Design and Analysis of a Space Gravitational-Wave Observation Telescope with Long Exit Pupil

For gravitational wave observation, an optical design of the telescope has been implemented to perfectly match the laser interferometry system, and stray light analysis is used to quantify the impact of mirror roughness noise on interferometric measurement sensitivity. An off-axis six-mirror afocal optical design, comprising a parabolic primary, hyperbolic secondary and plane-parabolic collimation group, delivers an optical system with a 400 mm entrance pupil, 100× expansion ratio, and λ/30@1064 nm Root Mean Square (RMS) wavefront quality. To obtain a feasible exit pupil position which can easily integrate the laser interferometer, the theoretical mathematical relationships among the exit pupil position, primary–secondary mirror spacing, and radius of curvature of the secondary mirror and the sixth mirror are derived. Accordingly, the effective exit pupil position is extended to 174 mm to match the laser interferometer. The sixth mirror is the primary source of backscattered light. With the RMS roughness of the primary, secondary, and three folding mirrors set to 6.4 Å, 1.6 Å, and 3.7 Å, respectively, the Point Source Transmittance (PST) value can be kept below 3.6 × 10−10 when the RMS roughness of the sixth mirror is less than 1.1 Å. Tolerance analysis is carried out to obtain the feasible engineering distribution of optical parameters, and the statistical results show that the system RMS wavefront error is smaller than 0.01λ@1064 nm.