Expansion piston for the BEBC test chamber and calibration of Gargamelle lenses

With a bubble chamber such as Gargamelle, which had a volume of several cubic meters, major errors were liable to occur because of optical distortion in the lenses. In the worst conditions these errors could be as great as 3 cm. It was therefore essential to know the extent of distortion exactly in order to be able to correct for it when processing the data. The eight lenses of Gargamelle were being calibrated at CERN — work began in August 1969 and was planned finished by February 1970. The lenses were made by Sopelem (France) and costed nearly 80 000 Swiss francs each. The measurement procedure was to fix a squared panel perpendicular to the axis of the lens, and to photograph this on a film which is then scanned. The position of the lines on the panel (to a precision of 0.01 mm) and the measurement precision of the film (±3 microns) were known. Measurements were taken at 1000 points for two different positions of the plate. The results were then punched on to cards and processed by the CDC 6600.