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The TianQin project is a planned space-based gravitational wave observatory in China, consisting of a formation of three spacecraft, each equipped with two telescopes for laser beam transmission and reception. The TianQin mission utilizes heterodyne interferometry to achieve precise distance measurements between test masses. The optical telescopes transmit measurement beams between the spacecraft, forming the long arms of the heterodyne interferometer. Due to the distinct objectives, the telescope system design for the space-based gravitational-wave observatory have slightly different design criteria compared to ordinary telescopes. In addition to meeting the requirements for diffraction-limited imaging quality, maintaining optical path stability is crucial. Wavefront aberrations caused by the telescopes and angular misalignment due to field of view jitter introduce changes in the optical path signal, inevitably generating tilt-to-length coupling noise. Relevant research indicates that the coordinate offset of the chief rays on the pupil plane will cause the TTL noise to exceed the expected level in the interferometer measurement system. While rarely mentioned in conventional optical systems, this system evidently provides a typical application for pupil aberrations. Specifically, the pupil aberration is the preferred option for evaluating telescope aberrations, understanding the requirements for optical path stability, and suppressing tilt-to-length coupling noise. Based on the theory of traditional imaging aberration and pupil aberration theory, the initial structure of the telescope is established, and the automatic correction of pupil aberration and image plane aberration is achieved through macro programming in the commercial optical software Zemax, enabling the design of a high-performance spaceborne telescope. The design results show that the pupil aberration of the system has been corrected, the RMS wavefront error of the scientific field of view is less than λ/200. The maximum value of tilt-to-length coupling noise within a ±300 μrad field of view is 0.0144 nm/µrad, meeting the requirements of the Tianqin mission. The introduction of the concept of pupil aberrations has led to a rapid convergence of TTL noise, clearly providing designers with a new perspective to address the original design issue. Moreover, the pupil aberration evaluation metrics mentioned in this paper can offer an alternative optimization target for other systems requiring pupil aberration correction. This could potentially evolve into a conventional tool in optical design in the future. We believe that our design approach can provide valuable guidance for other space-based gravitational wave detection projects and the design of similar optical systems for space telescopes.
Fraunhofer's diffraction model for transmitting Gaussian beam
Far-field wavefront. (a) The diffraction intensity distribution; (b) The diffraction phase distribution; (c) The generated phase with wavefront error of λ/40; (d) The diffraction phase distribution of the far field with wavefront error
Modeling of wavefront aberration and TTL coupling noise
Function of the chief ray and marginal ray in pupil imaging
Schematic diagram of paraxial ray tracing of the initial design
Optical layout of the TianQin telescope design
Wavefront error over the scientific field of view
Chief ray spot diagram in the exit pupil
Curve of slope of pathlength signal with tilt angle