HIGH RESOLUTION IR SOLAR OBSERVATION

To complement the spectroscopic observations of the sun with high spectral resolution done at the Jungfraujoch International Scientific Station in the Swiss Alps, the Institute of Astrophysics of the University of Liege, from Belgium developed a balloon-borne instrument to observe in all spectral regions not accessible from the ground.

The instrument was composed by a 40-cm aperture Ritchey-Chretien telescope, associated with an Ebert-Fastie-type grating spectrometer of 2.5 meters of focal length. A gimbaled plane mirror directed the solar radiation within the telescope. A solar image of 56 mm diameter was produced in the focal plane of the telescope and its central part was then transferred and focussed onto the entrance slit of the spectrometer by two mirrors. The main mirror of the instrument was spherically shaped to a radius of curvature of 5 meters. Two different gratings were available for covering the 1.5 to 15 microns region. The working order selection was obtained with a set of interference filters mounted on a filter-wheel. All mirrors and mounts were made from light aluminium alloy, kanigen-coated and the optical surfaces were aluminized.

During development was decided to adopt a double-pass configuration, instead os a single one. This allowed to reduce the level of diffused light at the exit slit, to remove spurious grating defects and also to provide an instrumental profile free of secondary "aperture-diffraction maxima". For the 1.5 to 3.0 microns region, the incoming radiation was focussed onto a lead sulfide cell, cooled down to -70°C by thermoelectrical effect; a second PbS detector located near and parallel to the intermediate slit allowed to record simultaneously the spectrum in single pass.

During the scanning, the signals detected by the cells were synchroneously amplified and stored in an on-board magnetic tape recorder as well transmitted to the ground by telemetry for real-time monitoring and optimisation of the equipment by telecommand. A tungsten lamp, which could be placed temporarily in the optical path, was used for in-flight realignment of the spectrometer; it also allowed to determine spectroscopically, the amount of water vapor inside the instrument.

The instrument was mounted inside a 4.75 meter high balloon gondola which contained the optics and all components. The gondola, as well as the telescope and spectrometer frames were made out of aluminium honeycomb. The pointing process was acomplished in two steps. A first coarse pointing using silicon solar sensors which controled an inertial wheel on top of the gondola to direct the aperture of the instrument towards the sun, then the fine pointing was completed moving the main mirror which was mounted in a two-axis gimbaled system, and controlled by torque motors servo-controlled by two pairs of fine solar sensors. The total weight of the balloon equipment was approximately 1100 Kgs.

The program was active from 1969 until 1993, performing 27 balloon flights.

Balloon launched on: 4/6/1970 at 5:11 cst
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 2.900.000 cuft (1.5 Mil.) Stratofilm)
Flight identification number: 539P
End of flight (L for landing time, W for last contact, otherwise termination time): 4/6/1970
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): F 3 h 10 m
Landing site: Mt. Enterprise, Texas, US
Payload weight: 3065 lbs.

The flight was the second one of the program (ULG-2). The lead sulfide detector, whose liquid nitrogen cooling system had prevented to obtaining reliable scientific data on the first flight (ULG-1 performed in June 6 1969), was replaced by a thermo-electrically cooled PbS cell. The equipment behaved very well at 90 Kft altitude. Solar observations were scheduled from 8 AM to 5 PM but a leak in the balloon lead to an early termination of the flight after 3 hours of float. While the balloon was being kept between 90 and 75 Kft altitude by ballasting, solar observations and on board calibrations were carried out successfully over a 3 hours period. The quality of the solar spectrum recorded was exceptionally good. This was the first time that a double-pass spectrometer with a narrow intermediate slit and providing a practical resolving power higher than l35.000 at l.85 microns was operated successfully above 25 Km altitude

• NCAR Scientific Balloon Facility Annual Report, 1970 National Center for Atmospheric Research, February 1971
• Water vapor above 25 km altitude Pure and Applied Geophysics, December 1973, Volume 106, Issue 1, pp 1346