Kenji Mitsui

Beam Pattern Measurements of Millimeter-Wave Kinetic Inductance Detector Camera With Direct Machined Silicon Lens Array

We have developed 220 and 440-GHz cameras using microwave kinetic inductance detectors (MKIDs) for astronomical observations. The optical system of the MKID camera is based on double-slot antennas and extended hemispherical silicon lens arrays. The lens diameter is three times the target wavelength. The 220-GHz camera and the 440-GHz camera have 9 pixels and 102 pixels, respectively. The silicon lens array has been directly machined using a high-speed spindle on an ultra-precision machine. The shape fabrication error and the surface roughness of the top of the lens were typically less than 10 μm (peak-to-valley) and about 0.7 μm (rms), respectively. The beam patterns of the MKID camera were measured and are in good agreement with the calculations.

Developments of wide field submillimeter optics and lens antenna-coupled MKID cameras

Wide field cryogenic optics and millimeter-wave Microwave Kinetic Inductance Detector (MKID) cameras with Si lens array have been developed. MKID is a Cooper-pair breaking photon detector and consists of supercon- ducting resonators which enable microwave (~GHz) frequency multiplexing. Antenna-coupled Aluminum CPW resonators are put in a line on a Si substrate to be read by a pair of coaxial cables. A 220 GHz - 600 pixels MKID camera with anti-reflection (AR) coated Si lens has been demonstrated in an 0.1 K cryostat. A compact cryogenic system with high refractive index materials has been developed for the MKID camera.

Development of mid-infrared spectrometer with an image slicer (MIRSIS) for ground-based astronomy: developing optical and mechanical mounts

Mid-Infrared Spectrometer with an Image Slicer (MIRSIS) is a 10micron band spectrometer for ground-based observations. Based on the optical design reported in Okamoto et al. (2006), we recently developed most of optical elements and their mounts. There, we adopted designs based on an ultra-precision cut for the slice mirrors and the pupil mirrors. We also designed and partly manufactured the optical parts with switching/adjusting mechanism with cryogenic step motors. Since MIRSIS has a very complicated stereoscopic configuration of optical elements, we developed a method to adjust the optical alignment where relative positional markers and a three-dimensional measuring system are combined. We confirmed that we can achieve position and angular adjustment with error down to 0.1mm and 0.05degree through alignment test with a pair of mirrors.

Development of a test N-band image slicer: optical design

A spectrometer with integral field units on large optical/infrared telescopes enables efficient spectroscopy of moderately extended objects. In future mid-infrared observations with 30m class telescopes, where circumstellar disks larger than the spatial resolution will be major targets, such efficient observations are strongly desirable. Here we present an optical design of our new N-band image slicing spectrometer to test basic techniques for future image slicing spectrometers on larger telescopes. Our prototype image slicer follows the idea of the advanced image slicer considering not only object images but also pupil images and is optimized for the N-band (10 micron atmospheric window). Five slicing mirrors and five pupil mirrors are used to slice the field of view and make a rearranged pseudo slit image. The pseudo slit image is collimated, dispersed by a grating, and imaged on a Si:As 320x240 array. For the slicing mirrors, we plan to use polished stainless mirrors of 300 micron width. The spectral resolution is set as about 200. We plan to put an imaging optics module for target aquisition in addition to the simple image slicer module. The whole optics is designed to be compact (about 600mm x 450mm x 300 mm), which will allow us to make test observations easily with various telescopes.

Recent Progress in the Development of Mid-Infrared Medium Resolution Spectrometer (MRS) installed in SPICA/MCS

Mid-infrared Medium Resolution Spectrometer (MRS) is one of the key spectroscopic modules of Mid- Infrared Camera and Spectrometers (MCS) that will be onboard SPICA. MRS is an Echelle Grating spectrometer designed to observe a number of fine structure lines of ions and atoms, molecular lines, and band features stemming from solid particles and dust grains of the interstellar and circumstellar medium in the mid-infrared wavelength range. MRS consists of two channels; the shorter wavelength channel (MRS-S) covers the spectral range from 12.2 to 23.0 micron with a spectral resolution power of R~1900-3000 and the longer wavelength channel (MRS-L) covers from 23.0 to 37.5 micron with R~1100-1500 on the basis of the latest results of the optical design. The distinctive functions of the MRS are (1) a dichroic beam splitter equipped in the fore-optics, by which the same field of view is shared between the two channels, and (2) the small format image slicer as the integral field unit installed in each channel. These functions enable us to collect continuous 12-38 micron spectra of both the point-like and diffuse sources reliably with a single exposure pointed observation. In this paper, the specifications and the expected performance of the MRS are summarized on the basis of the latest results of the optical design. The latest progress in the development of the key technological elements, such as the Dichroic Beam Splitter and the Small Format Monolithic Slice Mirrors, are also reported.

Fabrication of 721-pixel silicon lens array of a microwave kinetic inductance detector camera

Abstract. We have been developed a lens-integrated superconducting camera for millimeter and submillimeter astronomy. High-purity silicon (Si) is suitable for the lens array of the microwave kinetic inductance detector camera due to its high refractive index and low dielectric loss at low temperatures. The camera is an antenna-coupled Al coplanar waveguide on a Si substrate. Thus the lens and the device are made of the same material. We report a fabrication method of a 721-pixel Si lens array with an antireflection (AR) coating. The Si lens array was fabricated with an ultraprecision cutting machine. It uses TiAlN-coated carbide end mills attached with a high-speed spindle. The shape accuracy was less than 50  μm peak-to-valley and the surface roughness was arithmetic average roughness (Ra) of 1.8  μm. The mixed epoxy was used as an AR coating to adjust the refractive index. It was shaved to yield a thickness of 185  μm for 220 GHz. Narrow grooves were made between the lenses to prevent cracking due to the different thermal expansion coefficients of Si and the epoxy. The surface roughness of the AR coating was Ra of 2.4 to 4.2  μm.

A design and trial production of the image slicer unit for the mid-infrared spectrograph

An image slicer is highly in demand for an integral field unit (IFU) spectrograph of the next generation infrared telescopes. This paper reports the results of the trial production of three key optical elements for a small format (number of slice; n=5) image slicer, i.e. monolithic slice mirrors, monolithic pupil mirrors and monolithic pseudo slit mirrors. We have demonstrated that sufficiently high processing accuracy and mirror surface accuracy for infrared observations are achieved for each optical element based on our super precision cutting techniques.

A trial production of the image slicer unit for next generation infrared instruments and the assembly of the evaluation system of the pseudo slit image quality

We have carried out the trial production of small format (n=5) image slicer aiming to obtain the technical verification of the Integral Field Unit (IFU) that can be equipped to the next generation infrared instruments such as TMT/MICHI and SPICA/SMI. Our goal is to achieve stable pseudo slit image with high efficiency. Here we report the results of the assembly of the image slicer unit and the non-cryogenic evaluation system of the pseudo slit image quality in the infrared.

Broadband Corrugated Horn Array with Direct Machined Fabrication

We have developed a broadband corrugated horn in the 120-270 GHz and a horn array in the 80-180 GHz bands. The geometry of corrugations is so simple that the horn array can be directly machined from a bulk of aluminum with an end mill. The cross polarization and near sidelobe levels are less than -20 and -30 dB, respectively. The return loss is less than -15 dB in most design frequency bands, and the beam pattern is symmetric. The beam pattern and the return loss are measured in the 120-170 GHz range at room temperature. They are in good agreement with the simulation. It is possible to reduce reflection at the aperture surface and to reduce the weight by carving the unnecessary part. This design provides an octave bandwidth of the corrugated horn array at reasonable machining time.

Restraint deformation and corrosion protection of gold deposited aluminum mirrors for cold optics of mid-infrared instruments

We report the restraint deformation and the corrosion protection of gold deposited aluminum mirrors for mid-infrared instruments. To evaluate the deformation of the aluminum mirrors by thermal shrinkage, monitoring measurement of the surface of a mirror has been carried out in the cooling cycles from the room temperature to 100 K. The result showed that the effect of the deformation was reduced to one fourth if the mirror was screwed with spring washers. We have explored an effective way to prevent the mirror from being galvanically corroded. A number of samples have been prepared by changing the coating conditions, such as inserting an insulation layer, making a multi-layer and overcoating water blocking layer, or carrying out precision cleaning before coating. Precision cleaning before the deposition and protecting coat with SiO over the gold layer seemed to be effective in blocking corrosion of the aluminum. The SiO over-coated mirror has survived the cooling test for the mid-infrared use and approximately 1 percent decrease in the reflectance has been detected at 6-25 microns compared to gold deposited mirror without coating.