South Pole Telescope optics

Fabrication of a monolithic 5 m aluminum reflector for millimeter-wavelength observations of the cosmic microwave background

We have demonstrated the fabrication of a monolithic, 5 m diameter, aluminum reflector with 17.4 µm root-mean-square surface error. The reflector was designed to avoid the problem of pickup due to scattering from panel gaps in a large, millimeter-wavelength telescope that will be used for measurements of the cosmic microwave background.

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 optical design for a 60 m submillimeter telescope

We present a practical wide-field optical design for a 60 m aperture submillimeter telescope, which is currently under conceptual design study in China. The telescope is specified to operate over a wavelength range of 0.65-3 mm and provide a wide field of view (FOV) of 1° in diameter. We designed an F/6 Ritchey Chrétien (RC) system with a quasi-planar tertiary corrector, which cancels all spherical, coma, and astigmatism aberrations. It also achieves a good balance among the mirror sizes, central obscuration, and focal-plane curvature. The problems of focal surface curvature and nontelecentricity are treated in the subfield instrumental design, which employs a simple silicon wedge prism to obtain flat and telecentric focal planes for each subfield instrument module. Our studies show that by such a design, more than ${{10}^5}$10⁵ detector pixels can be efficiently and uniformly fed at the shortest wavelength band with Strehl ratios above 0.85 across the entire 1° FOV. Several practical issues related to the telescope optics are also discussed.

Broadband, millimeter-wave antireflection coatings for large-format, cryogenic aluminum oxide optics

We present two prescriptions for broadband ($ {\sim} 77 - 252\;{\rm GHz} $), millimeter-wave antireflection coatings for cryogenic, sintered polycrystalline aluminum oxide optics: one for large-format (700 mm diameter) planar and plano-convex elements, the other for densely packed arrays of quasi-optical elements-in our case, 5 mm diameter half-spheres (called "lenslets"). The coatings comprise three layers of commercially available, polytetrafluoroethylene-based, dielectric sheet material. The lenslet coating is molded to fit the 150 mm diameter arrays directly, while the large-diameter lenses are coated using a tiled approach. We review the fabrication processes for both prescriptions, then discuss laboratory measurements of their transmittance and reflectance. In addition, we present the inferred refractive indices and loss tangents for the coating materials and the aluminum oxide substrate. We find that at 150 GHz and 300 K the large-format coating sample achieves $ (97 \pm 2)\% $ transmittance, and the lenslet coating sample achieves $ (94 \pm 3)\% $ transmittance.

Detection of CMB-Cluster Lensing using Polarization Data from SPTpol

We report the first detection of gravitational lensing due to galaxy clusters using only the polarization of the cosmic microwave background (CMB). The lensing signal is obtained using a new estimator that extracts the lensing dipole signature from stacked images formed by rotating the cluster-centered Stokes QU map cutouts along the direction of the locally measured background CMB polarization gradient. Using data from the SPTpol 500  deg^{2} survey at the locations of roughly 18 000 clusters with richness λ≥10 from the Dark Energy Survey (DES) Year-3 full galaxy cluster catalog, we detect lensing at 4.8σ. The mean stacked mass of the selected sample is found to be (1.43±0.40)×10^{14}M_{⊙} which is in good agreement with optical weak lensing based estimates using DES data and CMB-lensing based estimates using SPTpol temperature data. This measurement is a key first step for cluster cosmology with future low-noise CMB surveys, like CMB-S4, for which CMB polarization will be the primary channel for cluster lensing measurements.

Designs for a large-aperture telescope to map the CMB 10× faster

Current large-aperture cosmic microwave background (CMB) telescopes have nearly maximized the number of detectors that can be illuminated while maintaining diffraction-limited image quality. The polarization-sensitive detector arrays being deployed in these telescopes in the next few years will have roughly 10⁴ detectors. Increasing the mapping speed of future instruments by at least an order of magnitude is important to enable precise probes of the inflationary paradigm in the first fraction of a second after the big bang and provide strong constraints on cosmological parameters. The CMB community has begun planning a next generation "Stage IV" CMB project that will be comprised of multiple telescopes with between 10⁵-10⁶ detectors to pursue these goals. This paper introduces the new crossed Dragone telescope and receiver optics designs that increase the usable diffraction-limited field-of-view, and therefore the mapping speed, by an order of magnitude compared to the upcoming generation of large-aperture instruments. Polarization systematics and engineering considerations are presented, including a preliminary receiver model to demonstrate that these designs will enable high efficiency illumination of >10⁵ detectors in a next generation CMB telescope.

Large-aperture wide-bandwidth antireflection-coated silicon lenses for millimeter wavelengths

The increasing scale of cryogenic detector arrays for submillimeter and millimeter wavelength astrophysics has led to the need for large aperture, high index of refraction, low loss, cryogenic refracting optics. Silicon with n=3.4, low loss, and high thermal conductivity is a nearly optimal material for these purposes but requires an antireflection (AR) coating with broad bandwidth, low loss, low reflectance, and a matched coefficient of thermal expansion. We present an AR coating for curved silicon optics comprised of subwavelength features cut into the lens surface with a custom three-axis silicon dicing saw. These features constitute a metamaterial that behaves as a simple dielectric coating. We have fabricated silicon lenses as large as 33.4 cm in diameter with micromachined layers optimized for use between 125 and 165 GHz. Our design reduces average reflections to a few tenths of a percent for angles of incidence up to 30° with low cross polarization. We describe the design, tolerance, manufacture, and measurements of these coatings and present measurements of the optical properties of silicon at millimeter wavelengths at cryogenic and room temperatures. This coating and lens fabrication approach is applicable from centimeter to submillimeter wavelengths and can be used to fabricate coatings with greater than octave bandwidth.

Millimeter-wave center of curvature test for a fast paraboloid

We describe a technique for measuring the surface profile of a radio telescope with a fast paraboloidal primary. The technique uses a sensor, at the center of curvature of the primary, consisting of a millimeter-wave source and an array of receivers to measure the field in the caustic. The sensor is mounted on the telescope enclosure and it moves with the telescope, so the measurements can be used for continuous, slow, closed-loop control of the surface. Sensor decenter and despace errors, due to wind buffeting and thermal deformation of the sensor support, do not compromise the surface measurements because they result in profile errors that are mainly translation, which has no effect on astronomical observations, or tilt and defocus, which can be measured using astronomical sources. If the position of the sensor is known to 20 μm rms, the surface can be measured to ~1  μm rms at λ=3 mm.