Surface figure compensation of the primary mirror in a Ritchey–Chrétien space telescope by spring preloads

Shuqiang Jia; Shuqiang Jia; Wei Huang; Mingfei Xu; Xin Qin; Xin Qin

doi:10.1364/AO.477606

Applied Optics
Vol. 62,
Issue 3,
pp. 635-643
(2023)
•https://doi.org/10.1364/AO.477606

Surface figure compensation of the primary mirror in a Ritchey–Chrétien space telescope by spring preloads

Shuqiang Jia, Wei Huang, Mingfei Xu, and Xin Qin

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Shuqiang Jia, Wei Huang, Mingfei Xu, and Xin Qin, "Surface figure compensation of the primary mirror in a Ritchey–Chrétien space telescope by spring preloads," Appl. Opt. 62, 635-643 (2023)

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Abstract

To reduce the surface figure error induced by mechanical strains during the integration process of a high-precision mirror, a cost-efficient compensation method by spring preloads is proposed. The study is based on the primary mirror of a Ritchey–Chrétien space telescope with a focal length of 1200 mm. First, the surface figure degradation of the mirror during the assembly process is expressed and analyzed. Then, a finite element model of the mirror and its mounting structure is established, and surface deformations caused by different preloading forces are simulated. An optimized combination of different preloads was obtained through data fitting, and the influence of the combined preload on the mirror was analyzed. The simulation results show that ring preloads mainly affect spherical aberration and high-order spherical aberrations, while quadrupole preloads mainly affect astigmatism, and the optimized preload can compensate for the surface figure error from ${0.120}\lambda$ RMS to ${0.088}\lambda$ RMS. Last, the surface figure error of the mirror is measured by experiments under optimized preloads, and the result is ${0.084}\lambda$ RMS, which verifies the correctness of the analysis process and effectiveness of the compensation method.

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Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Part	Material	$D e n s i t y / (g \cdot {c m}^{- 3})$	Young’s Modulus E/MPa	Poisson’s Ratio	CTE
Primary mirror	Schott Zerodur	2.53	90,300	0.24	$0.5 \times 1 0^{- 7}$
Central hub	Super Invar(4J32)	8.13	148,000	0.29	$1.0 \times 1 0^{- 7}$
Mount	Titanium 6A14V	4.43	114,000	0.34	$8.8 \times 1 0^{- 6}$

	$0^{\circ} \sim 90^{\circ}$	$90^{\circ} \sim 180^{\circ}$	$180^{\circ} \sim 270^{\circ}$	$270^{\circ} \sim 0^{\circ}$
Ring preloads	$F_{R i n g}$
Quadrupole preloads	$F_{Q u a d}$	$- F_{Q u a d}$	$F_{Q u a d}$	$- F_{Q u a d}$

	6°–96°	96°–186°	186°–276°	276°–366°
Ring preloads	108 N
Quadrupole preloads	10.5 N	$- 10.5 N$	10.5 N	$- 10.5 N$
Combined preloads	37.5 N	16.5 N	37.5 N	16.5 N

Spring No.	1	2	3	4	5	6	7	8	9	10	11	12
Angle	21°	51°	81°	111°	141°	171°	201°	231°	261°	291°	321°	351°
Preload/N	12.5	12.5	12.5	5.5	5.5	5.5	12.5	12.5	12.5	5.5	5.5	5.5

Part	Material	$D e n s i t y / (g \cdot {c m}^{- 3})$	Young’s Modulus E/MPa	Poisson’s Ratio	CTE
Primary mirror	Schott Zerodur	2.53	90,300	0.24	$0.5 \times 1 0^{- 7}$
Central hub	Super Invar(4J32)	8.13	148,000	0.29	$1.0 \times 1 0^{- 7}$
Mount	Titanium 6A14V	4.43	114,000	0.34	$8.8 \times 1 0^{- 6}$

Abstract

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Applied Optics