filtering in many current and next generation interferometers. instrumentation; interferometers - methods; observational - techniques; interferometric I N T R O D U C T I O N Minimizing measurement uncertainties in visibility observations with optical/IR interferometers is one of the major challenges facing any designer of a modern interferometric array. These uncertainties arise from both instrumental and atmospheric effects. The instrumental effects result from aberrations in the optical train and are usually fixed or slowly changing. Atmospheric effects are a result of turbulence which causes rapidly varying phase corrugations in stellar wavefronts. These corrugations corrupt the measured amplitudes and phases of the interference fringes. The use of the closure phase allows most of the object phase information to be recovered, and closure phase accuracies of a few degrees can be achieved after several seconds of averaging on a bright source. In contrast, the fringe amplitude or visibility is much harder to measure accurately. For example, the mean square visibility of a point source, which ought to have a constant value of 100 per cent, is typically observed to be less than this value and to vary by 10–50 per cent on time-scales of minutes to hours. This reduction in fringe visibility is because of mismatches in the shapes of the two wavefronts being interfered, caused by atmospheric and instrumental effects. The atmospheric mismatches vary on millisecond time-scales but even the mean effect of these mismatches taken over several seconds varies because of changes in the quality of the seeing. Some improvement can be obtained by observing a nearby point source and using this to estimate the visibility reduction. However, the use of a calibration source depends on the assumption that the visibility losses remain constant over the several minutes required to switch between calibrator and science objects. This is a poor assumption for atmospheric effects and as a result the calibrated visibilities still show variations at around the 10 per cent level. Furthermore, the constant switching between science and calibration sources dramatically reduces the usable observing time for an interferometer, and hence the amount of science that can be done with an instrument. Thus any system which stabilizes the visibility losses is valuable because it can reduce the reliance on a calibration source. A major step in stabilizing visibility losses was made when it was realized that spatially filtering the beams entering the beam combination system would remove the spatial phase perturbations across the incoming wavefronts (Shaklan & Roddier 1988) and would hence remove the major atmospheric contribution to visibility loss. Initial results with this technique have been extremely promising, reducing calibration errors on visibility measurements by as much as two orders of magnitude (Coude´ du Foresto et al. 1998). Such high-precision measurements are required for many of the most exciting astrophysical programmes for current and future arrays such as direct measurement of Cepheid pulsation. Consequently, spatial filtering is being actively pursued and several interferometer projects are now using or designing spatial filtering systems. Most of the work on spatial filters has been based on the use of single-mode optical fibres. In this paper we present a detailed analysis of a competing approach, where spatial filtering is provided by focusing a collimated beam on to a pinhole (see Prasad & Loos 1992; St. Jacques 1998). Despite the relative simplicity of this approach the use of pinholes has been largely ignored by the astronomical community under the impression that they provide inferior results. Our analysis compares the per (...truncated)


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J.W. Keen, D.f. Buscher, P.J. Warner. Numerical simulations of pinhole and single-mode fibre spatial filters for optical interferometers, Monthly Notices of the Royal Astronomical Society, 2001, pp. 1381-1386, 326/4, DOI: 10.1111/j.1365-2966.2001.04718.x