UV TELESCOPE

The mission served as the initial phase of a research program initiated in mid 1960's decade at the Geneva Observatory aimed at studying ultraviolet radiation from stars using stratospheric balloons. The initiative was fueled by the limitations of ground-based observations due to atmospheric absorption, particularly in the ultraviolet spectrum. The program emphasized broad-band photometry, a method to analyze the spectral energy distribution of starlight, providing insights into stellar properties such as luminosity, temperature, and chemical composition. These observations are essential for understanding stellar evolution, especially for hot stars whose ultraviolet emission is particularly informative.

In the image at left can be seen an scheme of the scientific instruments used (click to enlarge). The gondola, constructed from an aluminum frame with triangulated structures and a height of 2.1 meters, featured conical joints allowing rapid assembly and replacement of damaged parts. Weighing 18 kg, it included a sealed, thermostatically controlled cylindrical compartment for most of its equipment. Measurements were made using instruments such as a Thommen meteorological probe for pressure, temperature, and humidity, a direct-read barometer, thermistors, and a potentiometric accelerometer for mechanical stress analysis. The gondola's attitude was monitored by an HRA 31 magnetometer, measuring Earth's magnetic field across three orthogonal axes with one-degree accuracy.

A Maksutov telescope with a 7 cm aperture was designed for ultraviolet sky photography. Its optical system included a quartz corrector meniscus, a spherical mirror, and a secondary reflection mirror coated with aluminum optimized for 90% reflection at 2000 Å. The telescope's focal plane featured a photographic chamber loaded with 30 meters of 35 mm film. A dual optical system allowed for simultaneous ultraviolet and visible light imaging, with one field filtered to block wavelengths below 3700 Å. The entire telescope, mounted on the upper plate of the sealed compartment, was aligned at a 20° angle relative to the gondola's vertical axis and was equipped with a heated ring to prevent frost formation on the meniscus.

Data recording utilized a Siemens camera photographing instrument panels and telemetry data for redundancy in case of radio transmission failure. The gondola's programmed operations, initiated every 30 seconds, controlled film advancement, thermometric probe switching, and reference signal transmission.

Balloon launched on: 12/8/1964 at 22:30 UTC
Launch site: Centre de Lancement de Ballons CLBA, Aire Sur L'Adour, Landes, France  
Balloon launched by: Centre National d'Etudes Spatiales (CNES)
Balloon manufacturer/size/composition: Zero Pressure Balloon Zodiac - 35.000 m3
End of flight (L for landing time, W for last contact, otherwise termination time): 12/9/1964
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): ~ 4 h
Landing site: 250 km from the launch site

The balloon was launched from the CNES base on Aire Sur L'Adour on December 8, 1964, at 22:30 UTC under excellent weather conditions. The gondola, suspended by 120 meters of nylon rope, ascended steadily for two hours, reaching a peak altitude of approximately 35 kilometers, with external temperatures dropping to -60°C. During the ascent, the gondola's movements were initially chaotic, but once at its cruising altitude, it began a slow rotational oscillation relative to the southern horizon, allowing the onboard Maksutov telescope to sweep the Milky Way. However, the flight ceiling was maintained for only half an hour due to strong easterly winds, typical of winter at such altitudes, which propelled the gondola eastward at a speed of 250 km/h. To prevent a challenging recovery in the Alps, the flight was terminated via a timer mechanism, which released the balloon and allowed the gondola to descend using a parachute 250 km from the launch site, No damage was experienced by the telescope thanks to the fiberglass shock absorbers that sourrounded its structure.

All instruments operated successfully, and data were collected both via radio transmission and onboard photographic records. Observations revealed high-quality and original insights into the gondola's behavior at altitude and the prevailing meteorological conditions. This mission demonstrated the robustness of the gondola's structure, the reliability of its subsystems, and the feasibility of using stratospheric balloons for advanced astronomical research.

• Mesures astronomiques par ballons stratosphériques (I) Orion : Zeitschrift der Schweizerischen Astronomischen Gesellschaft, 10 (1965)
• Mesures astronomiques par ballons stratosphériques (II) Orion : Zeitschrift der Schweizerischen Astronomischen Gesellschaft, 10 (1965)