  The protoMIRAX hard X-ray imaging telescope is a balloon-borne experiment
developed as a pathfinder for the MIRAX satellite mission. The experiment
consists essentially in a coded-aperture hard X-ray (30-200 keV) imager with a
square array (13$\times$13) of 2mm-thick planar CZT detectors with a total area
of 169 cm$^2$. The total, fully-coded field-of-view is $21^{\circ}\times
21^{\circ}$ and the angular resolution is 1$^{\circ}$43'. In this paper we
describe the protoMIRAX instrument and all the subsystems of its balloon
gondola, and we show simulated results of the instrument performance. The main
objective of protoMIRAX is to carry out imaging spectroscopy of selected bright
sources to demonstrate the performance of a prototype of the MIRAX hard X-ray
imager. Detailed background and imaging simulations have been performed for
protoMIRAX balloon flights. The 3$\sigma$ sensitivity for the 30-200 keV range
is ~1.9 $\times$ 10$^{-5}$ photons cm$^{-2}$ s$^{-1}$ for an integration time
of 8 hs at an atmospheric depth of 2.7 g cm$^{-2}$ and an average zenith angle
of 30$^{\circ}$. We have developed an attitude control system for the balloon
gondola and new data handling and ground systems that also include prototypes
for the MIRAX satellite. We present the results of Monte Carlo simulations of
the camera response at balloon altitudes, showing the expected background level
and the detailed sensitivity of protoMIRAX. We also present the results of
imaging simulations of the Crab region. The results show that protoMIRAX is
capable of making spectral and imaging observations of bright hard X-ray source
fields. Furthermore, the balloon observations will carry out very important
tests and demonstrations of MIRAX hardware and software in a near space
environment.
