What becomes the primary limitation on astrometric precision when telescopes escape Earth's atmosphere?

The stability of the telescope itself, specifically structural drift from thermal expansion or internal stresses

When instruments are placed in space, the dynamic noise generated by atmospheric distortion is eliminated. However, the systematic noise source merely shifts to the spacecraft hardware. Astrometry requires measuring tiny angular displacements over extensive periods, sometimes spanning years. If the structure supporting the optics, detectors, or focal plane undergoes even minute physical changes—caused by thermal cycling (expansion/contraction), internal stresses, or impacts from micrometeoroids—this physical drift translates directly into systematic, long-term errors in the measured angles, masquerading as the star's actual proper motion.