Mark Silver

Experimental characterization of deployable outer barrel assemblies for large space telescopes

Several variations of large space-based observatories have been hypothesized using different approaches to deploying the primary and secondary mirrors on orbit. Careful consideration must also be given to the design and implementation of the shield that protects these observatories from thermal extremes, micro-debris, and controls stray light entry into the optical train. One approach to the shield architecture is use of an Optical Barrel Assembly (OBA), such as that used on the Hubble Space Telescope (HST). For space telescopes much larger than the HST, an OBA will need to be deployed or assembled to form an adequately large structure to fully shield both the primary mirror and secondary mirror. This paper describes the design, prototyping, characterization tests, and test results from two different OBA development efforts. The first design is a combined barrel and secondary mirror support structure. This system was designed for a fixed primary mirror and deploys straight upward along the optical axis, carrying the Secondary Mirror Assembly (SMA) with it. The second OBA design is of a structurally independent OBA that deploys out from behind the Primary Mirror Assembly (PMA) (itself deployed or assembled) and extends forward along the optical axis to completely enclose the optical train, pulling along the shroud material. Examples of both systems were built out of prototype materials, tested, and the test results were compared against modeled predictions of system performance. The designs, test procedures, and test results are presented along with recommendations for future work.

Vector antenna and maximum likelihood imaging for radio astronomy

Radio astronomy using frequencies less than ~100 MHz provides a window into non-thermal processes in objects ranging from planets to galaxies. Observations in this frequency range are also used to map the very early history of star and galaxy formation in the universe. Much effort in recent years has been devoted to highly capable low frequency ground-based interferometric arrays such as LOFAR, LWA, and MWA. Ground-based arrays, however, cannot observe astronomical sources below the ionospheric cut-off frequency of ~10 MHz, so the sky has not been mapped with high angular resolution below that frequency. The only space mission to observe the sky below the ionospheric cut-off was RAE-2, which achieved an angular resolution of ~60 degrees in 1973. This work presents alternative sensor and algorithm designs for mapping the sky both above and below the ionospheric cutoff. The use of a vector sensor, which measures the full electric and magnetic field vectors of incoming radiation, enables reasonable angular resolution (~5 degrees) from a compact sensor (~4 m) with a single phase center. A deployable version of the vector sensor has been developed to be compatible with the CubeSat form factor. Results from simulation as well as ground testing of the vector sensor are presented. A variety of imaging algorithms, including expectation-maximization (EM), space-alternating generalized expectation-maximization (SAGE), projected gradient ascent maximum likelihood (PGAML), and non-negative least squares (NNLS), have been applied to the data. The results indicate that the vector sensor can map the astronomical sky even in the presence of strong interfering signals. A conceptual design for a spacecraft to map the sky at frequencies below the ionospheric cut-off is presented. Finally, the possibility of using multiple vector sensors to form an interferometer is discussed.

Recent Developments in Precision High Strain Composite Hinges for Deployable Space Telescopes

Future large space-based telescopes will require precision mechanisms for deployment or alignment after they reach orbit. This research effort seeks to develop deployable primary-mirror telescope technologies that leverage recent advances in mirror phasing, actuation and deployable structure technologies. One way to obtain a precise deployed position of the optical components is with structures incorporating precision High-Strain Composite (HSC) hinges. This paper focuses on characterizing the deployment precision of individual HSC hinges and of a support frame utilizing multiple HSC hinges. Measurements of certain individual HSC hinges demonstrate position precision as low as 0.6 μm and angular precision as low as 6 μrad. A deployable frame concept produced axial precision of 0.2 μm, piston precision of 2.3 μm and angular precision of 6.3 μrad. A few modifications to the frame test setup are proposed that may improve the piston precision results.