Optical path length stability testing of the LISA telescopes

The LISA telescopes have the unique requirement that the overall optical path length through them must be stable to within 1pm/√Hz in the LISA detection band. At University of Florida we developed a facility and techniques to perform pm-precision measurements of meter scale structures over hours long time scales, exactly to verify that this requirement can be met by LISA telescope prototypes. From Summer 2023 we have been applying this capability to test the LISA telescope Structural Thermal Model (STM). We are further developing these capabilities in conjunction with NASA GSFC to prepare for testing the LISA telescope Engineering Development Units (EDUs), and eventually flight Qualification Models (QMs). In this talk we will briefly describe the key features of the facilities and techniques used for the measurements, summarize the STM test results, and sketch out the plans for EDU and QM tests in the future.