The NASA Stratospheric Observatory for Infrared Astronomy (SOFIA) is scheduled to begin routine flight operations from Ames in early 2003. To facilitate installation and integration of science instruments with the observatory, a Telescope Assembly Alignment Simulator (TAAS) is being designed and built. Such a facility is required
because of the high flight rate, frequent instrument change-outs, and limited access to the telescope cavity.
The TAAS will be an essential part of the Preflight Integration Facility in the SOFIA Science and Mission Operations Center. Before an instrument flies on SOFIA, it will first be mounted on the TAAS to:
(a) conform all mechanical and electrical interfaces;
(b) prepare the instrument for flight and assess its operational readiness; (c) optically align the instrument with respect to the telescope; and (d) measure the weight and moments of the instrument for use in balancing the telescope assembly. Because the TAAS has such a fundamental role in the preparation of science instruments for flight, a second unit will be permanently stationed in the Southern Hemisphere for use during deployments.
Figure 1 shows an advanced design for the main mechanical structure of the TAAS. Science instruments mount on the instrument-mounting flange, and their associated electronics are housed in a rack attached to the counterweight plate. In order to provide an accurate mechanical reproduction of the telescope interface, the flange assembly and counterweight plate will be exact duplicates of those on SOFIA. The bearing unit assembly allows the science equipment to be rotated through the full range of elevation angles appropriate for SOFIA. The drive system powers the rotation and maintains a selected elevation angle. Three different infrared light sources have been designed to mount on the rear of the TAAS, for use in instrument alignments. A bore-sight camera assembly will be located in the horizontal tube of the TAAS; it will record focus and bore-sight information and facilitate its transfer to the telescope.
Patch panels that are identical to those on the aircraft will be mounted on the sides of the counterweight rack. Through these panels, the instrument will be connected to all essential services, such as vacuum and gas lines, electrical power, and computer communications, to allow a full operational evaluation of the system. The communications will include connection to the computer simulator of the observatory for protocol evaluation and testing of critical software interfaces.
The TAAS is mounted on load cells so that it can measure the weight and moments of instrument focal-plane packages. These measurements will be used to infer the distribution of counterweights required to balance the SOFIA telescope about all three rotational axes for the given instrument configuration. With this information, the balancing procedure aboard the observatory should take less than an hour.
Preliminary design concepts have been developed for all TAAS subsystems, culminating in a successful preliminary design review in September 1999. Some critical subsystems have been prototyped in the laboratory, including the load cells and a special-purpose controller that modulates the signals from the alignment sources.
Point of Contact: M. Haas
(650) 604-5511
haas@cygnus.arc.nasa.gov
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