In this paper, analysis of variance on designed experiments with full factorial design was applied to determine the optimized machining parameters for ultra-precision fabrication of the secondary aspheric mirror, which is one of the key elements of the space cryogenic infrared optics. A single point diamond turning machine (SPDTM, Nanotech 4μpL; Moore) was adopted to fabricate the material, AL6061-T6, and the three machining parameters of cutting speed, feed rate and depth of cut were selected. With several randomly assigned experimental conditions, surface roughness of each condition was measured by a non-contact optical profiler (NT2000; Vecco). As a result of analysis using Minitab, the optimum cutting condition was determined as following; cutting speed: 122 m/min, feed rate: 3 mm/min and depth of cut: 1 μm. Finally, a 120 mm diameter aspheric secondary mirror was attached to a particularly designed jig by using mixture of paraffin and wax and successfully fabricated under the optimum machining parameters. The profile of machined surface was measured by a high-accuracy 3-D profilometer(UA3P; Panasonic) and we obtained the geometrical errors of 30.6 nm(RMS) and 262.4 nm(PV), which satisfy the requirements of the space cryogenic infrared optics.
Infra-Red (IR) objective achieves a few micrometers of spatial resolution with high Numerical Aperture (NA) of about 0.75, for example, in mid-IR. However, submicron resolution is hard to achieve in Mid-IR because of the long wavelength compared to the visible range. To overcome the limitation, a solid immersion lens (SIL) is incorporated into the conventional objective so that the high refractive index of SIL contributes to obtain the high spatial resolution image of sample immersed in SIL. Germanium is a typical material of SIL in the infrared wavelengths because of the high refractive index and the high transmittance. In our study, we fabricated a Germanium-SIL using the quantified parameters of the ultra precision machining. The parameters are tool rake angle, cutting speed, feed rate, and depth of cut. The surface shape of the fabricated SIL was measured with the accuracy of 0.0376 μm in RMS and 0.3159 μm in P-V. We applied the fabricated SIL to a custom IR objective to investigate the improvement of its spatial resolution. Optical performance of the IR objective was evaluated with and without SIL. As results, the IR objective with SIL achieved 1.23 μm of the spatial resolution from the 3.9 μm of IR objective without SIL