Towards the first light for a gamma-ray lens

The "gamma-ray lens" collaboration is presently preparing for a fundamental ly new type of telescope, which is to be flown on a statospheric balloon by the French Space Agency CNES. The instrument features a Laue diffraction l ens, a detector module with a 3x3 germanium array, and a balloon gondola st abilized to 15" pointing accuracy. The first flight is planned for the 2 en d of 1999 from Leon, Spain. The instrument's lens focusses gamma-ray photon s from its 600 cm area onto a small solid state detector, with only 18 cm(3 ) equivalent volume for background noise. Besides its excellent sensitivity , the telescope has outstanding angular and spectral resolution. The primar y objective for its first balloon Right is to detect the Crab nebula and to measure its angular extent at 170 keV with an unprecedented angular resolu tion of about 1 are minute. For the first time in gamma-ray astronomy, the statistics will be dominated by the signal: for a residual atmosphere of 5. 5 g/cm(2) and during a balloon Right at mid-latitudes, the observation of t he Crab is expected to be essentially background free. The background will be only a few count/hour, with a detected source Bur of 25 to 40 photons pe r hour at the Crab culmination. Scientific objectives for further flights i nclude collapsed objects, SNRs, and broad class annihilators.