Application of Ronchi test in rapid calibration of long effective focal length collimators

In this paper, we propose a method for rapid calibration of long effective focal length collimators by the Ronchi test. Long effective focal length collimators were often used to calibrate remote sensing optical payloads or telescopes. In order to accurately verify optical system’s collimation, the effective focal length of the collimator must be at least three times longer than the optical system under test. Traditionally, people usually used interferometer to calibrate collimator. However, this method could accurately detect the collimation of the optical system, but the inspection process was quite complex and required a high-precision 6-axis moving platform to recode the interferometer position. Therefore, we propose a method for rapid calibration of long effective focal length collimators by the Ronchi test. The Ronchi test module was composed of a light source, a Ronchi ruling and a camera. We would use the module to verify the alignment of an optical system with a focal length of 10,500mm and a f-number of approximately 14.3. If the distance between the primary mirror and the secondary mirror would be the same as the ideal optical path, the position of the focal plane didn't change. By measuring changes in focal plane position, collimation could be quickly confirmed and adjusted. According to experiment results, the measurement error of the focal plane of this module is approximately ± 55 um. The collimator of emission angle error less than 1.75E-4 mrad. Form the above experiment results, the module could perform collimation verification at lower cost and faster speed. In the future, we could further reduce the measurement error by adding multi-wavelength light sources, gratings of different period frequencies and analyzing various interference fringe characteristics.