Study of CCD Mosaicking Technique Based on Error Accumulation Model for WFST

The prime camera with a mosaic charge-coupled device (CCD) array is the imaging terminal of the wide field survey telescope (WFST), and the flatness of its focal plane array (FPA) is one of the critical indicators that determine the imaging quality of the telescope. The FPA works at
$- 100~^{\circ }$
C in a vacuum Dewar to suppress the dark current of CCDs. A large-size FPA of high flatness is hard to realize due to the influence of multiple factors, such as geometric tolerance, force, and temperature deformation. To address this issue, a CCD mosaicking technique based on an error accumulation model is proposed. First, the error source of FPA is analyzed with high-precision measurement and finite element simulation. Second, an assembly deviation simulation calculation model is established based on the actual contact surface morphology characteristics, and the error transfer process is analyzed by combining the small displacement torsor (SDT) and homogeneous transformation matrix (HTM). Then, the effects of force and temperature deformation on each CCD are superimposed independently by HTM, and the feasibility is verified by finite element simulation. Finally, the mosaic FPA of the prime focus camera (PFC) is completed based on this model. Comparing the simulated and measured results, the flatness of the FPA at room temperature reaches a PV of
$15.53~\mu $
m. The confidence interval for the flatness of the FPA is determined as [13.48, 17.60]
$\mu $
m, with a confidence level of 95%. Also, upon comparing the distribution of CCDs between measurement and simulated assembly, it is evident that only a subset of CCDs shows a height deviation of
$0.8~\mu $
m, thus affirming the effectiveness of the simulated assembly process. The FPA suffers a significant bow-shaped deformation at low temperatures, and 99.18% of the measured points are within
$16.30~\mu $
m.