The efficiency of a Compton telescope operating in its "imaging mode" decreases rapidly with energy below a few hundred keV. At the same time, the achievable angular resolution in Compton mode also declines. The efficiency decrease is mainly due to the increasing photoabsorption cross sections at these low energies. The average distance between interaction sites for Compton events decreases with decreasing photon energy; this, together with the increasing impact of Doppler broadening, results in the instrument's angular resolution rapidly deteriorating as the photon energy decreases. These limitations of efficiency and angular resolution constitute essentially "fundamental" limits on the energy regime accessible to imaging Compton telescopes. At the same time, a Compton Telescope at balloon altitudes or in space is exposed to Cosmic diffuse and atmospheric photon backgrounds. The intensity of both of these components increases rapidly with decreasing energy, making photons < 100keV a possibly significant contributor to random coincident events or instrument dead time. Thus shielding of low-energy photon components is desirable. For the first balloon flight of the Nuclear Compton Telescope (NCT) we have simultaneously reduced low-energy background and enabled hard X-ray imaging by enshrouding the top of the instrument in a tin shield (bottom had BGO shielding), with part of the shield having strategically placed holes - a 10% open coded aperture mask working up to -80 keV in conjunction with the finely pixellated double-sided Ge strip detectors. We present pre-flight calibration results of this coded mask.
