A BALLOON-BORNE IMAGING TELESCOPE FOR HIGH-ENERGY ASTROPHYSICS

We describe an imaging telescope for observations of celestial sources in the energy range between 30 keV and 1.8 MeV onboard stratospheric balloons. The detector is a 41 cm diameter, 5 cm thick NaI(Tl) crystal coupled to 19 photomultipliers in an Anger camera configuration. It i s surrounded by a plastic scintillator 15 cm thick on the sides, 0.2 c m thick at the top and 20 cm thick at the bottom. The imaging device i s based upon a 19 x 19 element square MURA (Modified Uniformly Redunda nt Array) coded mask mounted in an one-piece mask-antimask configurati on. The detector's spatial resolution is about 10 mm at 100 keV. This is the first experiment to use such a mask pattern and configuration f or astrophysical purposes. The expected 3sigma sensitivity for an on-a xis source observed for 104 s at a residual atmosphere of 3.5 cm-2 is 1.44 x 10(-5) photons cm-2 s-1 keV-1 at 100 keV and 1.00 x 10(-6) phot ons cm-2 s-1 keV-1 at 1 MeV. The angular resolution is approximately 1 4 arcminutes over a 13-degrees field of view. The instrument is mounte d in an automatic platform with a capability for pointing and stabiliz ation in both azimuth and elevation axis with 2 arcmin accuracy.