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Freeform mirrors can be readily fabricated by a single point diamond turning (SPDT) machine. However, this machining process often leaves mid-frequency errors (MFEs) that generate undesirable diffraction effects and stray light. In this work, we propose a novel thin electroless nickel plating procedure to remove MFE on freeform surfaces. The proposed procedure has a distinct advantage over a typical thick plating method in that the machining process can be endlessly repeated until the designed mirror surface is obtained. This is of great importance because the sophisticated surface of a freeform mirror cannot be optimized by a typical SPDT machining process, which can be repeated only several times before the limited thickness of the nickel plating is consumed. We will also describe the baking process of a plated mirror to improve the hardness of the mirror surface, which is crucial for minimizing the degradation of that mirror surface that occurs during the polishing process. During the whole proposed process, the changes in surface figures and textures are monitored and cross checked by two different types of measurements, as well as by an interference pattern test. The experimental results indicate that the proposed thin electroless nickel plating procedure is very simple but powerful for removing MFEs on freeform mirror surfaces.

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We present the development of Linear Astigmatism Free -Three Mirror System (LAF-TMS). This is a prototype of an off-axis telescope that enables very wide field of view (FoV) infrared satellites that can observe Paschen-α emission, zodiacal light, integrated star light, and other infrared sources. It has the entrance pupil diameter of 150 mm, the focal length of 500 mm, and the FoV of 5.5°× 4.1°. LAF-TMS is an obscuration-free off-axis system with minimal out-of-field baffling and no optical support structure diffraction. This optical design is analytically optimized to remove linear astigmatism and to reduce high-order aberrations. Sensitivity analysis and Monte-Carlo simulation reveal that tilt errors are the most sensitive alignment parameters that allow ∼1 . Optomechanical structure accurately mounts aluminum mirrors, and withstands satellite-level vibration environments. LAF-TMS shows arXiv:2002.05361v1 [astro-ph.IM] 13 Feb 2020 Development of LAF-TMS 2 optical performance with 37 µm FWHM of the point source image satisfying Nyquist sampling requirements for typical 18 µm pixel Infrared array detectors. The surface figure errors of mirrors and scattered light from the tertiary mirror with 4.9 nm surface micro roughness may affect the measured point spread function (PSF). Optical tests successfully demonstrate constant optical performance over wide FoV, indicating that LAF-TMS suppresses linear astigmatism and high-order aberrations.