Active alignment metrology for multi-channel photometric survey telescope

Abstract

The Multi-channel Photometric Survey Telescope (Mephisto) have a 1.6m primary mirror with 1.3 focal ratio, a 2degree field of view. Mephisto is capable of imaging the northern sky in three colors simultaneously. Its 0.7m secondary mirror is quite sensitive for alignment. The field dependent image quality of the misaligned Mephisto optics would be characterized as asymmetric, continuously varying and nonlinear with the misalignments. When the real-time field-PSF is measured from the astronomical images, the residual field-PSF of nominal optical design is defined as the goal of the merit function, the misaligned values of Mephisto secondary mirror can be calculated, by using the PSO (Particle swarm optimization) algorithm. The truth is field-PSF can be numerically expressed with ellipticity of the imaging stars or distribution of the ellipticity. However, for resolving the misalignment values, the stable field-PSF modes are needed, instead of random modes. PCA (Principal component analysis) algorithm is used for eliminating the random perturbations, such as turbulence.