Optical design of the extreme ultraviolet spectrometer (EUS) on board Solar Orbiter

We present optical designs for the Extreme Ultraviolet Imaging Spectrometer (EUS) proposed for Solar Orbiter. We summarise the scientific requirements for EUS and show how they translate into an instrument specification and optical design. Two design options are presented: one utilising a normal incidence telescope and one utilising a grazing incidence telescope. Both options use the same design of spectrometer, which incorporates a Toroidal Varied Line-Space (TVLS) grating, allowing high quality imaging at relatively large spectrometer magnifications. This results in a very compact yet high performance design. We give estimates of the instrument's optical performance and throughput and discuss briefly some heat management strategies.