Space-based astronomical instruments such as SIM, the Space Interferometry Mission [1, 2] require high-quality mirrors
whose zero-gravity surface figure is specified to nanometer accuracy. Testing, however, necessarily proceeds in 1g,
normal earth gravity. Extracting the zero-gravity surface has traditionally been done by combining measurements with
the mirror in a number of different orientations, so that the effects of gravity cancel. Here we examine some refinements
to that technique, showing that for precise results it is necessary that mount forces as well as gravity forces cancel over
the different orientations. These ideas are illustrated with lab experience obtained with a 343 mm diameter spherical
mirror (PT-M1), which is a prototype SIM compressor mirror.
SIM PlanetQuest (SIM) is a large (9-meter baseline) space-borne optical interferometer that will determine the position and distance of stars to high accuracy. With microarcsecond measurements SIM will probe nearby stars for Earth-sized planets. To achieve this precision, SIM requires very tight manufacturing tolerances and high stability of optical components. To reduce technical risks, the SIM project developed an integrated thermal, mechanical and optical testbed (TOM3) to allow predictions of the system performance at the required high precision. The TOM3 testbed used full-scale brassboard optical components and picometer-class metrology to reach the SIM target performance levels. During the testbed integration and after one of the testbed mirrors, M1, was bonded into its mount, some surface distortion dimples that exceeded the optical specification were discovered. A detailed finite element model was used to analyze different load cases to try to determine the source of the M1 surface deformations. The same model was also used to compare with actual deformations due to varied thermal conditions on the TOM3 testbed. This paper presents the studies carried out to determine the source of the surface distortions on the M1 mirror as well as comparison and model validation during testing. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
The increasing demand to improve focusing accuracy and to accommodate higher frequencies in space communications and radio astronomy has created significant challenges for improving the capability of the constituent systems in radio antennas and telescopes. One important system is the radio antenna/telescope backup structure connections. The backup structure is a key element in providing a stable, precise and rigid support for the reflective surface. The ideal connection for these types of structures is rigid and concentric resulting in minimal deformation with stress/strain curves that are linear, repeatable and exhibiting no hysteresis over the entire service load range. Conceivably such a connection could be designed so that the stress/strain curve mimics the stress/strain characteristics of the connecting member in both tension and compression. When this is achieved then such joints can be said to be "invisible" in the global behavior of the backup structure. At that point, overall reflector deflection becomes more linear and highly predictable. In conjunction with this advantage, optimized backup structure geometries, adaptive reflectors and compensating
algorithms can best be applied in producing an instrument of unparalleled performance. This paper introduces Co-Axial Joint (CAJ) technology as the practical and economical means to produce an invisible connection.
Conference Committee Involvement (8)
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III
10 June 2018 | Austin, Texas, United States
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation
26 June 2016 | Edinburgh, United Kingdom
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation Posters
26 June 2016 | Edinburgh, United Kingdom
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation
23 June 2014 | Montréal, Quebec, Canada
Modern Technologies in Space- and Ground-based Telescopes and Instrumentation II
1 July 2012 | Amsterdam, Netherlands
Modern Technologies in Space- and Ground-based Telescopes and Instrumentation
27 June 2010 | San Diego, California, United States
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