Design and preliminary validation of a low stray light and stable telescope with coaxial entrance and exit pupils for...

The spaceborne gravitational-wave (GW) detection telescope demands unprecedented stability and signal fidelity, with stringent requirements including a wavefront error <λ/40 at λ=1064nm, tilt-to-length (TTL) noise<25pm/µrad, and backward stray light suppression <10^-10. To overcome instability and stray light limitations of the conventional off-axis four-mirror structure with different entrance and exit pupil axes and small deflection angles of mirrors, we proposed what we believe to be two new off-axis five-mirror designs (S-type and δ-type) with coaxial entrance and exit pupils. Compared to the δ-type off-axis five-mirror design, the S-type off-axis five-mirror structure achieves the wavefront error better than λ/80 RMS at λ=1064nm in the acquisition field of view (FOV) (±200µrad) and a TTL maximum coupling noise value of less than 2pm/µrad with a tilt angle of ±600µrad in the central FOV. Theoretical analyses using the Harvey-Shack (HS) and Mie scattering models, combined with ABg-fitted component-level bidirectional reflectance distribution function (BRDF) data, confirm through system-level simulation that the S-type off-axis five-mirror configuration exhibits superior stray light suppression performance. The simulation results demonstrate a 55% reduction in backward stray light energy compared to the off-axis four-mirror structure. Consequently, the S-type off-axis five-mirror design serves as a viable solution for space-based GW detection.