  The Transient High Energy Sources and Early Universe Surveyor is an ESA M5
candidate mission currently in Phase A, with Launch in $\sim$2032. The aim of
the mission is to complete a Gamma Ray Burst survey and monitor transient X-ray
events. The University of Leicester is the PI institute for the Soft X-ray
Instrument (SXI), and is responsible for both the optic and detector
development. The SXI consists of two wide field, lobster eye X-ray modules.
Each module consists of 64 Micro Pore Optics (MPO) in an 8 by 8 array and 8
CMOS detectors in each focal plane. The geometry of the MPOs comprises a square
packed array of microscopic pores with a square cross-section, arranged over a
spherical surface with a radius of curvature twice the focal length of the
optic. Working in the photon energy range 0.3-5 keV, the optimum $L/d$ ratio
(length of pore $L$ and pore width $d$) is upwards of 50 and is constant across
the whole optic aperture for the SXI. The performance goal for the SXI modules
is an angular resolution of 4.5 arcmin, localisation accuracy of $\sim$1 arcmin
and employing an $L/d$ of 60. During the Phase A study, we are investigating
methods to improve the current performance and consistency of the MPOs, in
cooperation with the manufacturer Photonis France SAS. We present the optics
design of the THESEUS SXI modules and the programme of work designed to improve
the MPOs performance and the results from the study.
