The Einstein Probe (EP) is a mission of the Chinese Academy of Sciences (CAS) dedicated to time-domain high-energy astrophysics. Its primary goals are to discover high-energy transients and monitor variable objects. The ESA Science Programme Committee (SPC) approved on 19 June 2018 the participation of ESA to the CAS EP mission as a mission of opportunity. Among other elements, CAS has requested ESA participation for the provision of the mirror modules of the follow-up x-ray telescope (FXT).

FXT is a pair of Wolter-I telescopes operating in the 0.5-10 keV energy range, inheriting the design from eROSITA [2][3]. It provides field of view of about 1 deg diameter. The source localization error will be of 5-15 arcsec depending on the source strength [1]. The FXT is responsible for the quick follow-up observations of the triggered sources and will also observe other interested targets during the all-sky survey at the rest time.

Three FXT mirror modules were produced: structural and thermal model (STM), qualification model (QM) and flight model (FM). Media Lario could leverage on the manufacturing and integration infrastructure still available at its premises from the eROSITA programme [3][4], including the complete set of 54 mandrels needed for the mirror repliforming, property of MPE.

The abbreviation “eXTP” represents the enhanced x-ray timing and polarimetry, which is a key science mission initiated by the Chinese scientists, designed to study the state of matter under extreme conditions of density, gravity and magnetism [1]. Various payloads would be on board of the satellite. The SFA, namely the spectroscopy focusing array, consisting of nine identical x-ray telescopes working in the energy range of 0.5-10 keV, will be the focus here [1]. SFA has a field-of-view of 12 arcmin for each and a collecting area of 900 cm² and 550 cm² for each at 2 keV and 6 keV respectively [1].

This paper starts with a brief introduction of the general optics, and then goes across some important design aspects. It covers contents from the structural and thermal designs to the CAE analyses as well as the current status. The large diameter and huge focal length of the optics will definitely bring big issues to the robustness of the carrying structure under the severe conditions given by the launcher.

According to the current design, the mirror assembly will have 3 feet and 24 spokes. Vibration tests were already performed on a few prototypes by IHEP, and a preliminary evaluation on the feasibility of the design has been achieved. It clearly stated that the current design with only a single spider can probably survive the vibration tests assuming a compromised test condition somewhere. CAE models were adjusted thereafter to match the test results, which could be used for further assessments in a near future.