The Far Infrared Telescope was a second generation instrument developed by the Goddard Institute for Space Studies (GISS) for observations of the Galactic center and other sources in the far infrared domain. It evolved from an original design of a smaller instrument developed and flown by the same scientific group between 1966 and 1968. At left we can see a basic scheme of the instrument.
The telescope was an f/5 Newtonian with a parabolic pyrex primary mirror and a tube 1.5 m long including a 61 cm baffle to provide shielding from off-xis objects such as the Earth, the Moon, and the Sun. The f/5 beam was reflected off the secondary diagonal into a liquid helium dewar at the side through a polyethylene window. The field optics consisted of a 5 mm diameter crystal quartz lens which imaged the primary on the detector. The field of view in the sky was determined by a 6 mm aperture stop in front of the lens which gave a field of view of 12 arc min.
The filtering system which defined the 100 micron passband consisted of the 2 mm thick quartz lens, the 1.6 mm thick white polyethylene window, two layers of black polyethylene, 0.15 mm thick each, a 1.6 mm thick plate of cold teflon, and a No. 300 electroformed mesh.
To subtract off the emission from the sky and the telescope itself was used the technique of beam switching. This was done by having the beam alternately switched between two positions in the sky by vibrating the secondary mirror. The mirror was percussively switched between two adjacent positions in the sky separated by 18 arc min. The balloon gondola and telescope were designed and fabricated with prime consideration given to reliability, durability, and very rapid turnaround between flights. For this reason, an elevation-azimuth axis system was chosen with the elevation referred to the vertical as determined by Earth's gravity and the azimuth referred to the Earth's magnetic field. This system has been proven to satisfy the reliability requirement, to have a turnaround time of only six days, and to have achieved a stabilization and pointing accuracy of 3 arc min.
The overall height of the gondola was 5 m and the weight 2200 kg. The telescope, complete with cryogenically cooled detector, was counterbalanced by the insulated electronic unit mounted on the opposite side of the central mechanical bearing and drive assembly. The main horizontal shaft could be commanded to any elevation angle in half degree steps. It could also be commanded to scan up and down about a central position through an arc 2.5° to 20° in size. Alongside the main vertical support rod was a magnetometer mounted on a post above a motor controlled table which by command could orient the magnetometer in half degree steps relative to the gondola. The magnetometer was operated in a null servo system which drove the gondola in azimuth against the inertia wheel hung immediately below the main bearing. Excess angular momentum of the inertia wheel was transferred to the balloon. The gondola could be commanded to scan in azimuth over a range 2.5° to 10°.
Hanging directly below the inertia wheel was the telemetry transmitter and ballast hopper supplied by the National Center for Atmospheric Research (NCAR). Upon flight termination these two units were dropped onto a 20 ft long cord and the telescope was automatically stowed in a horizontal position so that it would land on the crush pads mounted directly onto the telescope and electronics structure. In addition, the vertical support rod was separated just above the bearing assembly in order to provide a flexible coupling at landing.
Balloon launched on: 4/7/1971
Launch site: Columbia Scientific Balloon Facility, Palestine, Texas, US
Balloon launched by: National Center for Atmospheric Research (NCAR)
Balloon manufacturer/size/composition: Zero Pressure Balloon Winzen 940.000 cuft (1.5 Mil.Stratofilm)
Flight identification number: 604P
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 18 h 30 m
Payload weight: 1256 lbs.
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