Details of the balloon and launch operations
Launch site:Columbia Scientific Balloon Facility, Palestine, Texas, US
Launch team: National Scientific Balloon Facility (NSBF)
Balloon: Open balloon (zero pressure) Raven 195.387 m3 (17.78 Microns - X-124) / N15SI-7/10/10T-6.90
Serial number: R6.90-2-101
Flight identification number: 1294P
Campaign: No Data
Payload weight: 2438 Kgs.
Gondola weight: -
Overall weight: -
Description of the payload or experiment
102 CM FAR INFRARED TELESCOPE
Responsable institution: Center for Astrophysics, Harvard College Observatory / Smithsonian Astrophysical Observatory / Lunar and Planetary Laboratory, University of Arizona
Principal Investigator: Dr. Giovanni Fazio
The 102-cm Far Infrared Telescope was born as a result of a Cooperative program between the Smithsonian Astrophysical Observatory and the University of Arizona to develop a balloon-borne telescope capable of carrying out far-infrared (40~250 µ) observations.
It was flown nineteen times since the inception of the program in 1972 until the moment it was cancelled in 1989. The telescope was one of the most sensitive and versatile balloon-borne instruments ever used for observations in the far-infrared region of the spectrum, producing high resolution maps of large sky areas including molecular clouds, dark cloud complexes, planets, asteroids, and leading to the discovery of many new IR sources.
The payload was designed and constructed by the Solar Satellite Engineering Group, Harvard College Observatory while the University of Arizona contributed the primary and secondary mirrors and the infrared detectors.
The core of the instrument was a Cassegrain telescope composed by a spherical primary mirror measuring 102 cm of diameter, constructed of Tenzalloy (an aluminum alloy) along with a 18-cm secondary mirror, made of Pyrex, and figured to match the primary mirror. The Cassegrain focus occured behind the primary mirror where the infrared beam was reflected by a dichroic beam splitter that passed visible light. A second beam splitter directed half the optical light onto an N-slit mask at a second focal plane. Light passing through the mask was focused onto a photomultiplier tube.
The secondary mirror was mounted by means of a central bolt to a solenoid-driven chopper mechanism further mounted on a commandable-focus drive, which made the mirror to oscillate in the azimuthal direction. This beam switching technique was particular usefull to cancel the background radiation from the sky and the mirror. The whole secondary system was supported by four sheet-metal spiders to the external support ring and then through conventional tubular trusses to the central telescope ring. The weight of the telescope assembly (including optics and instrumentation) was 400 kg.
The infrared detection system used on the focal plane of the instrument in the firsts flights consisted of four gallium-doped germanium bolometers, cooled to 1.8 degrees Kelvin in a liquid helium dewar vented to ambient atmospheric pressure. The cooled optics, consisted of a sandwich of 0.86 mm crystalline quartz and 1 mm calcium flouride. Each of the four bolometer signals was amplified by a voltage preamplifier, and several postamplifiers with different gains. The signal lines were then connected directly to the telemetry system through teflon coaxial cables, digitized and transmitted to the ground station by PCM telemetry.
The telescope was mounted in a rectangular aluminum-frame gondola 5.1 m high and 3.4 X 2.9 m wide. The entire system weighted approximately 1814 kg. Of primary importance were the heavy structural elements used throughout the central portion of the gondola to maintain the integrity of the telescope tube, the elevation and azimuth axes, and the payload electronics. The instrument was stabilized and pointed by means of positional servo controls on the elevation and azimuth axes. The entire gondola moved in azimuth, but the telescope motion in the elevation direction was with respect to the gondola frame. The driving element for each axis was a DC torque motor mounted directly on the axis without gearing. In elevation, the reaction mass was the main frame of the suspended gondola. Reaction forces for the azimuth position control were provided by a large reaction wheel mounted on the gondola center line below the telescope. To control the reaction-wheel speed and to isolate the gondola from random and rapid balloon rotations the system used a "momentum dump" device: a bearing supporting the shaft passing up to the balloon, whose outer race was driven in sinusoidal excursions.
Positioning the telescope optical line of sight was accomplished in two modes: first, an acquisition mode, determined with respect to the horizontal component of the earth's magnetic field in azimuth and with respect to the local vertical in elevation; and second, an inertial mode, determined by a two-axis gyroscope system mounted on the telescope tube, that gave stability in inertial space. For the acquisition mode, a null magnetometer was mounted on a table and servo-driven in azimuth so as to remain fixed to local magnetic north.
The N-slit photometer, was used also for fine-position determination of the telescope. As Stars transited across the three branches of the N mask, this information was telemetered to the ground, indicating with high precision the elevation and azimuth of the optical object with respect to the star field during scanning activities.
An independent device for postflight pointing verification was the star-field camera, a 35-mm-sequence camera mounted on the telescope, which provided an effective field of view of about 15°. The camera photographed star images and recorded a projected reticle pattern, along with data indicating frame number, time, and status of the payload. In addition to taking pictures on command from the ground, the camera was automatically triggered during the various payload scanning functions.
In the event of failure of one or more major systems in the primary operation of the telescope pointing, -something that occured in the first flights- a simple backup control system was available to be activated by tone commands. This system served two functions: automatic stow and crude pointing control of the telescope in azimuth and elevation.
The main power for the payload was supplied by a silver zinc battery pack while the telemetry system used was provided by the National Scientific Balloon Facility. This allowed to process the information, to store it digitally in tapes, and also giving a glimpse of the data in real-time through the output printed in a teletypewriter.
During the entire duration of the program a combination of six different instruments have been flown at the focal plane of the telescope, broadening the scope of research and increasing the amount of institutions that benefited of the use of the platform. Althought no serious incidents occured during the two decades that the program lasted at some point the gondola, associated electronics and wiring started to be too unreliable. Althought funds were requested from NASA for a complete refurbishment of the instrument, the upgraded never materialized. Thus, rather than risk a failure the program was terminated in 1989 and the system was put in storage. In 1996 the Smithsonian Astrophysical Observatory transfered the gondola to the National Air and Space Museum in Washington, D.C. where several components of it are in display.
As inheritance, the innovative design of the gondola was taken by three different scientific groups whom further developed the concept in brand new instruments for astronomical and astrophysical research.
Performance in flight and data obtained
No data obtained sur to the failure of the balloon.
External references and bibliographical sources
- Far-infrared sources in the vicinity of the supernova remnant W28 Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 279, April 1, 1984, p. 162-165
- Today whereabout of the 102 cm far infrared telescope at the National Air and Space Museum Web Site