Optical measurements of the silicon vacuum window with anti-reflective sub-wavelength structure for ASTE Band 10.

For millimeter and submillimeter-wave astronomy, it is highly desirable to have vacuum windows within the receiver cryostat that exhibit low reflection, low loss, and a wide bandpass. The use of antireflective (AR) sub-wavelength structures (SWSs) on substrates has expanded the possibilities for creating new vacuum windows. Recently, a novel method of fabricating AR SWS on a silicon-on-insulator wafer has been proposed, and a vacuum window with a two-layer AR SWS has been developed for use with the Atacama Submillimeter Telescope Experiment Band 10 receiver. To thoroughly assess the characteristics of the silicon window sample, we conducted transmittance measurements using terahertz time-domain spectroscopy, and noise and beam measurements using an Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 receiver. We found that the silicon window sample exhibits characteristics comparable to the quartz window of the ALMA Band 10 receiver. The result strongly encourages applications of AR silicon windows in receivers with wider bandwidths.

Analytical expression of aperture efficiency affected by Seidel aberrations.

The effect of aberrations on the aperture efficiency has not been discussed analytically, though aberrations determine the performance of a wide field-of-view system. Expansion of a wavefront error and a feed pattern into a series of the Zernike polynomials enables us to calculate the aperture efficiency. We explicitly show the aperture efficiency affected by the Seidel aberrations and derive the conditions for reducing the effects of the spherical aberration and coma. In particular, the condition for coma can reduce a pointing error. We performed Physical Optics simulations and found that, if the Strehl ratio is higher than 0.8, the derived expression provides the aperture efficiencies with a precision of <2%.

Wide field-of-view crossed Dragone optical system using anamorphic aspherical surfaces.

A side-fed crossed Dragone telescope provides a wide field of view. This type of telescope is commonly employed in the measurement of cosmic microwave background polarization, which requires an image-space telecentric telescope with a large focal plane over broadband coverage. We report the design of a wide field-of-view crossed Dragone optical system using anamorphic aspherical surfaces with correction terms up to the 10th order. We achieved a Strehl ratio larger than 0.95 over 32×18 square degrees at 150 GHz. This design is an image-space telecentric and fully diffraction-limited system below 400 GHz. We discuss the optical performance in the uniformity of the axially symmetric point-spread function and telecentricity over the field of view. We also address the analysis to evaluate the polarization properties, including the instrumental polarization, extinction rate, and polarization angle rotation. This work is a part of a program to design a compact multi-color wide-field-of-view telescope for LiteBIRD, which is a next-generation cosmic microwave background (CMB) polarization satellite.

Real-time point-diffraction interferometer and its analytical formulation.

We propose a novel wavefront sensor and study its performance with an analytical formulation. The sensor has a polarizing point-diffraction beam splitter. Using transmitted and reflected beams, we can build a real-time point-diffraction interferometer with high precision and efficiency. Our analytical studies reveal that wavefront errors might be measured incorrectly and that less precise estimates of wavefronts appear as the pinhole radius Rpin is increased. An investigation of propagating uncertainties shows that the wavefront measurement can be calibrated by estimating the pinhole effects and the polarizing properties with a precision of a few percent. Based on these studies, Rpin should be smaller than half of the Airy disk for better performance.

Measurement of complex amplitude with a point-diffraction interferometer.

We propose a pupil-plane wavefront sensor to measure instantaneous phase and amplitude aberrations, developing the configuration of the point-diffraction interferometer proposed by Imada et al. [Appl. Opt. (2015), accepted]. The previous configuration allows us to instantaneously acquire four phase-shifted interferograms, from which cosine and sine functions of the relative phase difference between the test and reference beams can be derived. Here, because the wavefront phase can be reconstructed from only its sine function when the aberration of the input beam is smaller than half that of the sensing wavelength, we directly measure the intensities of the test and the reference beams instead of two of the four interferograms. Using this proposed configuration enables reconstruction of not only the wavefront phase but also its amplitude. The conceptual design of the proposed sensor is described, and its performance is evaluated compared with the original point-diffraction interferometer through numerical simulations.