KEYWORDS: Near field, Antennas, Holography, Near field optics, Signal detection, Prototyping, Telescopes, Phase retrieval, Phase measurement, Fourier transforms
B-mode polarization detection in the Cosmic Microwave Background would provide crucial information on the inflationary universe. This is the main goal of the LiteBIRD JAXA-led space mission, to be launched by the beginning of the 2030s. To do so it is necessary to control instrumental systematics such as contamination from the far sidelobes of the telescopes. Following the studies that have been done for LiteBIRD’s Low Frequency Telescope (LFT), we measured near-field antenna patterns of a High Frequency Telescope (HFT) optical prototype to showcase the capabilities for the HFT of near-field antenna pattern measurement as well as holographic phase retrieval. We were able to characterize the far-field antenna pattern up to θ = 60° with a dynamic range of -80 dB. The residuals were estimated to be at most 5 dB at the -80 dB detection level. We detected far sidelobes at θ ≈ 30° with a relative intensity of -50 dB, in line with previous simulations of the HFT. Holographic phase retrieval and other time-frequency analysis of the data show promising results for the characterization of the HFT.
The Atacama Large Millimeter/submillimeter Array (ALMA) Wideband Sensitivity Upgrade (WSU) will at least double the observation bandwidth and improve the sensitivity and scientific capabilities. The WSU requires upgrading the receiver front-end and the associated analog and digital electronics including the correlator, as well as the ALMA software. In line with the WSU mission, the National Astronomical Observatory of Japan (NAOJ) initiated the ALMA Band 8 (385-500 GHz) version2 receiver upgrade project, aiming to build an upgrade of the currently existing receiver cartridge with a substantially improved second-generation version. This project originated from significant advances in receiver technologies and a variety of our previous wideband studies in the last decade. This paper briefly summarizes an overview of this project, scientific needs in this frequency range, and the technical readiness and challenges for critical components and subsystems.
LiteBIRD, the Lite (Light) satellite for the study of B-mode polarization and Inflation from cosmic background Radiation Detection, is a space mission for primordial cosmology and fundamental physics. JAXA selected LiteBIRD in May 2019 as a strategic large-class (L-class) mission, with its expected launch in the late 2020s using JAXA's H3 rocket. LiteBIRD plans to map the cosmic microwave background (CMB) polarization over the full sky with unprecedented precision. Its main scientific objective is to carry out a definitive search for the signal from cosmic inflation, either making a discovery or ruling out well-motivated inflationary models. The measurements of LiteBIRD will also provide us with an insight into the quantum nature of gravity and other new physics beyond the standard models of particle physics and cosmology. To this end, LiteBIRD will perform full-sky surveys for three years at the Sun-Earth Lagrangian point L2 for 15 frequency bands between 34 and 448 GHz with three telescopes, to achieve a total sensitivity of 2.16 μK-arcmin with a typical angular resolution of 0.5° at 100 GHz. We provide an overview of the LiteBIRD project, including scientific objectives, mission requirements, top-level system requirements, operation concept, and expected scientific outcomes.
LiteBIRD has been selected as JAXA’s strategic large mission in the 2020s, to observe the cosmic microwave background (CMB) B-mode polarization over the full sky at large angular scales. The challenges of LiteBIRD are the wide field-of-view (FoV) and broadband capabilities of millimeter-wave polarization measurements, which are derived from the system requirements. The possible paths of stray light increase with a wider FoV and the far sidelobe knowledge of -56 dB is a challenging optical requirement. A crossed-Dragone configuration was chosen for the low frequency telescope (LFT : 34–161 GHz), one of LiteBIRD’s onboard telescopes. It has a wide field-of-view (18° x 9°) with an aperture of 400 mm in diameter, corresponding to an angular resolution of about 30 arcminutes around 100 GHz. The focal ratio f/3.0 and the crossing angle of the optical axes of 90◦ are chosen after an extensive study of the stray light. The primary and secondary reflectors have rectangular shapes with serrations to reduce the diffraction pattern from the edges of the mirrors. The reflectors and structure are made of aluminum to proportionally contract from warm down to the operating temperature at 5 K. A 1/4 scaled model of the LFT has been developed to validate the wide field-of-view design and to demonstrate the reduced far sidelobes. A polarization modulation unit (PMU), realized with a half-wave plate (HWP) is placed in front of the aperture stop, the entrance pupil of this system. A large focal plane with approximately 1000 AlMn TES detectors and frequency multiplexing SQUID amplifiers is cooled to 100 mK. The lens and sinuous antennas have broadband capability. Performance specifications of the LFT and an outline of the proposed verification plan are presented.
Sapphire, alumina, and silicon present the following characteristics that make them suitable as optical elements for millimeter and sub-millimeter applications: low-loss, high thermal conductivity at cryogenic temperatures, and high refractive index ~3. However, the high index also leads to high reflection. We developed a technique to machine sub-wavelength structures (SWS) as a broadband anti-reflection coating on these materials through laser ablation. We describe here the status of our development: transmission measurements of fabricated samples in a diameter of 34.5 mm agree with predictions, and we are now focusing on increasing the fabrication area with high processing rate. This is motivated by the need of ~500 mm diameter optical elements for the next-generation cosmic microwave background polarization experiments. We show our large area machining method on the alumina and sapphire over an area of < 5200 mm2 with the processing rate of < 4:0 mm3=min:, and the transmission measurements are consistent with the predictions.
LiteBIRD is a JAXA-led Strategic Large-Class mission designed to search for the existence of the primordial gravitational waves produced during the inflationary phase of the Universe, through the measurements of their imprint onto the polarization of the cosmic microwave background (CMB). These measurements, requiring unprecedented sensitivity, will be performed over the full sky, at large angular scales, and over 15 frequency bands from 34 GHz to 448 GHz. The LiteBIRD instruments consist of three telescopes, namely the Low-, Medium-and High-Frequency Telescope (respectively LFT, MFT and HFT). We present in this paper an overview of the design of the Medium-Frequency Telescope (89{224 GHz) and the High-Frequency Telescope (166{448 GHz), the so-called MHFT, under European responsibility, which are two cryogenic refractive telescopes cooled down to 5 K. They include a continuous rotating half-wave plate as the first optical element, two high-density polyethylene (HDPE) lenses and more than three thousand transition-edge sensor (TES) detectors cooled to 100 mK. We provide an overview of the concept design and the remaining specific challenges that we have to face in order to achieve the scientific goals of LiteBIRD.
LiteBIRD is a JAXA strategic L-class mission devoted to the measurement of polarization of the Cosmic Microwave Background, searching for the signature of primordial gravitational waves in the B-modes pattern of the polarization. The onboard instrumentation includes a Middle and High Frequency Telescope (MHFT), based on a pair of cryogenically cooled refractive telescopes covering, respectively, the 89-224 GHz and the 166-448 GHz bands. Given the high target sensitivity and the careful systematics control needed to achieve the scientific goals of the mission, optical modeling and characterization are performed with the aim to capture most of the physical effects potentially affecting the real performance of the two refractors. We describe the main features of the MHFT, its design drivers and the major challenges in system optimization and characterization. We provide the current status of the development of the optical system and we describe the current plan of activities related to optical performance simulation and validation.
We present a breadboard model development status of the polarization modulator unit (PMU) for a low-frequency telescope (LFT) of the LiteBIRD space mission. LiteBIRD is a next-generation cosmic microwave background polarization satellite to measure the primordial B-mode with the science goal of σr < 0.001. The baseline design of LiteBIRD consists of reflective low-frequency and refractive medium-and-high-frequency telescopes. Each telescope employs the PMU based on a continuous rotating half-wave plate (HWP) at the aperture. The PMU is a critical instrument for the LiteBIRD to achieve the science goal because it significantly suppresses 1/f noise and mitigates systematic uncertainties. The LiteBIRD LFT PMU consists of a broadband achromatic HWP and a cryogenic rotation mechanism. In this presentation, we discuss requirements, design and systematic studies of the PMU, and we report the development status of the broadband HWP and the space-compatible cryogenic rotation mechanism.
We report the development of the achromatic half-wave plate (AHWP) at millimeter wave for cosmic microwave background polarization experiments. We fabricate an AHWP consisting of nine A-cut sapphire plates based on the Pancharatnam recipe to cover a wide frequency range. The modulation efficiency and the phase are measured in a frequency range of 33 to 260 GHz with incident angles up to 10 deg. We find the measurements at room temperature are in good agreement with the predictions. This is the most broadband demonstration of an AHWP at millimeter wave.
LiteBIRD is a candidate for JAXA’s strategic large mission to observe the cosmic microwave background (CMB) polarization over the full sky at large angular scales. It is planned to be launched in the 2020s with an H3 launch vehicle for three years of observations at a Sun-Earth Lagrangian point (L2). The concept design has been studied by researchers from Japan, U.S., Canada and Europe during the ISAS Phase-A1. Large scale measurements of the CMB B-mode polarization are known as the best probe to detect primordial gravitational waves. The goal of LiteBIRD is to measure the tensor-to-scalar ratio (r) with precision of r < 0:001. A 3-year full sky survey will be carried out with a low frequency (34 - 161 GHz) telescope (LFT) and a high frequency (89 - 448 GHz) telescope (HFT), which achieve a sensitivity of 2.5 μK-arcmin with an angular resolution 30 arcminutes around 100 GHz. The concept design of LiteBIRD system, payload module (PLM), cryo-structure, LFT and verification plan is described in this paper.
The Lite satellite for the studies of B-mode polarization and Inflation from the cosmic microwave background
(CMB) Radiation Detection (LiteBIRD) is a next generation CMB satellite dedicated to probing the inflationary
universe. It has two telescopes, Low Frequency Telescope (LFT) and High Frequency Telescope (HFT) to cover
wide observational bands from 34 GHz to 448 GHz. In this presentation, we report the optical design and
characterization of the LFT. We have used the CODE-V to obtain the LFT optical design based on a cross-
Dragonian telescope. It is an image-space telecentric system with an F number of 3.5 and 20 x 10 degrees2 field
of view. The main, near and far side lobes at far-field have been calculated by using a combination of HFSS and
GRASP 10. It is revealed that the LFT telescope has good main lobe properties to satisfy the requirements. On
the other hand, the side lobes are affected by the stray light that stems from the triple reflection and the direct
path from feed. In order to avoid the stray light, the way to block these paths is now under study.
KEYWORDS: Space operations, Cryogenics, Radiative energy transfer, Space telescopes, Telescopes, Solar radiation, Cryocoolers, Satellites, Optical properties
The conceptual thermal design of the payload module (PLM) of LiteBIRD utilizing radiative cooling is studied. The thermal environment and structure design of the PLM strongly depend on the precession angle α of the spacecraft. In this study, the geometrical models of the PLM that consist of the sunshield, three layers of Vgrooves, and 5 K shield were designed in the cases of α = 45° , 30° , and 5° . The mission instruments of LiteBIRD are cooled down below 5 K. Therefore, heat transfers down to the 5 K cryogenic part were estimated in each case of α. The radiative heat transfers were calculated by using geometrical models of the PLM. The conductive heat transfers and the active cooling with cryocoolers were considered. We also studied the case that the inner surface of the V-groove is coated by a high-emissivity material.
We present our design and development of a polarization modulator unit (PMU) for LiteBIRD space mission. LiteBIRD is a next generation cosmic microwave background (CMB) polarization satellite to measure the primordial B-mode. The science goal of LiteBIRD is to measure the tensor-to-scalar ratio with the sensitivity of δr < 10-3. The baseline design of LiteBIRD is to employ the PMU based on a continuous rotating half-wave plate (HWP) at a telescope aperture with a diameter of 400 mm. It is an essential for LiteBIRD to achieve the science goal because it significantly reduces detector noise and systematic uncertainties. The LiteBIRD PMU consists of a multi-layered sapphire as a broadband achromatic HWP and a mechanism to continuously rotate it at 88 rpm. The whole system is maintained at below 10K to minimize the thermal emission from the HWP. In this paper, we discuss the current development status of the broadband achromatic HWP and the cryogenic rotation mechanism.
LiteBIRD is a satellite project to measure the polarization of the CMB with an unprecedented accuracy. LiteBIRD observes all sky for three years at the sun-earth second Lagrange point. The goal of LiteBIRD is to observe the B-mode polarization at large angular scales and to measure the tensor-to-scaler ratio r with an accuracy less than 0.001, exploring the energy scale of the inflation. In order to mitigate the system 1/f noise and systematics, we plan to use continuous rotating half-wave plates (HWPs) as a polarization modulator at each aperture of two telescopes. One of the telescopes, called a low frequency telescope (LFT), covers the frequency range from 34 to 270 GHz, requiring the HWP to have a high modulation efficiency in the wide bandwidth. We employ a Pancharatnam-type achromatic HWP (AHWP) to achieve the broadband coverage. The AHWP consists of nine layer stacked HWPs with the optic axes mutually rotated by the angles optimized for the LFT bandwidth. In this paper, we report our development status of the nine layer AHWP and measurement results on the modulation efficiency and the phase as a function of frequency.
The LiteBIRD satellite aims at detecting a signature imprinted on the cosmic microwave background (CMB) by the primordial gravitational wave predicted in inflation, which is an exponentially expanding era before the hot big bang. The extraction of such weak spiral polarization patterns requires the precise subtraction of our Galaxy’s foreground emission such as the synchrotron and the dust emission. In order to separate them from the CMB by using their spectral shape differences, LiteBIRD covers a wide range of observing frequencies. The main telescope, Low Frequency Telescope (LFT), covers the CMB peak frequencies as well as the synchrotron emission. Based on the required sizes of optical elements in the LFT, an order of one meter, the telescope will consist of reflectors rather than lenses since the latter is limited in size availabilities of the corresponding materials. The image quality analysis provides the requirements of reflector surface shape errors within 30um rms. The requirement on surface roughness of 2μm rms is determined from the reflectance requirement. Based on these requirements, we have carried out tradeoff studies on materials used for reflectors and their support structures. One possibility is to athermalize with aluminum, with the expected thermal contract of 0.4% from room temperature to 4-10 K. Another possibility is CFRP with cyanate resin, which is lighter and has negligibly small thermal contraction. For the reflector surface shape measurements including in low temperature, photogrammetry is a strong candidate with suitable accuracy and dynamic range of measurements.
We propose a novel wavefront sensor for radio telescopes with a point diffraction interferometer. A point-like object is set at a pupil plane and the electric field at the focal plane is measured. A receiver dedicated to the novel sensor is prepared which has delay lines to make interferograms. A procedure to estimate the electric field at the pupil is shown analytically. Numerical simulation reveals that the proposed system allows us to measure the phase of the electric field at the pupil with a precision of about λ/28.
LiteBIRD is a next generation satellite aiming for the detection of the Cosmic Microwave Background (CMB) B-mode polarization imprinted by the primordial gravitational waves generated in the era of the inflationary universe. The science goal of LiteBIRD is to measure the tensor-to-scaler ratio r with a precision of δr < 10-3♦, offering us a crucial test of the major large-single-field slow-roll inflation models. LiteBIRD is planned to conduct an all sky survey at the sun-earth second Lagrange point (L2) with an angular resolution of about 0.5 degrees to cover the multipole moment range of 2 ≤ ℓ ≤ 200. We use focal plane detector arrays consisting of 2276 superconducting detectors to measure the frequency range from 40 to 400 GHz with the sensitivity of
3.2 μK·arcmin. including the ongoing studies.
LiteBIRD aims to detect the footprint of the primordial gravitational wave on the Cosmic Microwave Background (CMB) in a form of polarization pattern called B mode. In order to separate CMB from the Galactic emission, our
measurements cover 35 GHz to 450 GHz. The LiteBIRD optics consists of two telescopes: a crossed Dragone type for
lower frequencies, which provides a compact configuration with a wide field of view, and a refractor type for higher frequencies. The whole optical system is cooled down to around 5 K to minimize the thermal emission. We use two kinds of approaches of designing calculations as well as the experimental confirmation particularly for the lower
frequency telescope.
A wide-field Nasmyth optical system that connects a planned 10-m Ritchey-Chrétien telescope to a submillimeter camera is reported. This diffraction-limited system has a 1-deg field of view at 850 GHz, filled with a more than 20,000-pixel camera. The system enables us to carry out large field surveys of distant galaxies within reasonable time scales. The size of the Nasmyth optics is reasonably compact and its cryogenic part including the vacuum window, cryogenic lens, and IR block filters can be built using existing technologies at a reasonable cost. This type of optical system can be applied for the optical design of millimeter, terahertz, and other submillimeter instruments.
Anti-reflection (AR) is very important for high-throughput optical elements. The durability against cooling is
required for the AR structure in the cryogenic optics used for mid-infrared astronomical instruments. Moth-eye structure is a promising AR technique strong against cooling. The silicon lens and grism with the moth-eye structure are being developed to make high-throughput elements for long-wavelength mid-infrared instruments. A double-sided moth-eye plano-convex lens (Effective diameter: 33 mm, Focal length: 188 mm) was fabricated. By the transmittance measurement, it was confirmed that its total throughput is 1.7± 0.1 times higher than bare silicon lenses in a wide wavelength range of 20{45 μm. It suggests that the lens can achieve 83±5% throughput in the cryogenic temperature. It was also confirmed that the moth-eye processing on the lens does not modify the focal length. As for the grism, the homogeneous moth-eye processing on blaze pattern was realized by employing spray coating for the resist coating in EB lithography. The silicon grism with good surface roughness was also developed. The required techniques for completing moth-eye grisms have been established.
We designed wide FoV (1 degree) Nasmyth optics which transformed the f/6 Nasmyth focus to f/1 at a 850GHz superconducting camera for a planning 10-m Ritchey-Chrétien telescope. This optical system consists of reflecting mirrors at room temperature and a refractive lens at 4K. It enables us to carry out wide FoV imaging observations at the diffraction limit (Strehl ratio < 0.89) with a more than 100,000 pixel camera equipped in a 10-m telescope. The size of this system is reasonably compact (whole size:1.6 mx3.3 mx2.6 m, cryogenic part:0.7 mx0.7 mx1.0 m). The cryogenic part of this system such as vacuum window, cryogenic lens and IR block filters can be made with existing technologies at reasonable cost. The optical system can extend to the millimeter wave and the terahertz domain.
We have developed a 30-cm submillimeter-wave telescope intended to survey the Milky Way in 500 GHz emission lines at the Dome Fuji station in Antarctic plateau. Transportability and low power consumption are required while keeping low system noise temperature for the operation in Antarctica. The telescope is designed to be divided into five components and to operate with less than 2.5 kW of electric power. Its receiver noise temperature is less than 85 K in SSB at 461 and 492 GHz. We succeeded in operating the telescope at -30◦C in laboratory that is a typical temperature of the Dome Fuji in summer.
We propose a new high contrast imager for Kyoto 4m segmented telescope called SEICA (Second-generation
Exoplanet Imager with Coronagraphic Adaptive optics), aiming at detection and characterization of selfluminous
gas giants within 10AU around nearby stars. SEICA is aggressively optimized for high performance
at very small inner working angle, 10-6 detection contrast at 0".1 in 1-hour integration. We start the on-sky
commissioning test in 2016 and the science observations in 2017. Since it is the first time to realize the highcontrast
imaging on the segmented telescope, SEICA is an important step toward future high contrast
sciences on Extremely Large Telescopes (ELTs). This paper presents an overall of the SEICA program and
the conceptual design for ultimate performance under given atmospheric conditions.
Mid-infrared, 25 - 45 microns, is a very important wavelength region to investigate the physics of lower temperature
environments in the universe. There are few transparent materials in the range of mid-infrared except
silicon. However, the reflection on a silicon surface reaches 30 % because of its high refractive index (~3.4).
To apply silicon to mid-infrared astronomical instruments, we need a way of antireflection and have adopted
a moth-eye structure. This structure keeps durable under cryogenic environments, which is advantageous to
mid-infrared instruments. We have fabricated three samples of the moth-eye structure on plane silicon surfaces
by electron-beam photo-lithograph and reactive ion etching. The structures consist of many cones standing on
silicon surfaces. We have substantiated the transmittance of 96 % or higher in the wide range of 20 - 50 microns
and higher than 98 % at the maximum. The transmittance of moth-eye surfaces, however, is theoretically expected
as 100 %. We have examined the discrepancy between the transmittance of the theory and fabrications
with electromagnetic simulations. It has been revealed that shapes of the cones and gaps at the bottom of the
cones seriously affect the transmittance. We have estimated a few tolerances for manufacturing the moth-eye
structures achieving sufficient transmittance of nearly 100 %.
We have been developing high-throughput optical elements with the moth-eye structures for mid-infrared optical
systems. The moth-eye structures are optimized for the wavelength of 25-45μm. It consists of cones with a
height of 15-20μm arranged at an interval of 5μm. They are formed on silicon substrate by electron-beam
lithography and reactive ion etching. As a verification of the usefulness of moth-eye, a double-sided moth-eye
silicon plane was fabricated. It shows a transmittance increase of 60% compared with the unprocessed silicon
plane. As the first trial of the moth-eye optical element, two silicon lenses with single-sided moth-eye were
fabricated. One is a plane-convex lens with the moth-eye on the convex surface. The size of the moth-eye
formed region is 30 mm x 30 mm. Its focal length is 186 mm. The other one is a biconvex lens with moth-eye
formed region of Φ 33 mm and a focal length of 94 mm. Uniform moth-eye pattern was fabricated especially
for the second lens sample. Imaging test with the first sample showed that neither image degradation nor focal
length variation was induced by the moth-eye fabrication. As a step to grism with moth-eye, a moth-eye grating
sample was fabricated. The grating pattern (Grating constant: 124.9μm, Blaze angle: 4 deg) was successfully
fabricated with anisotropic etching. Moth-eye patterns were fabricated on the grating surface. Although the
resulted moth-eye was successfully fabricated in the most regions, some non-uniformity was found. It can be
attributed to unevenness of resist coating, and improvement of coating method is needed.
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