KEYWORDS: Space telescopes, Optical instrument design, Mirrors, Optical design, Telescopes, Aerospace engineering, Space robots, Space operations, Optical fabrication, James Webb Space Telescope
On-orbit Assembling Space Telescope (OAST) technology, which was firstly presented by NASA around early 2000s, is one of the most feasible paths to the implement of 10m-class aperture large scale space telescope. Unlike traditional monolithic aperture telescope (like HST) or deployable space telescope (like JWST), OAST is modularly designed, modularly manufactured and modularly launched, the telescope modules are assembled and co-phased to diffraction limit in spatial environment. Considering the development of OAST involves multiple interfaces and new subsystems, on-orbit assembling and aligning procedure, the top-level design methods and considerations are quite different from monolithic space telescopes. this article introduces and discusses several op-level design consideration including optical and mechanical structure, on-orbit assembling and aligning method, and on-orbit testbed developing method.
A systematic design method for three-mirror anastigmatic (TMA) telescopes with curved image surface is proposed in this paper. The initial structure parameters are solved analytically by the paraxial optical theory and Seidel aberration theory. On this basis, the design method is introduced. Two design examples for on-axis TMA telescope are conducted. One example is curved image surface for mitigate defocus during fine stabilization operations. The other example is curved image surface for configuration design with low misalignment sensitivities. The results demonstrate the feasibility of the design method.
Segmented mirror space telescopes have many advantages in both observation capacity and engineering feasibility. However, the alignment procedures for them are particularly complicated. Meanwhile, global alignment is one of the most important steps, in which the misalignments of each segment should be determined and corrected before image stacking is performed. Therefore, segment-level wavefront sensing is needed in this process. At present, traditional iterative phase retrieval algorithm is used to recover the segmented-level wavefront phase. However, the efficiency of this algorithm is comparatively low, especially given that there is an array of segment-level wavefront maps that need to be recovered. In addition, the magnitudes of misalignments are comparatively large in this stage and the iterative phase retrieval algorithm can be trapped in a local minimum for large-scale wavefront sensing. An analytic approach is proposed to estimate the segment-level wavefront aberrations based on the analysis of the geometrical features of one defocused point spread function (PSF) image. Meanwhile, some aberration properties of the misaligned system are also utilized. Simulations and an experiment are performed to verify the effectiveness of the proposed approach. This work can not only improve the efficiency and robustness of the global alignment of segmented mirror space telescopes, but also provide an intuitive and in-depth understanding for the mechanism of aberration calculation using PSF image features.
The aberration fields of misaligned on-axis telescopes can be described by nodal aberration theory. However, traditional nodal aberration theory cannot directly apply to pupil-offset off-axis systems. In our previous work, the net aberration fields of pupil-offset off-axis two-mirror astronomical telescopes induced by lateral misalignments were investigated by extending nodal aberration theory to include pupil-offset off-axis telescopes with a system-level pupil coordinate transformation through simulation. An experimental study on the net aberration fields of pupil-offset off-axis three-mirror anastigmatic (TMA) telescopes induced by lateral misalignments is further presented. Specifically, the astigmatism and coma aberration fields as well as their inherent relations are analytically expressed, simulated, and quantitatively validated with a real pupil-offset off-axis TMA telescope. Meanwhile, the differences between the aberration fields of misaligned off-axis and on-axis TMA telescopes are revealed and explicated. Our work not only contributes to a deep understanding of the net aberration fields of pupil-offset off-axis TMA telescopes induced by lateral misalignments but also represent an important validation for the extension of nodal aberration theory to pupil-offset off-axis telescopes.
Active optics usually uses the computation models based on numerical methods to correct misalignments and figure errors at present. These methods can hardly lead to any insight into the aberration field dependencies that arise in the presence of the misalignments. An analytical alignment model based on third-order nodal aberration theory is presented for this problem, which can be utilized to compute the primary mirror astigmatic figure error and misalignments for two-mirror telescopes. Alignment simulations are conducted for an R-C telescope based on this analytical alignment model. It is shown that in the absence of wavefront measurement errors, wavefront measurements at only two field points are enough, and the correction process can be completed with only one alignment action. In the presence of wavefront measurement errors, increasing the number of field points for wavefront measurements can enhance the robustness of the alignment model. Monte Carlo simulation shows that, when −2 mm ≤ linear misalignment ≤ 2 mm, −0.1 deg ≤ angular misalignment ≤ 0.1 deg, and −0.2 λ ≤ astigmatism figure error (expressed as fringe Zernike coefficients C5 / C6, λ = 632.8 nm) ≤0.2 λ, the misaligned systems can be corrected to be close to nominal state without wavefront testing error. In addition, the root mean square deviation of RMS wavefront error of all the misaligned samples after being corrected is linearly related to wavefront testing error.
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