What systematic error is created by a slow, steady warping of the primary mirror cell over a mission?

A spurious long-term trend in catalog positions masquerading as valid proper motion data

Instrumental drift arises from subtle, non-dynamic changes in the telescope's internal geometry, even in space. If a critical structural component, like the primary mirror cell, warps slowly and steadily over the course of years due to thermal cycling or internal mechanical stresses, this minute physical deformation directly introduces a systematic shift in the measured angles. Because this drift is often constant or changes very slowly, it becomes extremely difficult to distinguish this instrumental artifact from a star's genuine, slow proper motion, demanding exceptional mechanical and thermal engineering standards to maintain nanometer-level stability.