Study On The Cryogenic Off-axis Reflective Collimator

With the continuous progress in space exploration technology, the Space Optical System is developing in the direction of high resolution. As the necessary instruments for space optical system testing, collimator has been made with large-aperture, large-field and high-quality. In this paper, a cryogenic off-axis reflective collimator is designed with Φ400 mm diameter,3200mm focal length working at 0.4um-14um wavelength. The technique for testing the core optical elements is studied. And the methods for interpreting the image quality of the system under the normal and cryogenic conditions are researched.According to the specifications of the collimator with field of view 1° x 1°, the off-axial Three-mirror Anastigmat(TMA) optical system is proposed, based on the geometrical optics theory and primary aberration theory. The method is proposed to obtain the initial structure of the on-axial three-mirror anstigment system by determining the distance between the mirrors. The off-axis optical system is optimized by Zemax software. The PV value of wavefront abberration is better than 0.046λ (λ is the wavelength), RMS value is better than 0.0037λ., the maximum spot radius of confusion is less than 1.1 um. All above turn out the system designed match the specifications.Since the collimator will work under the environment with the temperature lower than 150K, the stress of the supporting structure will change the relative position of three mirrors. The relative changes between the mirrors are calculated and analyzed, and the result show that the errors are within the range of tolerance when the Invar is used, the system designed can match the requirement under the cryogenic condition at 150K.According to the surface parameters of the primary mirror and third mirror, the testing method are proposed with compensators and plate mirrors. The corresponding compensators are degisned by Zemax respectively, and the dynamic interferometer is used to test the primary mirror and third mirror with auto-collimation method. Finally, the RMS values are less than λ/40, that match the design requirements of the single mirror.