Research progress of telescopes for space-based gravitational wave missions

The optical telescopes for space-based gravitational wave missions play an important role in the measurement, which both expand the beam going to the far spacecraft and efficiently collect the beam sent from the far spacecraft. The telescope, as part of the interferometric path, directly affects the measurement noise. Compared with the imaging system, the telescope for space gravitational wave observatory not only has high requirements on wavefront quality, but also has extremely high requirements on stray light performance and optical path stability. In addition, the coupling of the wavefront aberration resulting from the telescope and pointing error can produce tilt-to-length (TTL) noise. One of the design goals is to minimize the TTL coupling in the transceivers. In terms of the optical system, the design baseline is an off-axis four-mirror optical system, which can meet the wavefront quality and stray light specification requirements. The optimization of the optical system is to further reduce the TTL noise and improve compatibility with the optical bench. The opto-mechanical structure of the telescope is the physical carrier to realize the optical, mechanical, and thermal design functions. The current design baseline tends to use Zerodur or ULE materials for all optical components and structures, which can ensure the optical path stability of the telescope. However, there are currently few studies on the vibration tolerance of telescopes during the launch stage. In terms of stray light, through optimization of the optical system and strict control of the surface quality of optical components, backscattered stray light can be reduced to 10⁻¹⁰ of the transmission power. Stray light caused by contamination of the optical surface is more serious than stray light caused by the roughness of the optical surface. Stringent anti-pollution measures need to be taken during development and storage. In terms of the thermal design, the main external heat flow disturbance in the 0.1 mHz - 1 Hz frequency band comes from the fluctuation of the solar constant. The thermal control of the telescope is closely related to the configuration of the spacecraft and the distribution of heat sources. Thermal design and optimization of telescopes need to be carried out integrated with the spacecraft. The research progress of the telescope's optical system, optomechanical structure, space environment and thermal design, stray light simulation and suppression, and stability measurement are reviewed, which can provide a reference for the development of space gravitational telescopes in our country.