The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) was a NASA Small Explorer mission launched in 2002 to investigate the physics of solar flares through high‑resolution X‑ray and gamma‑ray imaging spectroscopy. The spacecraft carried nine cryogenically cooled high‑purity germanium detectors that provided precise measurements over an energy range of approximately 3 keV to 17 MeV. RHESSI operated for more than sixteen years, delivering detailed observations of particle acceleration, energy release, and high‑energy radiation from the solar atmosphere.
Raw Level‑0 packed telemetry data for the entire mission are stored in FITS files, each typically covering one 90‑minute spacecraft orbit. These files contain the time of arrival and measured energy of every detected hard X‑ray photon in the original compressed telemetry format. The RHESSI software in SSW is required to unpack these data streams and construct calibrated spectra, time profiles, and images. Level‑0 files follow the naming convention hsi_yyyymmdd_hhmmss_ver.fits, where the date and time specify the start time of the file and ver is a version number.