The coded mask is a rectangle of absorbing elements of tungsten settled in a pseudo-random pattern determined by a mathematical procedure. It permits to reconstruct images of the sky.

The mask elements are 1.5 cm thick and their size is 9.5 x 9.5 mm. The dimensions of the coded mask are 27.8 x 29.7 cm. The coded mask is held in place at a distance of 2.5 m above the position detector by an aluminium tube.

The position detector is based on the Anger scintillation gamma camera principle used in nuclear medicine.

It consists of a fragile sodium iodide crystal (1.25 cm thick and 57 cm diameter) mounted on an optical glass disk and analysed by 61 hexagonal photomultiplier tubes. The active surface area of the position detector is 794 square centimeters.

Electronics associated with the detector gives the position of the interaction point of a gamma photon with the sensitive sodium iodide crystal and its energy.

The active shielding eliminates the background noise induced by the charged particles. It consists of 31 cesium iodide crystals analysed by 62 photomultiplier tubes, arrayed around the detector.

The principle is the one of the anticoïncidence : an event is ignored when it is detected simultaneously by both the active shielding and the position detector.

The active cesium iodide anticoïncidence shield surrounding the position detector is also used to detect gamma-ray bursts.Passive shielding

The background noise is also eliminated by limiting the field of view of the telescope with passive shielding, which is constituated of an association of three metals (0.5 mm lead, 0.1 mm tantalum and 0.4 mm tin).

The passive shielding is wrapped around the aluminium tube and reduces the flux of photons coming from all other directions in the sky outside the field of view of the telescope.

A plastic scintillator is placed in the telescope aperture to eliminate the charged particles without adversely affecting the indispensable transparency to those photons coming from sources in the field of view.