Beam combination: all with all

Abstract

ABSTRACT One of the concepts of radio interferometr y which is very difficult to apply to the visible domain is phase closure. The main difficulty is the spatial requirement, namely that all pencil beams will interfere with all other beams on a flat detector. We use a pair-wise combination method using anamorphic stretching of the beams. All beams are lined up, imaged through a cylindrical lens into a square where each beam is now spread into a parallel line. The comb of lines is made to interfere with a copy of itself rotated at 90 0 . A rotation shear interferometer is employed for that stage, and the cross pattern of apertures is imaged on the detector. The diagonal shows interference of each beam with itself, for intensity calibration purposes. An extended source clearly reduces contrast on some off-axis patterns, in a symmetric manner. We have already tested two designs in the laboratory using lasers and white light. Keywords: Astronomical interferometry, multiple beam combination, phase closure 1. INTRODUCTION While many astronomical interferometers have been built over the years in the visible and in fra-red, there was always a strong limitation which prevented their full usage when compared to radio interferometers. This is the limitation of beam combination. It is very important to combine as many beams in the interferometer simultaneously, creating actually a big telescope. True, this telescope is highly diluted, but the addition of all beams creates a highly speckled image, which can now be treated in the computer and combined with other time-l apse images for a fuller picture. The speckles are the result both of the sparsely-filled telescope and the non-common path-length variation between the beams. But because of the fact that many beams are added up, these variations can be cancelled, using the technology of phase closure