Elimination of the field-dependent aberrations of the JWST-like space telescopes in the multi-field fine-phasing process.

This paper investigates the alignment strategies for eliminating the field-dependent aberrations of the class of large three-mirror anastigmatic (TMA) space telescopes with a segmented primary mirror (PM) like the James Webb Space Telescope (JWST) in the multi-field fine-phasing process based on the framework of nodal aberration theory. During the single-field (on-axis field) fine-phasing process, the individual segment tip, tilt, and piston errors, as well as the de-space of the secondary mirror, are well corrected, and the PM is also adjusted to compensate for those aberrations induced by the misalignments of other mirrors at the center of the science field of view. However, interrogating off-axis field points can reveal the presence of large wavefront errors due to mirror misalignments. Eliminating these field-dependent aberrations is the main goal of the multi-field fine-phasing process. This paper first presents an analytic study on an established alignment strategy used for eliminating the field-dependent aberrations. While it is demonstrated that this alignment strategy has the ability to reduce the field dependency of the wavefront errors, it will, however, also be revealed that this strategy still exhibits some problems, and its alignment efficiency is low. Then, a new alignment strategy with higher alignment efficiency is further proposed. Detailed simulations with a TMA telescope that has similar parameters with the JWST are performed to illustrate the efficiency and rationality of the proposed strategy. This work can not only contribute to an in-depth understanding of the multi-field fine-phasing process, but also present a possibility to improve the efficiency of this process.