The gondola is made with welded aluminium square tubes and a careful design prevents important deformations of the optical design in the presence of strength. The two mirrors and the cryostat are fixed to the frame. The pivot connects the flight chain of the balloon to the payload through a thrust bearing, providing the necessary degree of freedom for payload spin. It includes a torque motor that acts against the flight chain to spin the payload. The rotation of the payload is controlled via a vibrating structure rate gyroscopes that can detect angular speeds as low as 0.1 deg/s.
A custom star sensor has been developed for pointing reconstruction in order to be fast enough to work on a payload rotating at 2-3 rpm. We have developed a simple night sensor, based on a telescope with photodiodes along the boresight of the mm-wave telescope. Thus, like the millimeter telescope, the star sensor scans the sky along a circle at an elevation of 41 deg. A linear array of 46 sensitive photodiodes were placed in the focal plane of a 40 cm diameter, 1.8 m focal length parabolic optical mirror. The line of photodiodes is perpendicular to the scan and covers 1.4 degrees in elevation on the sky. We can observe during one rotation of the payload stars up to magnitude 7 i.e. between 50 and 100 stars per turn during night time. An optical filter allows this star sensor to yield at least a few detected stars even in the presence of low elevation Sun. Pointing reconstruction is done a posteriori by comparing star candidates and a dedicated star catalog. The precision of the pointing solution is better than 1 arcminute rms for the Trapani test flight and 2 arcmin. for the Kiruna 2001 flight.