Yukinobu Okura

High-performance astronomical gratings by Canon

Canon is developing wide variety of gratings that can be effective solutions for high precision spectroscopy for the next-generation ground-based and space telescopes. In this paper, we focus on our development of infrared immersion grating, which is one of the most demanding devices among various gratings. We use CdZnTe for mid-infrared (MIR) application and KRS5 for near-infrared (NIR) to MIR application. In particular, CdZnTe immersion grating is the key-device for the MIR high-resolution spectrograph for the space infrared telescope SPICA. Using our diamond cutting (planing) technique, grooves are shaped on the hypotenuse area (30 mm x 10 mm) of a CdZnTe prism with the spacing accuracy of < 5 nm (rms) and the surface roughness of < 5 nm (rms). We also performed cutting of KRS5 disk and confirmed that excellent grooves can be shaped on this material.

Development of micro image slicer of integral field unit for spaceborne solar spectrograph

We present an innovative optical design for image slicer integral field unit (IFU) and manufacturing method which overcome optical limitation of metallic mirrors. Our IFU consists of micro image slicer of 45 arrayed highly-narrow flat metallic mirrors and a pseudo pupil mirror array of off-axis conic aspheres forming three pseudo slits of re-arranged slicer images. A prototype IFU demonstrates their optical quality high enough for a visible light spectrograph. The each slicer mirror is 1.58 mm in length and 30μm in width with surface roughness < 1 nm rms, edge sharpness < 0.1μm, etc. This IFU is small-sized and can be implemented in a multi-slit spectrograph without any moving mechanism and fore optics in which one slit is real and the others are of pseudo slits from the IFU. Those properties are well suitable for space-borne spectrograph to be aboard such as a next Japanese solar mission SOLAR-C.

Development of compact metal-mirror image slicer unit for optical telescope of the SOLAR-C mission

To realize an integral field unit (IFU) for a one-meter class optical telescope (SUVIT) on board Japanese next solar mission (SOLAR-C), we studied an optical design and manufacturing method to attain high optical performances for IFU, using a novel manufacturing technique developed by Canon. The IFU consists of micro-image slicer of 45 arrayed 30-micron-thick metal mirrors and a pseudo-pupil mirror array for making three pseudo-slits, providing possible optical configuration for a coexistence with a usual slit spectrograph without movable mechanism. The IFU mirrors were deposited by a protected silver coating for high reflectivity in visible and near IR wavelength region. We present the optical design, performance of prototype IFU and space qualification tests of the silver coating.

Commercial availability of astronomical machined gratings by Canon

A machining grating is remarkable for infrared use because it is difficult to get an ideal shape by traditional ruling engine. Canon has many kinds of gratings for the infrared on sale now by our processing technology. Our commercial grating is not only a conventional (reflection) grating but also an immersion grating. An immersion grating is very powerful and key device for the infrared high-resolution spectroscopy. It is sure that an immersion grating becomes next generation standard diffractive device for the infrared. However, there was no opportunity, which can be obtained easily before we enter this field. Our machined grating has very high absolute diffraction efficiency by the actual measurement. Furthermore, our surface flatness is the excellent compared before us. These are the performances already proved through our productions. Those will be the standard performance of a grating for astronomy. In this paper, we report that the performances and restrictions of machined grating in Canon.

Three types of immersion grating for next-generation infrared spectrometer

Since an immersion grating provides n (n: refractive index of its material) times higher spectral resolution compared to a conventional reflective grating of the same size, an immersion grating is a powerful optical device for the infrared high-resolution spectrometer. Recently a high-resolution spectrometer in the infrared wavelength range is increasing the importance increasingly for observations of relating with H2O, NHx, NOx and organic molecules. Higher spectral resolution allows us to detect weak lines without spectral line confusion. On the other hands, there is no practical immersion grating for high-resolution spectrometer except Si immersion grating by anisotropic etching. It was very difficult for a fragile IR crystal to manufacture a diffraction grating precisely by machining. Our original free-forming machine has accuracy of a few nano-meter in positioning and stability. We succeeded in fabricating immersion gratings with three kinds of materials. Three materials are CdZnTe, germanium and InP, each refractive index is about 2.7, 4.0 and 3.2 respectively. By combining these devices, a spectrometer with immersion grating is realizable in the wavelength range of 1.5-20um. Thereby, the realization of these immersion gratings has led to a dramatic improvement in the operability and performance of next generation high-performance spectrometer. In this paper, we report performance of our immersion gratings and other possibility.

Compact integral field unit for optical telescope of the Solar-C mission

A Japan-led international solar mission “SOLAR-C” is being proposed for mid-2020s launch.

Development of INP immersion grating for the near- to mid-infrared wavelength

An Immersion grating is a powerful optical device for the infrared high-resolution spectroscope. We already fabricated the large CdZnTe(CZT) immersion grating (Sukegawa et al. (2012), Fig.1)[1][2][3] and Germanium(Ge) immersion grating (Sukegawa et al. (2015), Fig.2)[4]. Ge is the best material for a mid-infrared immersion grating because of Ge has very large reflective index (n=4.0).

Development of compact integral field unit for spaceborne solar spectro-polarimeter

A 1.5-m class aperture Solar Ultra-violet Visible and IR telescope (SUVIT) and its instruments for the Japanese next space solar mission SOLAR-C [1] are under study to obtain critical physical parameters in the lower solar atmosphere. For the precise magnetic field measurements covering field-of-view of 3 arcmin x3 acmin, a full stokes polarimetry at three magnetic sensitive lines in wavelength range of 525 nm to 1083 nm with a four-slit spectrograph of two dinesional image scanning mechanism is proposed: one is a true slit and the other three are pseudo-slits from integral field unit (IFU). To suit this configuration, besides a fiber bundle IFU, a compact mirror slicer IFU is designed and being developed. Integral field spectroscopy (IFS), which is realized with IFU, is a two dimensional spectroscopy, providing spectra simultaneously for each spatial direction of an extended two-dimensional field. The scientific advantages of the IFS for studies of localized and transient solar surface phenomena are obvious. There are in general three methods [2][3] to realize the IFS depending on image slicing devices such as a micro-lenslet array, an optical fiber bundle and a narrow rectangular image slicer array. So far, there exist many applications of the IFS for ground-based astronomical observations [4]. Regarding solar instrumentations, the IFS of micro-lenslet array was done by Suematsu et al. [5], the IFS of densely packed rectangular fiber bundle with thin clads was realized [6] and being developed for 4-m aperture solar telescope DKIST by Lin [7] and being considered for space solar telescope SOLAR-C by Katsukawa et al. [8], and the IFS with mirror slicer array was presented by Ren et al. [9] and under study for up-coming large-aperture solar telescope in Europe by Calcines et al. [10] From the view point of a high efficiency spectroscopy, a wide wavelength coverage, a precision spectropolarimetry and space application, the image slicer consisting of all reflective optics is the best option among the three. However, the image slicers are presently limited either by their risk in the case of classical glass polishing techniques (see Vivès et al. [11] for recent development) or by their optical performances when constituted by metallic mirrors. For space instruments, small sized units are much advantageous and demands that width of each slicer mirror is as narrow as an optimal slit width (< 100 micron) of spectrograph which is usually hard to manufacture with glass polishing techniques. On the other hand, Canon is developing a novel technique for such as high performance gratings which can be applicable for manufacturing high optical performance metallic mirrors of small dimensions. For the space-borne spectrograph of SUVIT to be aboard SOLAR-C, we designed the IFS made of a micro image slicer of 45 arrayed 30-micron-thick metal mirrors and a pseudo-pupil metal mirror array re-formatting three pseudo-slits; the design is feasible for optical configuration sharing a spectrograph with a conventional real slit. According to the optical deign, Canon manufactured a prototype IFU for evaluation, demonstrating high performances of micro image slicer and pupil mirrors; enough small micro roughness for visible light spectrographs, sharp edges for efficient image slices, surface figure for high image quality, etc. In the following, we describe the optical design of IFU feasible for space-borne spectrograph, manufacturing method to attain high optical performance of metal mirrors developed by Canon, and resulted performance of prototype IFU in detail.