Objective In recent years, as ground-based telescopes continue to advance toward larger apertures, wider fields of view, and higher imaging quality, the use of large-aperture corrector has become increasingly common. The MUST (MUltiplexed Survey Telescope) proposed by Tsinghua University is equipped with a wide field corrector (WFC), which consists of five lenses with a maximum aperture of 1.6 m. The WFC ranks among the most advanced internationally in terms of both size and complexity. To ensure the imaging quality of the telescope system, the alignment requirements for the WFC are extremely stringent: the decentering must be better than 20 µm and the tilt must be better than 9″. These demands pose significant challenges for optical alignment of the WFC. Currently, there is no centering instrument of such large aperture and high precision to guide the optical alignment of WFC of this scale. Therefore, the characteristics of MUST corrector is analyzed and an ultra-large-aperture centering instrument is designed in this paper.

Methods To meet the system precision requirements, a centering instrument based on reflective eccentricity is adopted in this paper (Fig.2). A phased alignment process based on this large-aperture centering instrument is proposed, which decouples the alignment degrees of freedom for individual lens, significantly reducing the complexity of system alignment (Fig.3). To fulfill the high-precision alignment demands of the WFC, the contributions of runout of turntable and straightness of guideway to measurement uncertainty are analyzed. In response to the large decentration caused by tilted mirrors with long radius of curvature, the requirements for the field of view of centering instrument are examined, and an alignment method for wedge prism pairs is proposed (Fig.5). Additionally, a selection strategy for a fixed-focus converging lens suitable for full-process imaging is presented (Tab.3).

Results and Discussions An air-bearing turntable with aperture of 2 m is selected, with main error source of runout. Considering the difficulty of regrinding, flatness of the turntable should be better than 20  μm, while radial and axial runouts are required to be better than 0.5  μm each. A precision linear guide with a 3-meter range is chosen, which requires straightness better than 10  μm over its full travel range. The alignment process involves up to 22 different focal lengths of converging lenses. During actual alignment, adjusting the object distance via the guide allows clear imaging within a certain range, thereby reducing the number of lenses to 10 (Tab.3). L3-B and L4-F are tilted spherical surfaces, which inducing decentrations of 29.24 mm and 21.49 mm, respectively. An industrial camera with a sensor size of 8.44 mm×7.06 mm is chosen to meet both the field-of-view and imaging quality. To ensure proper relative rotation of these wedge lenses, an alignment method is designed to mark the orientation of the tilt (Fig.5).

Conclusions To meet the high-precision alignment requirements of the ultra-large WFC of MUST, this study investigates the measurement principles of reflective eccentricity and proposes a phased alignment process. This study provides specific engineering implementation recommendations to ensure that the measurement uncertainty of a 2-meter-class centering instrument remains better than 2 μm. This research forms a complete technical solution encompassing theoretical modeling, alignment processes, and error control, offering an implementable theoretical and engineering framework for the high-precision alignment of the MUST WFC. It demonstrates practical engineering application value and broader significance for potential adoption in related fields.