SHORT ABSTRACTThe progress and results of the ongoing FP7-FISICA programme to re-asses the scientific goals of a Far-Infrared Space Interfereometer and push the development of some of its key technology elements are reported.
Many important astrophysical processes occur at wavelengths that fall within the far-infrared band of the EM spectrum, and over distance scales that require sub-arc second spatial resolution. It is clear that in order to achieve sub-arc second resolution at these relatively long wavelengths (compared to optical/near-IR), which are strongly absorbed by the atmosphere, a space-based far-IR interferometer will be required. We present analysis of the optical system for a proposed spatial-spectral interferometer, discussing the challenges that arise when designing such a system and the simulation techniques employed that aim to resolve these issues. Many of these specific challenges relate to combining the beams from multiple telescopes where the wavelengths involved are relatively short (compared to radio interferometry), meaning that care must be taken with mirror surface quality, where surface form errors not only present potential degradation of the single system beams, but also serve to reduce fringe visibility when multiple telescope beams are combined. Also, the long baselines required for sub-arc second resolution present challenges when considering propagation of the relatively long wavelengths of the signal beam, where beam divergence becomes significant if the beam demagnification of the telescopes is not carefully considered. Furthermore, detection of the extremely weak far-IR signals demands ultra-sensitive detectors and instruments capable of operating at maximum efficiency. Thus, as will be shown, care must be taken when designing each component of such a complex quasioptical system.
FISICA (Far-Infrared Space Interferometer Critical Assessment) was a three year study aimed at designing a far-infrared spatio-spectral double-Fourier interferometer concept. This paper describes a two-telescope (and hub) baseline optical design, that fulfills the requirements of the FISICA science case. Due the physical size of far-infrared wavelengths with respect to the size of the required optics, a number of different analysis techniques were required for the design work. Approximate simulation tools such as ray tracing and Gaussian beam mode (GBM) methods were employed for initial analysis, with GRASP PO (physical optics) used for final analysis for higher accuracy. The work builds on previous far-infrared double
Spectral-spatial interferometry pioneered in a narrow band in the near infrared has not enjoyed much exploitation as a technique. Proposed as a promising modulation method for a potential Far-infrared future satellite, a period of study was performed on two testbeds to improve and evolve this technique in the laboratory in order to simplify some of the technical aspects and the data analysis involved. Here we will present an update on the successful upgrade of a previous wideband millimetric (0.3-1.0 THz) testbed to a far-IR (11-14THz) one, as well as the ongoing progress on a broadband setup for an imaging system with a commercial thermal-or mid-IR (8 to 12 micron or 25-35 THz) camera currently working as imaging FTS. Source size, coherence and technical issues are discussed.
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