The Spectrometer/Telescope for Imaging X-rays (STIX) is one of the 10
instruments on-board the scientific payload of ESA's Solar Orbiter mission.
STIX provides hard X-ray imaging spectroscopy in the 4-150~keV energy range,
observing hard X-ray bremsstrahlung emission from the Sun. These observations
provide diagnostics of the hottest thermal plasmas (10~MK) and information
on the non-thermal energetic electrons accelerated above 10~keV during solar
flares. STIX has a spectral resolution of 1~keV, and employs the use of
in-direct bi-grid Fourier imaging to spatially locate hard X-ray emission.
Given that STIX provides critical information about accelerated electrons at
the Sun through hard X-ray diagnostics, it is a powerful contribution to the
Solar Orbiter suite and has a significant role to explore the dynamics of solar
inputs to the heliosphere. This chapter describes the STIX instrument, its
design, objectives, first observations and outlines the new perspectives STIX
provides over the mission lifetime of Solar Orbiter.