Far-Infrared Interferometric Telescope Experiment : I. Interferometer Optics

We have developed a far-infrared interferometer (Far-Infrared Interferometric Telescope Experiment: FITE). It will be the first astronomical infrared interferometer working in space. FITE is a balloon-borne telescope, and operated in the stratosphere (the altitude of 35 km). The aim of the FITE project is to achieve a high spatial resolution of 1 arcsecond at the wavelength of 100 micrometers. FITE is a Michelson stellar interferometer, and is able to realize a long base line beyond the size of the collecting mirror by using four plane mirrors. The first flight is scheduled for November 2008 in Brazil, and the aim is to measure the interference fringes with a spatial resolution of 2.5 arcseconds. In order to achieve this, the two beams must be focused within 2.5 arcsecond accuracy in the imaging quality, within 10 arcsecond accuracy in the beam alignment and within 30 micrometers accuracy in the optical path length between the two beams. In order to archive these accuracies, the structural parts of the telescope were made of carbon-fiber reinforced plastics, which have very low thermal expansion coefficient and large Young's modulus. During observation of a target, the optical alignment is actively adjusted and the orientation of the telescope is stabilized by the three-axis control.