the fundamental high-energy processes at the core of the solar flare problem. The primary scientific objective of RHESSI was to understand the following processes that take place in the magnetized plasmas of the solar atmosphere during a flare: Impulsive energy release Particle acceleration Particle and energy transport These high-energy processes play a major role at sites throughout the universe ranging from magnetospheres to active galaxies. Consequently, the importance of understanding these processes transcends the field of solar physics; it is one of the major goals of space physics and astrophysics. The high energy processes of interest include the following: The rapid release of energy stored in unstable magnetic configurations The equally rapid conversion of this energy into the kinetic energy of hot plasma and accelerated particles (primarily electrons, protons and ions) The transport of these particles through the solar atmosphere and into interplanetary space The subsequent heating of the ambient solar atmosphere These processes involve: Particle energies to many GeV Temperatures of tens or even hundreds of millions of degrees Densities as low as 100 million particles per square cm Spatial scales of tens of thousands of kilometers, and Magnetic containment times of seconds to hours It is impossible to duplicate these conditions in laboratories on the Earth. The acceleration of electrons is revealed by hard X-ray and gamma-ray bremsstrahlung while the acceleration of protons and ions is revealed by gamma-ray lines and continuum. The proximity of the Sun means not only that these high-energy emissions are orders of magnitude more intense than from any other cosmic source but also that they can be better resolved, both spatially and temporally. Imaging Since X-rays are not easily reflected or refracted, imaging in X-rays is difficult. One solution to this problem is to selectively block the X-rays. If the X-rays are blocked in a way that depends on the direction of the incoming photons, then it may be possible to reconstruct an image. The imaging capability of RHESSI was based on a Fourier transform technique using a set of 9 Rotational Modulation Collimators (RMCs) as opposed to mirrors and lenses. Each RMC consisted of two sets of widely spaced, fine-scale linear grids. As the spacecraft rotated, these grids blocked and unblocked any X-rays which may have come from the Sun modulating the photon signal in time. The modulation could be measured with a detector having no spatial resolution placed behind the RMC