Far-Infrared Interferometric Telescope Experiment: Optical Adjustment System

We developed a balloon-borne, astronomical far-infrared interferometer telescope experiment (FITE), which is a Fizeau-type two-beam interferometer consisting of two off-axis parabolic mirrors. Therefore, it is important to establish a method by which the two beams can be simultaneously adjusted. A conventional Hartmann test was originally employed; however, it has two disadvantages in that we cannot simultaneously measure two beams, and it is time-consuming. We developed a new optical adjustment system that can simultaneously measure and evaluate two beams using a Shack-Hartmann wavefront sensor. In the first stage, the field-of-view (FOV) of a wavefront sensor is adapted to a full beam size of 40 cm (the beam diameter), and the mirror surface accuracy and mirror alignment are measured and adjusted for one beam. After the adjustment of both beams, both focuses coincide at the input aperture hole of the far-infrared sensor system by expanding the FOV of the wavefront sensor so that it includes both beams. Using this new method, we can realize the real-time measurement and analysis of converging beams, and can also achieve fast switching between the single- and double-beam modes. We demonstrated this new adjustment method by performing laboratory measurements; we have also designed a new optical adjustment system for FITE. To manufacture this system, we required a large and precise spherical mirror as the reference mirror with R = 300 mm and D = 300 mm to simultaneously calibrate the two beams using the wavefront sensor. We also manufactured an F/1 lens for the collimation of the two beams, a Keplarian beam expander and a beam-switching system. We will soon assemble this optical adjustment system and apply it to the optical adjustment of the FITE interferometer system.