Design and preliminary validation of a low stray light and stable telescope with coaxial entrance and exit pupils for gravitational-wave detection

The spaceborne gravitational-wave (GW) detection telescope demands unprecedented stability and signal fidelity, with stringent requirements including a wavefront error <<i>λ</i>/40 at <i>λ</i>=1064<i>n</i><i>m</i>, tilt-to-length (TTL) <i>n</i><i>o</i><i>i</i><i>s</i><i>e</i><25<i>p</i><i>m</i>/µ<i>r</i><i>a</i><i>d</i>, 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 <i>δ</i>-type) with coaxial entrance and exit pupils. Compared to the <i>δ</i>-type off-axis five-mirror design, the S-type off-axis five-mirror structure achieves the wavefront error better than <i>λ</i>/80 RMS at <i>λ</i>=1064<i>n</i><i>m</i> in the acquisition field of view (FOV) (±200µ<i>r</i><i>a</i><i>d</i>) and a TTL maximum coupling noise value of less than 2<i>p</i><i>m</i>/µ<i>r</i><i>a</i><i>d</i> with a tilt angle of ±600µ<i>r</i><i>a</i><i>d</i> 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.