Long-term optical path-length stability testing of the LISA Telescope Structural Thermal Model*

The Laser Interferometry Space Antenna (LISA) is an upcoming space-based ESA lead gravitational wave (GW) detection mission, with high sensitivity in the milliHertz GW gap left between Earth based GW detectors and pulsar timing arrays. The mission consists of 3 spacecraft (SC) in heleocentric orbit, with exchanged laser beams between each pair of SC monitoring displacements at the level of picometers in the LISA band between housed test masses. This requires LISA's telescopes, provided by NASA, demonstrate picometer level stability along the optical path. University of Florida's LISA Telescope team will test the path-length stability of beams within the structural thermal model (STM) of the telescopes, a functional equivalent containing all bonds but none of the polished mirror surfaces. By introducing several cavity optics, we create two optical cavities within the STM which we will PDH lock to two beams. Beat notes of these against a laser locked to an ultrastable reference cavity should then provide the necessary CTE and stability information about the telescope optical path. Additionally, beatnotes with an iodine stabilized laser can yield infromation about long term drifts of the cavity length. In our presentation we go over the testing setup of these two frequency based stabilization techniques, and whatever results we can share from testing that are available to the public.