  The present paper describes the current baseline optical design of POLLUX, a
high-resolution spectropolarimeter for the future LUVOIR mission. The
instrument will operate in the ultraviolet (UV) domain from 90 to 390 nm in
both spectropolarimetric and pure spectroscopic modes. The working range is
split between 3 channels -- far (90-124.5 nm), medium (118.5-195 nm) and near
(195-390 nm) UV. Each of the channels is composed of a polarimeter followed by
an echelle spectrograph consisting of a classical off-axis paraboloid
collimator, echelle grating with a high grooves frequency and a cross-disperser
grating operating also as a camera. The latter component integrates some
advanced technologies: it is a blazed grating with a complex grooves pattern
formed by holographic recording, which is manufactured on a freeform surface.
One of the key features underlying the current design is the large spectral
length of each order ~6 nm, which allows to record wide spectral lines without
any discontinuities. The modelling results show that the optical design will
provide the required spectral resolving power higher than R ~ 120,000 over the
entire working range for a point source object with angular size of 30 mas. It
is also shown that with the 15-m primary mirror of the LUVOIR telescope the
instrument will provide an effective collecting area up to 38 569 cm 2. Such a
performance will allow to perform a number of groundbreaking scientific
observations. Finally, the future work and the technological risks of the
design are discussed in details.
