Ho-Soon Yang

Chemical Vapor Deposition Silicon Carbide (CVD-SiC) is a suitable material for satellite-borne optical payloads, such as space observation mirrors, owing to its exceptional mechanical and chemical properties, superior optical characteristics, and robust resistance to space radiation. However, its inherent high hardness and brittleness present substantial machining challenges, necessitating systematic selection of processing parameters tailored to the specific functional objectives of each grinding process. This study aims to quantitatively derive optimal machining conditions across the process stages of Electrolytic In-Process Dressing (ELID) grinding for CVD-SiC mirrors. ELID grinding experiments were conducted through three sequential phases: coarse grinding, medium finishing, and finishing/ultra-finishing. Based on the surface roughness and material removal rates, a Multi-Criteria Decision Making (MCDM) evaluation utilizing the Faire Un Choix Adéquat (FUCA) method was performed. Hybrid weights were synthesized by applying the Bonferroni operator to weights derived from various techniques, including EQUAL, RS, ROC, and MEREC. The proposed approach determined the optimal machining conditions that align with the specific technical goals of each process stage. These findings suggest that the FUCA-based MCDM evaluation integrated with Bonferroni fusion weights offers a useful approach for identifying optimal CVD-SiC ELID grinding conditions among those tested.

We present a systematic interferometric qualification procedure for a meniscus null lens that discriminates surface figure errors from decentration-induced aberrations. The procedure combines two complementary full-aperture measurement configurations: a back-surface reflection test and a transmitted wavefront test. These tests exhibit different sensitivities to alignment-induced and figure-induced wavefront errors. By exploiting this difference, the aberration measured in each configuration can be decomposed into its individual contributions, enabling unambiguous identification of the aberration origin without additional optics or hardware. The methodology was applied to the fabrication of a meniscus lens serving as a null optic in a modified Simpson–Oland–Meckel test. For the specific lens geometry considered in this work, the theoretical and simulated sensitivity ratios are ≈3.19 and ≈0.11, respectively. Throughout the fabrication process, the back-surface reflection test was used as the primary reference for corrective figuring. The null lens was then used in the fabrication of the target convex aspheric mirror, which reached a final root-mean-square wavefront error of 11.4 nm. The proposed approach is practical, implementable with standard interferometric equipment, and broadly applicable to meniscus lenses used either as null optics in surface testing or as optical elements in imaging and telescope systems.

Indoor airflow is a major source of uncertainty in interferometric testing of large optics, yet quantitative criteria for identifying conditions that support repeatable measurements remain limited. We simultaneously measured the INTENSE-based effective Fried parameter ( r ₀ ) and interferometric root-mean-square wavefront error (RMS WFE) of a 1 m-class mirror under four airflow conditions involving forced airflow, local mixing, and no fan operation. For each condition, the primary analysis used ten 1-min measurements, with five additional 1-min measurements used for reproducibility assessment. Session-mean effective r ₀ alone did not consistently indicate repeatability: the condition with the largest mean effective r ₀ differed between sessions, whereas the combined forced-airflow and local-mixing condition exhibited the smallest observed repeat-measurement fluctuations in both effective r ₀ and RMS WFE. Normalized effective- r ₀ fluctuation was used as a scale-adjusted descriptor for intersession comparison, whereas absolute fluctuation showed a stronger exploratory association with RMS WFE fluctuation. Analyses of low-order Zernike modes and temporal power spectral density provided complementary modal and spectral information. Within the present facility and sampled time scales, these results suggest that the fluctuation of effective r ₀ may complement its mean value and the corresponding RMS WFE statistics when evaluating measurement repeatability.

Accurate slope measurement in large-aperture optical systems is limited by drift and reference optics. We present DSM-LT—a direct slope measurement technique that employs a 2 × 2 mirror or collimator array with retro-reflected beams. Hardware-based differential detection suppresses common-mode pointing errors without requiring software-based tilt compensation. Wavefronts are reconstructed using a zonal least-squares method, enabling precise localized feature recovery. DSM-LT supports flexible scan patterns and can seamlessly resume after scan interruptions. Simulation results confirm nanoradian-level slope accuracy and robustness against noise. A dual-detector configuration with polarization-separated beams mitigates turbulence effects. DSM-LT provides a compact, resilient, and scalable metrology solution for large-aperture and space-based telescopes.

Chemically vapor-deposited silicon carbide (CVD-SiC) is a high-performance material that possesses excellent mechanical, chemical, and electrical properties, making it highly promising for components in the semiconductor, aerospace, and automotive industries. However, its inherent hardness and brittleness present significant challenges to precision machining, thereby hindering the commercialization of reliable, high-precision parts. Therefore, the application of CVD-SiC in fields that require ultra-precision shaping and nanometric surface finishing necessitates the exploration of machining methods specifically tailored to the material’s unique characteristics. This paper presents a comprehensive review of CVD-SiC machining—from traditional mechanical approaches to advanced hybrid and high-energy techniques—aimed at overcoming machining limitations from its material properties and achieving high-efficiency and nanometric-quality machining. The study discusses various grinding tools designed for superior surface finishing and efficient material removal, as well as machining techniques that utilize micro-scale removal mechanisms for ductile regime machining. Looking ahead, the integration of AI-based process optimization with enhanced machining methods is expected to fully exploit the superior properties of CVD-SiC and broaden its industrial application as a high-performance material.

For accurate optical testing of large mirrors with long radii of curvature, multiple measurements using single-shot phaseshifting interferometers are typically required to reduce the impact of air turbulence on air stratification prevention. This paper proposes a new technique that utilizes hierarchical clustering to classify acquired wrapped phases into similar pattern clusters, eliminating the influence of unknown noises by excluding minority patterns. For each cluster, the circular mean is used to calculate denoised wrapped phases, and the surface figure is obtained from the unwrapped phases. Since the number of phase-unwrapping algorithm runs equals the number of chosen clusters, the proposed technique is significantly faster than conventional procedures. In 500 measurements on a 1.5 m diameter spherical mirror, the proposed technique reduced measurement time and the root-mean-square error's standard deviation of the surface figure by approximately 23% and 15%, respectively, compared to conventional procedures.

CAS500-1 (Compact Advanced Satellite 500-1) launched in Mar. 2021 is the first Korean ground observation satellite localizing the entire mirror development, especially the primary mirror M1. M1 is a lightweight concave mirror with a diameter of 0.6 m, and it has 3 square bosses at the rim for flexure mounting. We present the whole development procedure of the flight model M1, including the optomechanical design, fabrication, environmental tests, and a coordinate mapping method for alignment and integration.

We propose an astigmatism correction method for the subaperture stitching Hindle test to measure hyperbolic convex aspheres. Astigmatic wavefront errors arise from misaligned Hindle setups, mechanical runout errors of the rotational motion for stitching, and the surface error of the target itself. Because these errors are combined, they cannot be separated in the conventional subaperture stitching Hindle tests. We exploited the rotational periodicity of each error to distinguish the surface figure error from other astigmatic error sources and rectified the Hindle test results with a third-order astigmatism. Using the subaperture stitching Hindle test, we averaged two sets of measurement data with a 180° rotational phase difference between them to calculate the astigmatic surface error. The proposed method was verified experimentally by comparing it with the results from a commercial stitching interferometer from QED Technologies; only subnanometer differences in the root-mean-square values were obtained. Therefore, the proposed method calibrated the system errors from the test surface wedge and the rotational decenter easily, thereby reducing the mechanical costs and alignment efforts and making it more accessible than a sophisticated mechanism.

The main factors that limit the performance of ring laser gyroscopes include random walk, quantization noise, and bias stability. The first two factors determine the alignment time for navigation systems, and the bias stability error can result in long-term accuracy. We show that a phase wrapping/unwrapping algorithm, which has been widely used in interferometry fields, can be utilized to remove lock-in and quantization errors by implementing a zero-lock-in ring laser gyroscope. Furthermore, the performance of the ring laser gyroscope with no mechanical dither is described, and mirror dithering, instead of mechanical dithering, is proposed.

The Ritchey–Common test is a widely used method for testing flat mirrors with a larger reference spherical mirror. However, with the increase in the size of the flat mirror, the fabrication of the spherical mirror becomes more time consuming and expensive. In this study, we developed a novel technique to test a large flat mirror with a smaller reference sphere using a dual subaperture stitching (DSS) method. One part of the DSS technique is a modified Ritchey–Common test, which uses a reference sphere smaller than the flat mirror. The other part involves scanning along the centerline of the flat mirror. The former can be used to determine the surface form error (SFE), except for rotationally symmetric components, such as power and spherical aberrations, which can be measured by the latter. To perform the stitching process using a smaller reference sphere, the flat mirror was repeatedly rotated by a fixed angular step. One of the advantages of the rotation of the flat mirror is that it can be used to identify the errors resulting from the reference sphere, which do not vary during the rotation of the flat mirror. We verified the DSS method using a 152 mm diameter optical flat mirror and determined the root-mean-square (rms) measurement error to be only 0.2 nm, which was comparable to the error of the full-aperture interferometric measurement. In addition, we tested a 1.2 m diameter flat mirror with a reference sphere with an aperture of 0.8 m and measured its SFE to be 11.9 ± 0.5 n m rms.