The dominant free space optical (FSO) system adopts an optical fiber as a light transfer unit for transmitting or receiving of communication or beacon signals. The optical fiber is usually placed in a pinhole fiber connector which prevents direct measurement of optical instruments. Traditional test method fails to guide the accurate alignment of an FSO system. Here we propose a new method for point spread function (PSF) modeling by fiber coupling efficiency measurement. First we show the convolution effect of a multimode fiber with a numerical simulation using a focal spot with uniform irradiation. The coupling efficiency map versus the lateral translation has a flattop at the center and drops to zero at the edge, by which the focal spot diameter can be determined. A further simulation shows that the beam profile of a focal spot with rotational symmetry can be derived from coupling efficiency map by deconvolution. We build the mathematical model of the deconvolution method and recover the beam profile with simulated data. With proper modeling and data smoothing, the PSF of a FSO system is recovered with great consistency to the simulation data. The recovered profile can be used for guidance with the alignment of the system. Although the simulated data is rotationally symmetric, the deconvolution method can be improved in the future to be compatible with focal spot with arbitrary beam profile. The method can also be useful in applications such as laser beam profiling, online system testing, phase retrieval and so on.
A large-aperture, wide field-of-view, three-mirror optical system for a space borne astronomical survey telescope has been designed. The unobstructed optical system with a circular pupil has a 2 meter aperture, 1.7 deg<sup>2</sup> FOV, and provides remarkably good imagery. Freeform surfaces and decentered/tilt surfaces are introduced to the system, which have the advantage of balancing the unsymmetrical aberrations, especially for the wide-field off-axis optical systems. The evaluation of image quality is that the value of the RMS wavefront error is 21 nm, the ellipticity is less than 6.5% in the main imaging area (about 1.1 deg<sup>2</sup>), and the maximum radius of Encircled Energy 80% (EE80) is less than 0.09arcsec.