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: 6/0/1965
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): 6/??/1965
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): ~ 5 h

This was the second mission of the program. The balloon was launched from the CNES base of Aire Sur L'Adour during June 1965. After a nominal ascent, the balloon reached 32,5 kilometers of altitude and remained at that level for two hours performing observations of stars from nine constellations.

The observations made were recorded on Agfa Isopan film using the Maksutov telescope equipped with a combination of two photometric filters. The observation program consisted of a systematic sky scan, completing one full revolution every 15 minutes. The telescope's elevation was set at 45 degrees for the first revolution and then increased by 3 degrees with each pass to the north. During the two hours of observations stars from the following constellations were captured in the telescope's field: Cepheus, Cygnus, Vulpecula, Aquila, Ophiuchus, Serpens, Boötes, Ursa Major, and Ursa Minor. The coincidence of the scanning motion, the elevation adjustments, and the rising motion of stars in the east allowed these stars to be observed at two, three, or even four consecutive elevations, separated by 3 degrees.

Of the 1,300 images exposed, 500 stellar traces were identified and cataloged, corresponding to stars up to the 6th magnitude. However, only about a hundred stars produced high-quality recordings. The reduction in usable observations was primarily due to weak stars causing a darkening too close to the background level of the film, preventing the weakest observable features from being detected. It was also caused by accidental defocusing due to the pressurization of the telescope.

• Mesures de l'ultra-violet stellaire à partir de ballons stratosphériques Archives des sciences, (22), 1969