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The CACTµS (Compact Array of Cadmium Telluride Micro Spectrometers) instrument was constructed as a prototype detection plane for the Coded Imager and Polarimeter for High Energy Radiation (CIPHER) telescope. The instrument, flown as a 'piggy-back' experiment on a stratospheric balloon as part of the Summer 2002 trans-mediterranean flight campaign from the Milo Italian balloon base in Sicily, was constructed in order to verify the feasibility of using this kind of position sensitive detector for hard X and soft gamma ray polarimetry and imaging, with the objective of studying the instrumental background at stratospheric balloon altitudes over the 20-1000 keV energy range. Of particular interest is the spectrum and distribution of Compton scattered events that trigger two pixel (double) events, the recognition of which is essential for providing a high sensitivity to linear polarisation. It was also used to evaluate the efficiency and the reliability of an off-line numerical algorithm to be applied to the CdTe flight data for signal compensation, which uses the amplitudes obtained by readout electronics with a double shaping filter stage.

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