Double Fourier Interferometer Research Articles

Quasi-optical analysis of a far-infrared spatio-spectral space interferometer concept

FISICA (Far-Infrared Space Interferometer Critical Assessment) was a three year study of a far-infrared spatio-spectral double-Fourier interferometer concept. One of the aims of the FISICA study was to set-out a baseline optical design for such a system, and to use a model of the system to simulate realistic telescope beams for use with an end-to-end instrument simulator. This paper describes a two-telescope (and hub) baseline optical design that fulfils the requirements of the FISICA science case, while minimising the optical mass of the system. A number of different modelling techniques were required for the analysis: fast approximate simulation tools such as ray tracing and Gaussian beam methods were employed for initial analysis, with GRASP physical optics used for higher accuracy in the final analysis. Results are shown for the predicted far-field patterns of the telescope primary mirrors under illumination by smooth walled rectangular feed horns. Far-field patterns for both on-axis and off-axis detectors are presented and discussed.

Demonstration of spectral and spatial interferometry at THz frequencies

A laboratory prototype spectral-spatial interferometer has been constructed to demonstrate the feasibility of the double-Fourier technique at far infrared (FIR) wavelengths (0.15-1 THz). It is planned to use this demonstrator to investigate and validate important design features and data-processing methods for future astronomical FIR interferometer instruments. In building this prototype, we have had to address several key technologies to provide an end-end system demonstration of this double-Fourier interferometer. We report on the first results taken when viewing single-slit and double-slit sources at the focus of a large collimator used to simulate real sources at infinity. The performance of the prototype instrument for these specific field geometries is analyzed to compare with the observed interferometric fringes and to demonstrate image reconstruction capabilities.

Realization and theoretical limits of the spectral resolution in double Fourier interferometric imaging with an optical telescope array

Abstract The realization and theoretical limits of the spectral resolution in double Fourier interferometric imaging (DFII) with a phase telescope array are presented. The realization and the effects of optical path difference (OPD) modulations are discussed. Two basic requirements for OPD modulations must be met in the design and operation of a double Fourier interferometer: symmetric OPD modulations about zero OPD and identical OPD modulations at all the available spatial frequencies. The Gibbs phenomenon due to OPD modulations affects the source's spectral brightness distribution, but, for a point source, not the complex visibility. Theoretically, without loss in spatial image fidelity, it is not possible to perform DFII with a telescope array when the aperture synthesis is done only by the Earth's rotation. In practice, DFII can be realized under approximation. In this case, relations between the spectral resolution, the interferometer's parameters and the observed star's parameters are derived. For a two-aperture interferometer with an east-west baseline of 100 m and telescope diameter of 8 m, the spectral resolution is limited to several thousands or less for a star having a magnitude of five, six or fainter, a field of view equal to or larger than the Airy disc of the diffraction pattern of the single telescope aperture and a signal-to-noise ratio of three in the interferogram.

Realization and theoretical limits of the spectral resolution in double fourier interferometric imaging with an optical telescope array

The realization and theoretical limits of the spectral resolution in double Fourier interferometric imaging (DFII) with a phase telescope array are presented. The realization and the effects of optical path difference (OPD) modulations are discussed. Two basic requirements for OPD modulations must be met in the design and operation of a double Fourier interferometer: symmetric OPD modulations about zero OPD and identical OPD modulations at all the available spatial frequencies. The Gibbs phenomenon due to OPD modulations affects the source's spectral brightness distribution, but, for a point source, not the complex visibility. Theoretically, without loss in spatial image fidelity, it is not possible to perform DFII with a telescope array when the aperture synthesis is done only by the Earth's rotation. In practice, DFII can be realized under approximation. In this case, relations between the spectral resolution, the interferometer's parameters and the observed star's parameters are derived. For a two-aperture interferometer with an east-west baseline of 100 m and telescope diameter of 8 m, the spectral resolution is limited to several thousands or less for a star having a magnitude of five, six or fainter, a field of view equal to or larger than the Airy disc of the diffraction pattern of the single telescope aperture and a signal-to-noise ratio of three in the interferogram.

Polarization Effects and Their Minimization in an Infrared Single-mode Fibre-optic Double-fourier Stellar Interferometer

In astronomical double-Fourier interferometry, stretching a single-mode fibre to generate optical path length modulation is an attractive alternative solution to the more classical bulk delay line. However, such an operation is not free from side effects, one of them being the perturbation of the radiation state of polarization by the waveguide. We present both theoretical and experimental investigations of the polarization behaviour of single-mode fibres in a double-Fourier interferometer. Changes in the state of polarization can be induced by asymmetries in the fibre parameters, and macrobending, twisting, etc., but the main cause of perturbation is the stretch of the fibre which is wrapped onto a piezoelectric tube driven by a high voltage. Not only are these fibre-stretch-induced polarization effects chromatic, but also they depend on the stretching state. The general consequences of the polarization perturbations for double-Fourier interferometric imaging are to distort the source's reconstructed spe...

Stellar Diameter Measurements with Fiber Optic Double Fourier Interferometry—Experimental Study

Long baseline optical interferometry has been successfully employed to measure the diameters of stars. In this technique, bandwidth smearing can affect the measurement accuracy. These bandwidth smearing effects can be, to some extent, eliminated by dividing the whole observing spectral band into sub-bands and calculating the star's diameter based on the visibilities and spatial frequencies at the corresponding sub-bands. In the visible range, dividing the whole spectral band can be implemented by introducing a spectrograph, while in the IR domain, this operation can be performed efficiently with the technique of double Fourier interferometry (DFI) without losing the advantage of multiplexing. In particular, the use of IR single-mode fiber optics for DFI will make the interferometer extremely compact, light, insensitive to surrounding conditions, etc. We established an IR single-mode fiber optic double Fourier interferometer in the laboratory, in which the optical path difference modulations are generated by stretching fiber arms and the beam combination is carried out with a fiber optic directional coupler. In this paper, we report on experiments and experimental results from measurements of the diameter of an artificial star with the technique of fiber optic DFI.