All reflective THz telescope design with an inflatable primary antenna for Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) mission

The Orbiting Astronomical Satellite for Investigating Stellar Systems (OASIS) is a 17 meter class space observatory concept that will perform heterodyne and high spectral resolution observations at terahertz wavelength ranging from 81 to 659 micrometers to observe the transition of water and its isotopologues and other molecular species[1]. The baseline design, in particular with an inflatable primary antenna achieves orders of magnitude larger photon collection area >120 m2 and diffraction limited performance at field of view (FOV) of +/- 0.05 deg with a simple tip/tilt scanner and over 0.2 degrees with an advanced scanning field lens design. The THz observatory with such an inflatable primary system involves an interesting challenge in optical design. The surface shape of the inflatable primary antenna, known as Hencky surface, induces 4th or higher order deformation of reflector surface which is corrected by following 3-mirror correction optics, with a power arrangement which is similar to Offner’s null corrector optics. The same optical architecture is also applicable for more parabola like inflatable antenna shape. The diffraction limited intermediate image field is scanned by a mirror tip-tilt scanner, alternatively for a larger FOV scanning a field lens, refractive or reflective, rigidly connected to the scanning mirror is employed. The design with thin reflective field lens or all refractive design increases overall photon throughput while accommodating broad band spectral range. Along with the 1st and 3rd order optical design procedure, in this presentation, We address challenges in optical design of such a large and inflatable antenna based photon collection system in THz astronomy, including correction of aberration from a membrane antenna, and relay optics to match mode field of antenna to that of THz heterodyne detectors.