Testing SPI imaging of high-energy and extended sources

INTEGRAL's main instruments employ coded apertures to obtain directional information on the incoming radiation. In order to experimentally better determine the imaging capabilities of the spectrometer SPI, the SPI Imaging Test Setup (SPITS) has been built at MPE. It consists of the SPI coded mask and two SPI-identical Ge detectors on an XY-table which allows us to move them to cover the 19 Ge detector positions. The SPI flight model imaging calibration only covered the energy range up to 2.7 MeV and did not include extended emission. SPITS was used to explore the performance of such a coded aperture system – combined with the SPI image analysis software – for higher-energy point sources and extended sources. We find that a 2.4° diameter disk emitting 511 keV emission is reconstructed well. For the high signal-to-noise ratios of laboratory measurements, positions of point sources above 4 MeV could be reconstructed to better than 0.1°.