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Abstract
Although a reflective optical system can theoretically realize ideal optical designs, it is not always the optimal choice compared with a refractive one because of the difficulty in achieving high wavefront accuracy. A promising solution is to build reflective optical systems by mechanically assembling all the optical and structural components made of cordierite, which is a ceramic with a very low thermal expansion coefficient. Interferometric evaluations of an experimental product demonstrated that diffraction-limited performance in the visible wavelength was achieved and maintained even after cooling to 80 K. This new technique may be the most cost-effective method for utilizing reflective optical systems, especially for cryogenic applications.
© 2023 Optica Publishing Group
1. INTRODUCTION
Reflective optical systems have many advantages over refractive ones. These advantages include no chromatic aberration, wide wavelength coverage obtained with metal coatings, compact optical layouts realized by folded optical path designs, high-order aberration correction using nonspherical (freeform) surfaces, availability of large (${\gt}{1}\;{\rm m}$) optical systems, and athermal optics employing the same material for optical and structural components.
Although a reflective optical system with these advantages seems superior to a refractive optical system in terms of realizing ideal optical designs, it is not always the optimal choice for demanding applications, for example, recent optical/infrared astronomical instruments, which typically demand diffraction-limited performance over a wide wavelength range and wide field of view under various ambient-temperature conditions. Refractive optical systems are frequently used for such demanding applications, even with the unavoidable disadvantage of chromatic aberration, because they can achieve high wavefront accuracy with limited development time and costs. For example, assuming that the refractive index of a lens material is 1.5, the requirements of surface accuracy, surface roughness, and alignment precision for refractive optics are 4 times relaxed compared with those for reflective optics. In addition, the coaxial shape of the lens allows us to adjust the alignment of refractive optics with high accuracy.
One idea to change this tendency is to build reflective optical systems by mechanically assembling all components (i.e., mirror substrates, mirror mounts, and optical benches) that are made of the same material and are shaped with high dimensional precision. Using advanced high-precision multi-axial machining processes, surfaces that act as references for alignment can be precisely processed to all parts. By assembling such parts mechanically based on reference surfaces, precise alignment can be realized without the traditional (bothering) iterative adjustment based on optical light-based inspections. Because the assembled reflective optics have no mismatch in the coefficient of thermal expansion (CTE) among the parts, efforts against temperature variation can be significantly reduced. The feasibility of this idea strongly depends on the material selection. Optical glasses (including glass ceramics), which are widely used as mirror substrates, are inapplicable because they are unsuitable structural materials and are difficult to shape with sufficient precision owing to their brittleness. Metals have sufficient machinability. However, metal mirrors exhibit surface scattering problems originating from the machining process of single-point diamond turning [1], which is a common technique for fabricating metal mirrors. In addition, metal optics are subject to thermal stability problems because their CTEs are high and internal stress can produce unpredictable or irreversible deformations when the ambient temperature varies [2]. Although aluminum alloys are considered to be favorable for the development of single-material athermalized systems, these problems limit their applications [1]. Silicon carbide (SiC) and its related materials (e.g., Si-SiC), which are types of ceramics, have also been used to develop single-material athermalized systems. These materials offer a smoother mirror surface and better thermal stability than aluminum alloys [1,3]. However, SiC materials have disadvantages in terms of manufacturability. Because bare mirror blanks are porous, they require a chemical vapor deposition (CVD) cladding process prior to polishing [4]. In addition, SiC materials require considerable time to polish or machine after sintering [5]. These problems lead to high development costs. Consequently, the applications would be limited to well-funded space projects. If such conventional materials are used, it is difficult to break away from the current unfavorable tendency.
Table 1. CTE and Physical Properties of Typical Materials Utilized for Mirrors for Astronomical Instruments
An attractive material for realizing this idea, that is, mechanically assembled single-material reflective optical systems, is cordierite [6], which is a ceramic material with a very low CTE. Ceramics have attracted attention as mirror substrate materials because of their moderate hardness for polishing. However, the application of ceramics has been hindered owing to their porous properties. Internal voids appear on the optical surface as significant digs after polishing. Recently, the elimination of voids via the hot isostatic pressing (HIP) process has progressed, and some commercial cordierite ceramics produced by the HIP process, such as CO-720 (KYOCERA Corporation) and NEXCERA (Kurosaki Harima Corporation), have offered polished surfaces with high optical quality comparable to that of glass mirrors [7,8]. Cordierite, originally used as a structural material, is available for mirror mounts and optical benches. Because ceramics, including cordierite, can be shaped via near-net shaping in soft states, similar to clay works, before sintering, they can form a complex 3D shape more easily than metal or glass materials. In addition, cordierite parts can be shaped as precisely as typical metal parts via post-sintering finish processing (the machined volume is minimized by the prior near-net-shaping process). The assembled all-cordierite reflective optical system exhibits an athermal property owing to its single-material structure as well as extreme thermal stability owing to its very low CTE, which is as low as zero thermal expansion glass/glass ceramics, for example, ULE, Zerodur, and Clearceram (Table 1). The ability of ceramics to form complex 3D shapes ultimately enables the fabrication of monolithic (i.e., seamless) optical systems.
All-cordierite reflective optical systems are expected to be widely used as the most cost-effective method for developing innovative optical systems that exploit the advantages of reflective optical systems. This represents a significant breakthrough in cryogenic optical systems for infrared and space instruments. To demonstrate the applicability of this idea to infrared astronomy, we developed an all-cordierite experimental optical system comprising three spherical mirrors. To the best of our knowledge, this is the first attempt to fabricate an entire optical system using only cordierite. Previous studies have applied cordierite to a part of an optical system, for example, mirrors, an optical bench, a large grating holder that utilizes the high thermal stability of cordierite [15,16], and a large lens barrel that utilizes both the high thermal stability and high specific stiffness of cordierite [17]. These previous applications were intended for use at around room temperature. In this paper, we report the optical performance of this experimental optical system at cryogenic temperatures in the visible wavelength range and discuss the feasibility and prospects of the proposed all-cordierite optical system for cryogenic applications.
2. EXPERIMENTAL OPTICAL SYSTEM
A. Offner Design
We adopted the Offner design [18] as the experimental optical system to evaluate the applicability of the all-cordierite reflective optical system (Fig. 1). An Offner optical system is a unit magnification reimaging system comprising two concentric spherical surfaces, a concave primary mirror, and a convex secondary mirror (curvature radius ratio of 2:1). The optical system is symmetric relative to the secondary mirror, which plays the role of the aperture, causing the system to have a slight astigmatic aberration but nearly no off-axial aberration. The optical system exhibiting diffraction-limited imaging performance with such simple configurations is widely used as relay optics in astronomical instruments. If any discrepancies are found in the optical performance between the design and evaluated results, their causes can be easily investigated. The primary mirror usually comprises a single mirror (the beam is reflected twice by the primary mirror, as shown in Fig. 1) to reduce the degree of freedom of alignment. However, for testing purposes, we divided the primary mirror into two to increase the chance of observing alignment errors caused by alignment work and cooling.
B. Fabricated System
Figure 2 shows the fabricated experimental optical system. The primary mirrors have a curvature radius (R) of 200 mm and an effective diameter ($\varphi$) of 36 mm, while the secondary mirror has ${\rm R} = {100}\;{\rm mm}$ and $\varphi = {15}\;{\rm mm}$. The mirrors constitute an optical system with an aperture ratio of F/8 and an effective field of view of 10 mm. The mirrors are made of cordierite CO720, and their polished surfaces are coated with Au. The surface accuracy and surface roughness of both mirrors are less than $\lambda /{8}$ PV ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$) and a few nm, respectively. Specifically, cordierite mirrors realize optical performance comparable to that of glass mirrors. The structural components (i.e., mounts and optical bench) are made of cordierite CO220 (Kyocera). Note that CO720 and CO220 have the same CTE value but different porosities (populations of voids), depending on whether the HIP process is performed. In CO720, voids are sufficiently reduced via the HIP process; thus, they can be used as mirror substrates. In contrast, CO220 is produced without HIP. Thus, CO220 is a low-cost material that is available in larger sizes than CO720; thus, it is suitable for structural components. The cordierite parts are joined with invar screws, whose CTE (${0.8} - {2.5} \times {{10}^{- 6}}\;{{\rm K}^{- 1}}$ at temperatures of 80−300 K [19]) is as low as that of cordierite. Note that the screw holes tapped in the cordierite parts are reinforced with helical coil inserts made of SUS owing to the material availability. The torques of the screws are carefully controlled to prevent the fracture of the screw holes.
Generally, an optical system is assembled by iteratively adjusting the alignments of its components based on inspections of the image or wavefront, which are generated by inputting controlled lights, for example, lasers. With the experimental optical system, all components to which the reference surfaces were processed (Section 1) were assembled mechanically using only gauges and jigs according to the following procedures. First, blocks acting as references of positions were set on the optical bench and aligned with a tolerance of $\pm {10}\;\unicode{x00B5}{\rm m}$ by monitoring their positions using a contact-type 3D measuring machine. Then, mirror units (mirrors integrated to mounts) were set such that their reference surfaces contacted those on the blocks. Finally, the positions of the mirror units were measured using a contact-type 3D measuring machine to confirm that the mirror surfaces had an alignment tolerance of $\pm {30}\;\unicode{x00B5}{\rm m}$, as expected. In a brief optical inspection, we observed the point spread function (PSF) using a CCD camera set on the focal plane by inputting an F/8 beam to the experimental optical system, where the beam was generated using a focusing collimated laser beam ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$) through an achromatic lens ($\varphi = {50}\;{\rm mm}$, ${f} = {250}\;{\rm mm}$) with a fully corrected spherical aberration. The observed PSFs indicated that the experimental optical system exhibited imaging performance at a diffraction-limited level throughout the field of view.
3. WAVEFRONT ACCURACY
A. Measurement at As-Built Condition
To measure the wavefront accuracy of the experimental optical system interferometrically at the visible wavelength ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$), a convex mirror with a curvature radius of 100 mm (surface irregularity ${\lt}\lambda /{20}$) was attached to the system at a position 100 mm off-focused from the image surface (Fig. 3). The convex mirror was custom-made using CO720. The beam entering the optical system is reflected back by the convex mirror to retrace the optical path (Fig. 3). Figure 4 shows the wavefront error map measured using a laser interferometer (Zygo). As can be observed, diffraction-limited optical performance (${\rm PV} \lt \lambda /{4}$, ${\rm rms} \lt \lambda /{14}$) is achieved at this wavelength, and this result demonstrates that precise alignment can be realized using only mechanical assembly.
Fig. 3. Convex mirror (non-coated) attached to measure the wavefront accuracy of the optical system interferometrically. The C-mount (Fig. 2) was detached for this configuration. Note that the attached convex mirror has a chip (lower right) caused by an accidental fault in the fabrication process.
Fig. 4. Wavefront error map of the experimental optical system measured in the as-built condition. The map was measured using a laser interferometer (Zygo) emitting an F/3.3 beam ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$) through a transmission sphere. The emitted beam directly entered the experimental optical system. The figure shows an area covered by an F/8 beam with masking the edge due to chipping of the attached convex mirror (Fig. 3).
B. Measurement at Cryogenic Temperature
To measure the wavefront accuracy at cryogenic temperatures, the experimental optical system with the attached convex mirror was installed in a liquid nitrogen (${{\rm LN}_2}$) cryostat, which was set on an isolation vibration table with a laser interferometer (Zygo) emitting an F/7.1 beam ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$) through a transmission sphere (Fig. 5). Here, the direction of the emitted beam was adjusted to coincide with the optical axis of the experimental optical system using two flat mirrors installed in kinematic holders placed on the optical path between the laser interferometer and cryostat. The distance of the optical path was adjusted such that the focus of the emitted beam coincided with the focal plane of the experimental optical system, using linear motion guides with a linear positioning stage attached to the cryostat mount. These adjustments were performed by monitoring the interference fringe produced by the beam reflected from a thin flat glass set at the focus of the experimental optical system. The above coincidences were confirmed by the fringe being the null pattern.
Fig. 5. Experimental configuration used to measure wavefront error at cryogenic temperatures. The top-left insert shows an observed fringe pattern. The bright circular pattern in the fringe comes from the cryostat window. The lack of the fringe image (top-left side) corresponds to the chipped area of the convex mirror attached for interferometric measurement (Fig. 3).
In the cryostat, the experimental optical system was mounted on a cold work surface (CWS) via an interface plate made of oxygen-free copper (Fig. 5). The optical bench was fixed to the interface plate using a single screw to prevent deformation owing to the CTE mismatch between them. The fixing position was set just below the intersection point between the optical axis and focal plane of the experimental optical system because this position can minimize changes to the optical layout relative to the interferometer pre- and post-cooling. To prevent a rotational shift of the experimental optical system, two pins set on the backside of the optical bench were inserted into slotted holes bored in the interface plate. The longer direction of the slotted holes aligns lines that pass the fixing position (and are orthogonal to each other) to ensure that the optical bench expands/shrinks relative to the fixing position. To cool the experimental optical system efficiently, the optical bench was connected to the CWS using two thermal straps, each comprising six pieces of a copper plate with a thickness of 0.3 mm.
Fig. 6. Temperature change of experimental optical system (optical bench and mount of the attached) after cooling began. The arrows show the timing of wavefront error measurements.
Fig. 7. Top, wavefront error maps ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$) of the experimental optical system measured at temperatures of 300 K (before cooling), 80 K (the first measurement at 80 K, see Fig. 6), 200 K, and 300 K (after cooling). The maps are in time order from left to right. The portions of the bright fringe pattern due to the cryostat window (Fig. 5) and the edge due to the chipping of the attached convex mirror (Fig. 3) are masked. The maps include wavefront errors due to the interferometer, fold mirrors, and cryostat window. Ideally, the leftmost map should be equivalent to Fig. 4. The difference in appearances is due to the fold mirrors and cryostat window. Bottom, difference of the wavefront maps relative to the map before cooling (initial condition). The sequential maps show a continuous change, i.e., an astigmatic pattern appears and increases in the difference maps. Note that horizontal stripe patterns found in the 80 K and 200 K maps are due to air fluctuations.
Wavefront errors of the experimental optical system in the cryostat were measured before, during, and after cooling with liquid nitrogen. For reference, the wavefront errors were measured in the same configuration, but without the cryostat window before and after cooling (the corresponding data are discussed in Section 4.A). The temperature change of the experimental optical system from the start of cooling to room temperature is shown in Fig. 6. Liquid nitrogen was supplied continuously until approximately one day after the experimental optical system had cooled completely; subsequently, no temperature control was performed until the system passively returned to room temperature. Although the thermal conductivity of cordierite is not as high as that of metal materials (Table 1), this cooling demonstrates that cordierite optical systems can be cooled in a reasonable time for practical application.
Figure 7 shows the wavefront error maps measured before cooling (300 K), under cryogenic conditions (80 K), during the warming process (200 K), and after warming (300 K). Note that the wavefront maps include wavefront errors owing to the interferometer, fold mirrors, and cryostat window. The measured maps show that the diffraction-limited optical performance at the visible wavelength was maintained at cryogenic temperature. As expected, these results demonstrate that all-cordierite optical systems offer excellent thermal stability.
4. DISCUSSION
A. Thermal Cycle Stability
Although the wavefront errors measured in the first thermal cycle were within a significantly small range for optical systems of typical astronomical instruments, they demonstrated a continuous change, that is, an increasing astigmatic pattern in their difference maps relative to the initial condition (Fig. 7). Whether the pattern increases as the thermal cycles repeat is a concern for practical applications because cryogenic instruments undergo thermal cycles for tests and maintenance. To address this issue, we made the experimental optical system undergo an additional three thermal cycles (Fig. 8) and then measured the wavefront error with the same configuration as the first measurement (Section 3.A). To evaluate the change in wavefront error owing to the additional three thermal cycles, we generated a difference map between the wavefront error obtained by this measurement and that before the first thermal cycle (Fig. 4). Figure 9 shows the difference map and compares it with the difference map obtained before and after the first thermal cycle (Section 3.B), where data obtained without the cryostat window were used. If any changes to the wavefront error occur with the additional three thermal cycles, they should be found in the difference between the difference maps. Ignoring the stripe patterns, which are owing to air fluctuations, the two maps are remarkably similar in appearance, which indicates that no clear change occurred with additional thermal cycles. Note that the PV and rms of the upper rightmost map in Fig. 7 are greater than those shown in Fig. 9(a1), which indicates that the wavefront error of the former map includes some biases owing to the fold mirrors and cryostat window. The actual wavefront error of the upper-rightmost map in Fig. 7 can be comparable to that in Fig. 9(a1) owing to the similarity between Figs. 9(a3) and 9(b3).
Fig. 8. Temperature change of the optical bench of the experimental optical system during additional thermal cycles (solid blue line). The temperature change during the first thermal cycle (Fig. 6) is also plotted for comparison (dashed–dotted black line). The CWS was warmed actively by a heater in the warming processes to reduce warming time. The arrows indicate the start of active warmings.
Fig. 9. Comparison of the differences in wavefront error maps ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$): (a1) after additional three thermal cycles; (a2) before the first thermal cycle (same as Fig. 4); (a3) difference between (a1) and (a2); (b1) after first thermal cycle; (b2) before the first thermal cycle; (b3) difference between (b1) and (b2). Note that (a1) and (a2) were taken under the configuration described in the caption of Fig. 4, and (b1) and (b2) were taken under the configuration shown in Fig. 5 without the cryostat window. The different configurations for measurements resulted in different map resolutions in (a) and (b). (b1)–(b3) should be equivalent to the upper rightmost, the upper leftmost, and bottom-right maps in Fig. 7, respectively. The slight differences in their appearances are due to air fluctuations and the cryostat window. The edges due to the chipping of the attached convex mirror are masked, as in Figs. 4 and 7. The masks near the centers of (a1), (a3), (b1), and (b3) exclude spike features due to some kind of deposit, e.g., dust. The horizontal stripe patterns in (b3) are due to air fluctuations. If the air fluctuation patterns are excluded, the difference between (a3) and (b3) represents change that occurred with the additional thermal cycles.
The reason why the wavefront error exhibited small changes only in the first thermal cycle can be interpreted by the alignment change of some optical components owing to irreversible changes in the conditions of the screw joints. The screw holes tapped in the cordierite parts were reinforced using helical coil inserts made of SUS. In the first cooling process, a tapped hole slightly expanded depending on the negative CTE of cordierite at low temperatures while a helical coil insert extended to fit the hole because of its elastic behavior as a spring. In the subsequent warming process, the expanding tapped hole would shrink to the initial size; however, the shrinkage would be resisted by the extending helical coil insert and would stop when its force balanced with the resisting force of the helical coil insert. This would result in a change in the screw joint condition before and after the initial cooling. This interpretation is supported by another experiment in which we cooled a mock part replicating the screw joint with metric M4 screws. For comparison, the mock part had screw holes with and without helical coil inserts. An inspection of the mock part after soaking in liquid nitrogen demonstrated that submicrometer swelling features occurred only on the lateral surfaces around the screw holes with helical coil inserts. The distance between the lateral surfaces and centers of the screw holes was 5 mm. Such irreversible changes in the screw joint conditions would not occur in the second and subsequent thermal cycles, which accounts for the observed wavefront error map not changing after additional thermal cycles.
Our experimental results and interpretations imply that the wavefront error will remain at a significantly low level with further thermal cycling; thus, the proposed all-cordierite optical system is sufficient for application in cryogenic instruments. In principle, screw joints can be replaced with seamless structures by exploiting the ability of ceramics to form complex 3D shapes; thus, even such small alignment changes owing to screw joints can be potentially eliminated.
B. Prospect Application in Cryogenic Optical Systems for Infrared Astronomy
In practice, an all-cordierite reflective optical system will significantly change the way optical systems are used in visible and infrared astronomical instruments. The proposed system will offer a significant breakthrough in cryogenic optical systems for infrared instruments, which are operated at cryogenic temperatures, and space instruments, which suffer from significant post-launch temperature changes. Note that cordierite has two additional advantages (compared with very low CTE glass materials) other than high-precision shaping and exceptionally low CTE. One advantage is high specific stiffness (Table 1), which benefits instruments with low weight requirements. The other advantage is high long-term stability [20], which benefits instruments that must support long-term operations, for example, journeys to the outer solar system. However, these details are omitted here because they are beyond the scope of this paper. The primary goal of this study was to apply an all-cordierite reflective optical system to cryogenic infrared instruments for ground-based telescopes. In recent ground-based observations with the largest class of optical telescopes (8–10 m in diameter), diffraction-limited images have been acquired at shorter wavelengths (e.g., down to near-infrared), owing to significant advances in adaptive optics (AO) technologies. In addition, a wider field of view (i.e., a wider band for spectroscopy) has been realized through the widespread use of large-format infrared arrays with high sensitivity. However, the opportunities resulting from such advancing technologies have not been fully utilized with present instruments that use conventional optical systems. We expect that this limitation will be improved by the proposed all-cordierite optical system, which can realize instruments that combine high wavefront accuracy, wide field of view, and high sensitivity. This optical system is expected to be widely used as a standard technique in future infrared instruments, as well as to increase the feasibility of planned large and complex optical systems and enable more challenging systems for next-generation extremely large optical telescopes (${\sim}{30}\;{\rm m}$ in diameter). As a first step, we have been developing a near-infrared high-resolution spectrograph that uses an all-cordierite optical system at the Laboratory of Infrared High-resolution spectroscopy (LiH) at the Koyama Astronomical Observatory, Kyoto Sangyo University [21].
5. CONCLUSION
In this study, to demonstrate the applicability of a mechanically assembled all-cordierite (a ceramic material with a very low CTE) reflective optical system in cryogenic optical systems to extend the capabilities of infrared astronomical instruments, we fabricated an experimental optical system with the following characteristics.
- (1) The experimental system included an Offner-type layout (F/8) comprising three isolated spherical mirrors, where the Offner primary mirror (curvature radius of 200 mm) was divided into two mirrors.
- (2) All components (mirror substrates, mounts, and optical bench) were made of cordierite CO720 or CO220 (KYOCERA Corporation). The cordierite parts were joined with invar screws, and the screw holes tapped in the cordierite parts were reinforced with helical coil inserts made of SUS.
- (3) All components were mechanically assembled using only gauges and jigs. Traditional iterative alignment adjustments using light were not performed.
By measuring the wavefront error of the experimental optical system with a laser interferometer ($\lambda = {0.633}\;\unicode{x00B5}{\rm m}$) under the as-built condition, we found that diffraction-limited optical performance can be achieved (the wavefront error was ${0.154}\;\lambda$ in PV and ${0.022}\;\lambda$ in rms) in the visible wavelength range using only mechanical assembly. In addition, through cryogenic measurements of the wavefront error of the experimental optical system in a liquid-nitrogen cryostat, we found that the diffraction-limited optical performance was maintained even after cooling to 80 K. The wavefront error exhibited a continuous change during the first thermal cycle (an astigmatic pattern increased in the difference maps relative to the initial condition); however, no further change was observed after additional three thermal cycles with cooling to 80 K. The behavior of the wavefront error can be explained by the interpretation that the alignment of the optical components was altered slightly as the conditions of their screw joints changed irreversibly in the initial thermal cycle. This interpretation is supported by the results of a cooling experiment conducted with a mock part replicating the screw joint. Therefore, the wavefront error can be maintained at a significantly low level with further thermal cycles; thus, the all-cordierite optical system is expected to exhibit high wavefront accuracy in practical applications in cryogenic instruments that undergo thermal cycles for tests or maintenance. Our experimental results demonstrate that the proposed all-cordierite optical system can easily achieve high wavefront accuracy at cryogenic temperatures because of the combination of precise alignment via mechanical assembly alone and excellent thermal stability, which cannot be realized by conventional optical materials. An all-cordierite optical system is a cost-effective way to realize high-performance optical systems that exploit the advantages of reflective optical systems. In particular, we expect the proposed system to provide a significant breakthrough in cryogenic optical systems for infrared and space instruments. This new technique still has room for improvement. We can further exploit cordierite’s ability to form complex 3D shapes to realize seamless structures (ultimately monolithic optical systems), which is expected to eliminate screw joints, assembly work, and subsequent alignment errors.
Funding
MEXT Supported Program for the Strategic Research Foundation at Private Universities, 2014–2018 (S1411028); Thirty Meter Telescope (TMT) Project, National Astronomical Observatory of Japan, through its funding program for research and development of TMT science instruments.
Acknowledgment
We would like to thank Takeo Manome, Kentaro Yanagibashi, Naoto Iida, Masahiko Horiuchi, Masatsugu Kamiura, and Shinji Mukai of KYOCERA Corporation for cooperation on fabricating the experimental optical system made entirely of cordierite. We are grateful to Kenji Oka and Sayumi Kaji of Kyoto Sangyo University for supporting experiments.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
REFERENCES
1. D. Vukobratovich, “Design and mounting of metallic mirrors,” in Opto-Mechanical Systems Design, 4th ed. (CRC Press, 2015), Vol. 2, pp. 255–342.
2. R. A. Paquin, “Metal mirrors,” in Handbook of Optomechanical Engineering (CRC Press, 1997), pp. 89–110.
3. D. A. Bath and E. A. Ness, “Applying silicon carbide to optics,” Opt. Photon. News 19(5) 10–13 (2008).
4. W. A. Goodman and P. R. Yoder Jr., “Emerging mirror technologies,” in Opto-Mechanical Systems Design, 4th ed. (CRC Press, 2015), Vol. 2, pp. 451–487.
5. J. S. Johnson, K. Grobsky, and D. J. Bray, “Rapid fabrication of lightweight silicon carbide mirrors,” Proc. SPIE 4771, 243–253 (2002). [CrossRef]
6. Y. Sarugaku, Y. Ikeda, N. Kobayashi, T. Manome, K. Yanagibashi, N. Iida, M. Horiuchi, S. Mukai, K. Oka, S. Kaji, S. Kondo, C. Yasui, and H. Kawakita, “Reflective optical system made entirely of ultra low thermal expansion ceramics: a possibility of genuine athermal cryogenic IR instrument,” Proc. SPIE 10706, 107063P (2018). [CrossRef]
7. Y. Nakahori, Y. Ikeda, S. Mukai, Y. Nishi, H. Himeno, S. Shiramomo, M. Horiuchi, T. Manome, and K. Masatsugu, “Alignment-free gapless segmented mirror for large telescope,” Proc. SPIE 10706, 107060B (2018). [CrossRef]
8. T. Kamiya and T. Mizutani, “Study on polishing characteristics of cordierite ceramics for ultra-lightweight and thermally stable mirror applications,” Proc. SPIE 10742, 1074205 (2018). [CrossRef]
9. P. R. Yoder Jr., “Opto-mechanical characteristics of materials,” in Opto-Mechanical Systems Design, 4th ed., (CRC Press, 2015), Vol. 1, pp. 95–186.
10. T. Kamiya and T. Mizutani, “Comparison of material properties between ultra low thermal expansion ceramics and conventional low thermal expansion glass,” Proc. SPIE 10372, 1037208 (2017). [CrossRef]
11. G. K. White, “Thermal expansion at low temperatures of glass-ceramics and glasses,” Cryogenics 16, 487–490 (1976). [CrossRef]
12. J. H. Burge, T. Peper, and S. F. Jacobs, “Thermal expansion of borosilicate glass, Zerodur, Zerodur M, and unceramized Zerodur at low temperatures,” Appl. Opt. 38, 7161–7162 (1999). [CrossRef]
13. OHARA corporation catalog, https://www.oharacorp.com/pdf/ccz-2013-nav.pdf.
14. M. Okaji, K. Nara, H. Kato, K. Michishita, and Y. Kubo, “Thermal expansion of some advanced ceramics applicable as specimenholders of high Tc superconductors,” Cryogenics 34, 163–165 (1994). [CrossRef]
15. T. Kotani, M. Tamura, H. Suto, et al., “Infrared Doppler instrument (IRD) for the Subaru telescope to search for Earth-like planets around nearby M-dwarfs,” Proc. SPIE 9147, 914714 (2014). [CrossRef]
16. Y. Ikeda, N. Kobayashi, S. Kondo, et al., “Highly sensitive, non-cryogenic NIR high-resolution spectrograph, WINERED,” Publications of the Astronomical Society of the Pacific 134, 015004 (2022). [CrossRef]
17. S. Miyazaki, Y. Komiyama, H. Nakaya, et al., “Hyper Suprime-Cam,” Proc. SPIE 8446, 84460Z (2012). [CrossRef]
18. A. Offner, “New concepts in projection mask aligners,” Opt. Eng. 14, 130–132 (1975). [CrossRef]
19. A. F. Clark, “Low temperature thermal expansion of some metallic alloys,” Cryogenics 8, 282–289 (1968). [CrossRef]
20. S. Nagata, Y. Nakahori, S. Mukai, J. Estrada, A. Shobe, M. Kamiura, and Y. Ikeda, “Cordierite design, manufacturing, and performance,” in OSA Technical Digest (2019), paper OT2A.1.
21. T. Arasaki, N. Kobayashi, Y. Ikeda, S. Kondo, Y. Sarugaku, S. Kaji, and H. Kawakita, “VINROUGE: a very compact 2-5µm high-resolution spectrograph with germanium immersion grating,” Proc. SPIE 9908, 990875 (2016). [CrossRef]
