SWIMS-IFU is an image-slicer integral field unit designed for Simultaneous-color Wide-field Infrared Multi-object Spectrograph (SWIMS) of the University of Tokyo Atacama observatory 6.5m telescope. Its field-of-view, slice width and slice number are 17.2 ′′ × 12.8 ′′, 0.4 ′′ and 26, respectively. Due to the space limitation inside SWIMS, the IFU should fit in the dimension of 60mm×170mm×220mm. After finishing development of optical design, we have conducted tolerance analysis. The results show that the probability of vignetting of less than 5% is ∼90%, although at a slice of one side it drops to 50%. We plan to fabricate the mirror arrays monolithically by a ball-end milling with an ultra-high precision machine tool, and have conducted a demonstration process to prove its feasibility. Our requirement for shape error is less than 100 nm P-V and that for surface roughness is less than 10 nm r.m.s. Results of the latest demonstration satisfies the requirement. We will fabricate the mirror arrays and the support structures in 2018, and the IFU will be installed into SWIMS in 2019.
The grism is a versatile dispersion element for an astronomical instrument ranging from ultraviolet to infrared. Major benefit of using a grism in a space application, instead of a reflection grating, is the size reduction of optical system because collimator and following optical elements could locate near by the grism. The surface relief (SR) grism is consisted a transmission grating and a prism, vertex angle of which is adjusted to redirect the diffracted beam straight along the direct vision direction at a specific order and wavelength. The volume phase holographic (VPH) grism consists a thick VPH grating sandwiched between two prisms, as specific order and wavelength is aligned the direct vision direction. The VPH grating inheres ideal diffraction efficiency on a higher dispersion application. On the other hand, the SR grating could achieve high diffraction efficiency on a lower dispersion application. Five grisms among eleven for the Faint Object Camera And Spectrograph (FOCAS) of the 8.2m Subaru Telescope with the resolving power from 250 to 3,000 are SR grisms fabricated by a replication method. Six additional grisms of FOCAS with the resolving power from 3,000 to 7,000 are VPH grisms. We propose “Quasi-Bragg grism” for a high dispersion spectroscopy with wide wavelength range.
The germanium immersion grating for instance could reduce 1/64 as the total volume of a spectrograph with a conventional reflection grating since refractive index of germanium is over 4.0 from 1.6 to 20 μm. The prototype immersion gratings for the mid-InfraRed High dispersion Spectrograph (IRHS) are successfully fabricated by a nano-precision machine and grinding cup of cast iron with electrolytic dressing method.
We describe overview of fabrication methods and measurement results of test fabrications of optical surfaces for an integral ﬁeld unit (IFU) for Simultaneous color Wide-ﬁeld Infrared Multi-object Spectrograph, SWIMS, which is a ﬁrst-generation instrument for the University of Tokyo Atacama Observatory 6.5-m telescope. SWIMS-IFU provides entire near-infrared spectrum from 0.9 to 2.5 μm simultaneously covering wider ﬁeld of view of 17" × 13" compared with current near-infrared IFUs. We investigate an ultra-precision cutting technique to monolithically fabricate optical surfaces of IFU optics such as an image slicer. Using 4- or 5-axis ultra precision machine we compare the milling process and shaper cutting process to ﬁnd the best way of fabrication of image slicers. The measurement results show that the surface roughness almost satisﬁes our requirement in both of two methods. Moreover, we also obtain ideal surface form in the shaper cutting process. This method will be adopted to other mirror arrays (i.e. pupil mirror and slit mirror, and such monolithic fabrications will also help us to considerably reduce alignment procedure of each optical elements.
We propose and demonstrate an optical component that overcomes critical limitations in our previously demonstrated high-speed multispectral videography—a method in which an array of periscopes placed in a prism-based spectral shaper is used to achieve snapshot multispectral imaging with the frame rate only limited by that of an image-recording sensor. The demonstrated optical component consists of a slicing mirror incorporated into a 4f-relaying lens system that we refer to as a spectrum slicer (SS). With its simple design, we can easily increase the number of spectral channels without adding fabrication complexity while preserving the capability of high-speed multispectral videography. We present a theoretical framework for the SS and its experimental utility to spectral imaging by showing real-time monitoring of a dynamic colorful event through five different visible windows.
A volume phase holographic (VPH) grating achieves high angular dispersion and very high diffraction efficiency for the
first diffraction order and for S or P polarization. However the VPH grating could not achieve high diffraction efficiency
for non-polarized light at a large diffraction angle because properties of diffraction efficiencies for S and P polarizations
are different. Furthermore diffraction efficiency of the VPH grating extinguishes toward a higher diffraction order. A
birefringence binary Bragg (3B) grating is a thick transmission grating with optically anisotropic material such as lithium
niobate or liquid crystal. The 3B grating achieves diffraction efficiency up to 100% for non-polarized light by tuning of
refractive indices for S and P polarizations, even in higher diffraction orders. We fabricated 3B grating with liquid crystal
and evaluated the performance of the liquid crystal grating. A quasi-Bragg (QB) grating, which consists long rectangle
mirrors aligned in parallel precisely such as a window shade, also achieves high diffraction efficiency toward higher orders.
We fabricated QB grating by laminating of silica glass substrates and glued by pressure fusion of gold films. A
quasi-Bragg immersion (QBI) grating has smooth mirror hypotenuse and reflector array inside the hypotenuse, instead of
step-like grooves of a conventional immersion grating. An incident beam of the QBI grating reflects obliquely at a
reflector, then reflects vertically at the mirror surface and reflects again at the same reflector. We are going to fabricate
QBI gratings by laminating of mirror plates as similar to fabrication of the QB grating. We will also fabricate silicon and
germanium immersion gratings with conventional step-like grooves by means of the latest diamond machining methods.
We introduce characteristics and performance of these gratings.
We describe the fabrication of a long mirror for focusing X-ray free electron lasers to nanometer dimension, for the production of high photon density beams. The focusing mirror has an elliptical curved shape with a length of 400 mm and focal length of 550 mm. Electrolytic in-process dressing grinding is used for first-step figuring and elastic emission machining is employed for final figuring and surface smoothing. Figure accuracy with a peak-to-valley height of 2 nm is achieved. A focusing test was performed at BL29XUL of SPring-8 and found the focused beam size to be approximately
75 nm at 15 keV, very similar to the theoretical value.
We have successfully fabricated germanium immersion gratings with resolving power of 45,000 at 10 μm by using a nano precision 3D grinding machine and ELID (ELectrolytic In-process Dressing) method. However the method spends large amount of machine times. We propose grooves shape with a new principle for a solid grating, which
achieves high performance and lower cost. We have developed volume phase holographic (VPH) grisms with zinc selenide (ZnSe) prisms for spectrograph of the Subaru Telescope and the other telescopes. While a VPH grism with high index prisms achieves higher dispersion,
diffraction efficiency of VPH grating decreases toward higher orders. A "quasi-Bragg grating" which inherits advantage of a VPH grating achieves high diffraction efficiency toward higher orders. Wavelength tuners with a pair of counter-rotation prisms for a VPH and quasi-Bragg grating obtain high diffraction efficiency over wide wavelength range. The novel immersion grating, VPH grism with high index prisms, quasi-Bragg
grating and wavelength tuners dramatically reduce volumes of astronomical spectrographs.
In this paper, we present a damage monitoring method to detect and track the evolution of structural damages based on the measurements acquired by vibratory gyroscopes. We propose a modal flexibility-based damage index that quantifies the local bending flexibility of the beam structures by conducting virtual local loading tests. We use vibratory gyroscopes to measure the angular mode shapes which are necessary to assess the proposed damage index. A time-frequency analysis-based simple scheme is developed to identify the time-varying mode shapes to detect and track the evolution of the damage index. Experimental results show that the proposed monitoring method is capable to indicate the location of the damages as well as the time when they occur.
We propose a method to make advanced thin-foil substrates for an X-ray telescope by press forming. Walter Type-I optics, which reflect X-rays twice with their confocal paraboloidal and hyperboloidal reflectors, have commonly been used in X-ray astronomy.
However, it is difficult to make substrates for a large X-ray telescope with a high imaging quality. By forming two reflectors from one thin substrate, the accuracy of the angle between two reflectors is determined by the accuracy of a mandrel or a stamping die.
We made a stamping die using a precision processing method at RIKEN. After several improvements of our pressing method, we have obtained the substrates with a figure error < 15 um, which corresponds to an angular resolution of about 2' at the focal plane, although a small waviness was found in the mirror surface. We also propose a new holding method of the foil to reduce a displacement of the foil positions. The foil is tightly held with three thin-supports with a thickness of 0.3 mm. We made the test model of a mirror housing, and found that the foil position was determined with σ~12μm.
The mid-infrared high dispersion spectrograph (IRHS; tentative name) with a resolving power of 200,000 at 10 μm is a candidate of the second-generation instrument for the 8.2m Subaru Telescope. A germanium immersion grating will be employed as a dispersing element for this instrument. Germanium immersion gratings for the prototype IRHS were successfully fabricated by using a nano precision 3D profile grinding/turning machine and ELID grinding method on diamond machining. As a result, the fabricated gratings observed to have grooves with ideal saw-tooth shape, smooth surface and acceptable wave front error of a diffraction beam at 10μm. In the present paper, we characterized the performance of the developed immersion gratings.
We have developed compound refractive prism for cold neutrons. To prevent an increase in neutron absorption, we have developed prism array like a Fresnel lens. The prism characteristics were investigated with experimental and numerical simulation studies. We achieved transmission of 0.75 and refractive angle of 7.5 mrad for 15 neutrons with 49 layered prism array.