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The mission of the InFocus balloon-borne telescope is to obtain high-resolution x-ray images of black holes at the center of our Milky Way galaxy. The InFocus project is a low-budget, fast-paced challenge, but with significant science payoff. The high-resolution payload requires pointing error less than 60 arcsec 90% CE, and jitter less than 1 arcsec 90% CE over 1 second. Attitude knowledge is required to be 10 arcsec 2σ during night-time observations. These pointing and knowledge requirements are significantly more stringent than for previous balloon-borne telescopes. ACS designed the pointing control system for InFocus to meet these requirements.
Balloon telescopes are typically pointed by a gimbal control system. The InFocus telescope pivots on an oil bearing mounted to a gondola. The oil bearing design is not new, though such a system has not been flown since about 1975. The oil bearing isolates the telescope from rotational motions of the gondola. We expect to achieve ten times better pointing accuracy with the InFocus system compared to previous gimbaled systems. The telescope is an open truss design and is about 8.3 meters long. The gondola is suspended from TBD meter long load train attached to a 400 × 300 meter balloon. The operating altitude of the balloon is approximately 41000 meters (135000 feet). The launch site is Alice Springs, Australia, and the anticipated launch date is April 2011. That date is ideal for night-time viewing (beneficial for the star trackers) but most importantly because the calm winds at that time of year facilitate launch of the balloon.
Because of tight schedule and budget constraints, the pointing control system is built using off-the-shelf commercial hardware, some of which will be qualified for the first time by InFocus.
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