Over half of the light incident on the Earth from the Universe falls within the Far-Infrared (FIR) region of the spectrum. Due to the deleterious effects of the Earth's atmosphere and instrument self-emission, astronomical measurements in the FIR require space-borne instrumentation operating at cryogenic temperatures. These instruments place stringent constraints on the mechanical and thermal properties of the support structures at low temperatures. With high stiffness, tensile strength, strength-to-mass ratio, and extremely low thermal conductivity, carbon fibre reinforced polymers (CFRPs) are an important material for aerospace and FIR astronomical applications, however, little is known about their properties at cryogenic temperatures. We have developed a test facility for exploring CFRP properties down to 4 K. We present results from our ongoing study in which we compare and contrast the performance of CFRP samples using different materials, and multiple layup configurations. Current results include an evaluation of a cryostat dedicated for materials testing and a custom cryogenic metrology system, and preliminary cryogenic thermal expansion measurements. The goal of this research is to explore the feasibility of making CFRP-based, lightweight, cryogenic astronomical instruments.
Proc. SPIE. 9904, Space Telescopes and Instrumentation 2016: Optical, Infrared, and Millimeter Wave
KEYWORDS: Telescopes, Telescopes, Mirrors, Mirrors, Data modeling, Interferometers, Polymers, Composites, Space telescopes, Space telescopes, Space mirrors, Finite element methods, Finite element methods, Cryogenics
The FP7 project, FISICA (Far Infrared Space Interferometer Critical Assessment), called for the
investigation into the suitability of Carbon fiber Reinforced Plastic (CFRP) for a 2m primary mirror. In this paper,
we focus on the major challenge for application, the development of a mirror design that would maintain its form at
cryogenic temperatures. In order to limit self-emission the primary is to be cooled to 4K whilst not exceeding a form
error of 275nm PV. We then describe the development of an FEA model that utilizes test data obtained from a
cryogenic test undertaken at the University of Lethbridge on CFRP samples. To conclude, suggestions are made in
order to advance this technology to be suitable for such an application in order to exploit the low density and
superior specific properties of polymeric composites.
The next generation of space-borne instruments for far infrared astronomical spectroscopy will utilize large diameter,
cryogenically cooled telescopes in order to achieve unprecedented sensitivities. Low background, ground-based cryogenic
facilities are required for the cryogenic testing of materials, components and subsystems. The University of Lethbridge
Test Facility Cryostat (TFC) is a large volume, closed cycle, 4 K cryogenic facility, developed for this purpose. This paper
discusses the design and performance of the facility and associated metrology instrumentation, both internal and external
to the TFC. Additionally, an apparatus for measuring the thermal and mechanical properties of carbon-fiber-reinforced
polymers is presented.
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.