Coded Mask

Figure 5: Illustration of the JEM-X coded mask pattern layout without the mechanical interface. The diameter of the coded mask is 535 mm. The mask has a transparency of 25%.

The mask is based on a Hexagonal Uniformly Redundant Array (HURA). For JEM-X a pattern composed of 22501 elements with only 25% open area has been chosen. The 25% transparency mask actually achieves better sensitivity than a 50% mask, particularly in complex fields with many sources, or in fields where weak sources should be studied in the presence of a strong source. A mask with lower transparency also has the advantage of reducing the number of events to be transmitted, while at the same time increasing the information content of the remaining events. Considering the telemetry allocation to JEM-X, this means an improved overall performance for the instrument, particularly for observations in the plane of the Galaxy. The mask height above the detector ( 3.4 m) and the mask element dimension (3.3 mm) define together the angular resolution of the instrument, in this case 3'. Figure 5 illustrates the JEM-X coded mask pattern.