of the telescope tube. The grid pairs modulate the transmission of solar flare X-ray and gamma ray emissions through to the detectors as the spacecraft spins around the axis of the telescope tube. The modulated count rates in the nine detectors are used in computers on the ground to construct images of solar flares in different energy bands. The five coarse grids (square) were constructed by Van Beek Consultancy in Netherlands. The four fine grids (round) were constructed by Thermo Electron Tecomet in Massachusetts. All grids were characterized both optically and with X-rays at Goddard Space Flight Center before being shipped to the Paul Scherrer Institute for integration into the imaging telescope assembly. The spectrometer contains nine germanium detectors that are positioned behind the nine grid pairs on the telescope. These artificially grown crystals, pure to over one part in a trillion, were manufactured by the ORTEC division of Perkin Elmer Instruments. When they are cooled to cryogenic temperatures and a high voltage is put across them (up to 4000 volts), they convert incoming X-rays and gamma-rays to pulses of electric current. The amount of current is proportional to the energy of the photon, and is measured by sensitive electronics designed at the Lawrence Lawrence Berkeley National Laboratory and the Space Sciences Laboratory, at Berkeley, California. The detectors are cooled with an electromechanical Stirling-cycle cryocooler (built by SunPower Inc., and flight qualified at Goddard Space Flight Center). It maintains them at the required operating temperature of , or 75° above absolute zero). Mission concept RHESSI was designed to image solar flares in energetic photons from soft X-rays (~3 keV) to gamma rays (up to ~20 MeV) and to provide high resolution spectroscopy up to gamma-ray energies of ~20 MeV. Furthermore, it had the capability to perform spatially resolved spectroscopy with high spectral resolution. Scientific objectives Researchers believe that much of the energy released during a flare is used to accelerate, to very high energies, electrons (emitting primarily X-rays) and protons and other ions (emitting primarily gamma rays). The new approach of the RHESSI mission was to combine, for the first time, high-resolution imaging in hard X-rays and gamma rays with high-resolution spectroscopy, so that a detailed energy spectrum can be obtained at each point of the image. This new approach enabled researchers to find out where these particles are accelerated and to what energies. Such information will advance understanding of