The original pointing accuracy requirement of the Stratospheric Observatory for Infrared Astronomy SOFIA was defined
at the beginning of the program in the late 1980s as very challenging 0.2 arcsec rms. The early science flights of the
observatory started in December 2010 and the observatory has reached in the mean time nearly 0.7 arcsec rms, which is
sufficient for most of the SOFIA science instruments. NASA and DLR, the owners of SOFIA, are planning now a future
4 year program to bring the pointing down to the ultimate 0.2 arcsec rms.
This may be the right time to recall the history of the pointing requirement and its verification and the possibility of its
achievement via early computer models and wind tunnel tests, later computer aided end-to-end simulations up to the first
commissioning flights some years ago. The paper recollects the tools used in the different project phases for the
verification of the pointing performance, explains the achievements and may give hints for the planning of the upcoming
final pointing improvement phase.
The SOFIA telescope has a silicon carbide secondary mirror and a six degree-of-freedom secondary mirror mechanism. Each of these high-technology items represents a single-point failure mode, because both are essential for operation of the observatory, and neither has a spare. Reduced-performance, relatively inexpensive “backup” hardware can enable a large fraction of the planned SOFIA science observations, and so can help to assure a highly reliable flight program. Accordingly, we have developed an aluminum secondary mirror and derived design requirements for a backup secondary mirror mechanism that will meet minimum performance needs.
The primary focus of this paper is to describe the development of a highly modified aircraft that carries a twenty ton telescope to the stratosphere and then loiters at this desired altitude to act as the observatory platform and dome. When the aircraft has reached its nominal cruise condition of Mach 0.84 in the stratosphere, a large cavity door opens (the dome opens), exposing a large portion of the interior of the fuselage that contains the telescope optics directly to the Universe. The topics covered in this paper include: the relevant criteria and the evaluation process that resulted in the selection of a Boeing 747-SP, the evolution of the design concept, the description of the structural modification including the analysis methods and tools, the aerodynamic issues associated with an open port cavity and how they were addressed, and the aeroloads/
disturbances imparted to the telescope and how they were measured in the wind tunnel and extrapolated to full size. This paper is complementary to a previous paper presented at the 2000 Airborne Telescope Systems conference which describes the challenges associated with the development of the SOFIA Telescope.
For completeness, this paper also provides a brief overview of the SOFIA project including the joint project arrangement between NASA and DLR, a top level overview of the requirements, and finally the current project status.
This paper describes some of the many challenges involved in trying to obtain 0.2 arc-second (1 micro-radian) pointing stability for a large telescope mounted in an open port cavity on board an aircraft flying in the stratosphere, specifically the Stratospheric Observatory For Infrared Astronomy Project. It includes an overview of the SOFIA project including the joint project arrangement between NASA and DLR, the project status, a top overview of the science objectives and the resulting requirements.
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