One of the highest scientific priorities for ESO's Extremely Large Telescope is imaging and characterising Earth-like planets around Sun-like stars, requiring contrast ratios of 10<sup>-9</sup> only achievable through a combination of extreme adaptive optics, advanced coronagraphy, and data post-processing. The dedicated planetary instrument ELT-PCS uses an integral field spectrograph to simultaneously obtain a homogeneous set of spectra over a two-dimensional field on the sky. Here we present the design and early results from prototyping activities for a lenslet-array and an image-slicer based IFU that are being studied in the context of the ELT-PCS R and D. In particular for the image slicer we investigate classical polishing techniques, like used for SINFONI and SWIFT, and diamond machining.
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.
Germanium Immersion Gratings (GIGs) may be an important component for a compact, high-resolution spectrograph for the infrared. Germanium’s large index of refraction reduces the length of the grating by a factor of four compared to conventional reflection gratings. Germanium transmits light from roughly 2 to 11.5 μm, which includes spectral regions largely unavailable from the ground because of molecules in Earth’s atmosphere. This combination makes GIGs a compelling technology for space missions focused on molecules in astrophysical environments. We are beginning testing of a GIG supplied by Canon, Inc., and anticipate eventual detailed testing of the Canon grating and a similar GIG supplied by LLNL. We also discuss potential science observations that demonstrate the significance of high-resolution, infrared spectroscopy from space.
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  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.<p> </p>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  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.<p> </p>So far, there exist many applications of the IFS for ground-based astronomical observations . Regarding solar instrumentations, the IFS of micro-lenslet array was done by Suematsu et al. , the IFS of densely packed rectangular fiber bundle with thin clads was realized  and being developed for 4-m aperture solar telescope DKIST by Lin  and being considered for space solar telescope SOLAR-C by Katsukawa et al. , and the IFS with mirror slicer array was presented by Ren et al.  and under study for up-coming large-aperture solar telescope in Europe by Calcines et al. <p> </p>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.  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.<p> </p>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.
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) and Germanium(Ge) immersion grating (Sukegawa et al. (2015), Fig.2). Ge is the best material for a mid-infrared immersion grating because of Ge has very large reflective index (n=4.0).
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 H<sub>2</sub>O, 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.
WINERED is a PI-type 0.9 – 1.35 μm high-resolution spectrograph developed by the Laboratory of Infrared highresolution Spectrograph (LiH) of the Koyama Astronomical Observatory at Kyoto Sangyo University, Japan. The scope of WINERED is to realize a high-resolution near-infrared (NIR) spectrograph with both wide coverage and high sensitivity. WINERED provides three observational modes called as the Wide, Hires-Y and Hires-J modes. The Wide mode simultaneously covers the z, Y and J-bands in a single exposure with R ≡ λ/Δλ = 28,000 and was commissioned for the 1.3 m Araki Telescope of Koyama Astronomical Observatory in 2013. We have been building alternative observational modes “Hires-Y” and “Hires-J”, providing R = 80,000 spectra in the Y- and J-bands, respectively. There are two choices for realizing a compact spectrograph with a high spectral resolution of R ≧ 50,000: an immersion grating (IG) or a highblazed echelle grating (HBG). Investigating the availabilities of both optical devices, we selected an HBG solution for λ < 1.5 μm because can be realized with currently available technology in earlier time. The optical parameters of WINERED’s HBGs are as follows: groove pitch = 90.38 μm, blaze angle = 79.32 °, and apex angle = 88°, which are determined to minimize vignetting in the optical system as well as aberrations with the spectral resolution of R = 80,000. Custom HBGs were made by CANON Inc. Because of the size the size limitation in fabrication process, we decided to use a mosaicked grating consisting of two HBGs. The alignment tolerances of the two HBGs are very tight (< 0.5 arcsec for the parallelism between grooves of the two gratings and 1.5 arcsec for the flatness between the two grating surfaces). To enable these fine alignments, we designed a grating holder with an adjustment mechanism with sub-μm positional resolution. We adapted cordierite CO-220 as the material for the grating holder, thereby reducing the misalignment generated by thermal expansions/compression with extremely low coefficient of thermal expansion (CTE < 2.0 ×10<sup>−8</sup> K<sup>-1</sup> at 23 °C). As a result of the measurement of the two HBGs installed in the grating holder, we confirmed the parallelism of < 0.1 arcsec. Finally, we evaluated the total optical performances of the Hires modes with the HBGs. The widths of the monochromatic slitimages obtained with a Th-Ar lamp were measured to be 1.7 – 2.3 pixels, which agreed well with the designed values (1.6 – 2.6 pixels). These results should guarantee the spectral resolution (R = 78,000) estimated from the measurement of the linear dispersion [pix / μm]. Because there was an avoidable degradation in reducing the two-dimensional spectrum using HBGs with a large γ angle, the final spectral resolution of the reduced one-dimensional spectrum results in R = 68,000.
Immersion gratings will play important roles for infrared astronomy in the next generation. We have been developing immersion gratings with a variety of kinds of materials and have succeeded in fabricating a high-efficiency germanium (Ge) immersion grating with both a reflection coating on the grating surface and an AR coating on the entrance surface. The grating will be installed in a K-, L-, and M-bands (2-5μm) high-resolution (R=80,000) spectrograph, VINROUGE, which is a prototype for the TMT MIR instrument. In this paper, we report the preliminary results on the evaluation of the Ge immersion grating. We confirmed that the peak absolute diffraction efficiency was in the range of 70-80%, which was as expected from the design, at both room and cryogenic temperatures.
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.
Immersion grating is a powerful optical device for thee infrared high-resolution spectroscope. Germanium (GGe) is the best material for a mid-infrared immersion grating because of Ge has very large reflective index (n=4.0). On the other hands, there is no practical Ge immersion grating under 5umm use. It was very difficult for a fragile IR crystal to manufacture a diffraction grating precisely. Our original free-forming machine has accuracy of a few nano-meter in positioning and stability. We already fabricated the large CdZnTe immersion grating. (Sukegawa et al. (2012), Ikeda et al. (2015)) Wee are developing Ge immersion grating that can be a good solution for high-resolution infrared spectroscopy with the large ground-based/space telescopes. We succeeded practical Ge immersion grating with the grooved area off 75mm (ruled direction) x 119mm (grove width) and the blaze angle of 75 degrees. Our astronomical large Ge immersion grating has the grooved area of 155mm (ruled direction) x 41mmm (groove width) and groove pitch off 91.74um. We also report optical performance of astronomical large Ge immersion grating with a metal coating on the diffraction surface.
In recent years, a calibration method for an astronomical spectrograph using an optical frequency comb (OFC) with a
repetition rate of more than ten GHz has been developed successfully [1-5]. But controlling filtering cavities that are
used for thinning out longitudinal modes precludes long term stability. The super-mode noise coming from the
fundamental repetition rate is an additional problem. We developed a laser-diode pumped Yb:Y2O3 ceramic oscillator,
which enabled the generation of 4-GHz (maximum repetition rate of 6.7 GHz) pulse trains directly with a spectrum
width of 7 nm (full-width half-maximum, FWHM), and controlled its optical frequency within a MHz level of accuracy
using a beat note between the 4-GHz laser and a 246-MHz Yb-fiber OFC. The optical frequency of the Yb-fiber OFC
was phase locked to a Rb clock frequency standard. Furthermore we also built a table-top multi-pass spectrograph with a
maximum frequency resolution of 600 MHz and a bandwidth of 1 nm using a large-size high-efficiency transmission
grating. The resolution could be changed by selecting the number of passes through the grating. This spectrograph could
resolve each longitudinal mode of our 4-GHz OFC clearly, and more than 10% throughput was obtained when the
resolution was set to 600 MHz. We believe that small and middle scale astronomical observatories could easily
implement such an OFC-calibrated 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.
We have been developing an immersion grating for high-resolution spectroscopy in the mid-infrared (MIR)
wavelength region. A MIR (12-18 µm) high-resolution (R = 20,000-30,000) spectrograph with the immersion
grating is proposed for SPICA, Japanese next-generation space telescope. The instrument will be the world's first
high-resolution spectrograph in space, and it would make great impacts on infrared astronomy. To realize a high-efficiency immersion grating, optical properties and machinability of bulk materials are the critical issues. There
are three candidate materials with good MIR transmittance; CdTe (n = 2.65), CdZnTe (n = 2.65), and KRS5 (n
= 2.30). From measurements of transmittance with FTIR and of homogeneity with phase-shifting interferometry
at 1.55 μm, we confirmed that CdZnTe is the best material that satisfies all the optical requirements. As for
machinability, by applying Canon's diamond cutting (planing) technique, fine grooves that meet our requirement
were successfully cut on flats for all the materials. We also managed to fabricate a small CdZnTe immersion
grating, which shows a high grating efficiency from the air. For the reflective metal coating, we tried Au (with
thin underlying layer of Cr) and Al on CdZnTe flats both by sputter deposition and vapor deposition. All samples
are found to be robust under 77 K and some of them achieve required reflectivity. Despite several remaining
technical issues, the fabrication of CdZnTe immersion grating appears to be sound.
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.
Wavefront aberrations of the projection optics induce unignorable focus and overlay errors dependent on the shape of the
device pattern and illumination settings. Thus, the 32nm node and the subsequent double patterning lithographic
generation require ever more stringent control of aberrations. For the most recent exposure tools with polarized
illumination and high throughput capabilities in particular, due attention needs to be paid to the influences of aberrations
caused by polarization and exposure load. A system for measuring and correcting polarization aberrations and lens
heating aberrations has been developed, and its technical details and application examples are presented in this paper.
Furthermore, improvement in aberration control on the next generation exposure tool compatible with double patterning
is stated as well.