The South Pole Telescope

TL;DR

This paper presents the design of the South Pole Telescope (SPT), a 10 m off-axis Gregorian instrument optimized for wide-area millimeter/submillimeter surveys of CMB temperature and polarization at the South Pole. It describes a fast, low-spillover optical system delivering a one-degree field of view at $\lambda = 2\ \mathrm{mm}$, with a large ground screen and a focal plane of ~1000 TES bolometers read by frequency-domain SQUID multiplexing, cooled by a closed-cycle refrigerator to around 250 mK. The initial science objective is a Sunyaev-Zel'dovich Effect (SZE) cluster survey over ~4000 deg$^2$, yielding thousands of clusters with a nearly redshift-independent mass selection that constrains the dark energy equation of state $w$. The paper also discusses site advantages, atmospheric-noise considerations, and multi-band foreground mitigation across bands from ~95 to ~350 GHz, noting that the SZE has a spectral null near $\nu \approx 220$ GHz and that the bands are chosen to separate CMB, SZE, KSZ/OV, and dusty-galaxy signals.

Abstract

A new 10 meter diameter telescope is being constructed for deployment at the NSF South Pole research station. The telescope is designed for conducting large-area millimeter and sub-millimeter wave surveys of faint, low contrast emission, as required to map primary and secondary anisotropies in the cosmic microwave background. To achieve the required sensitivity and resolution, the telescope design employs an off-axis primary with a 10m diameter clear aperture. The full aperture and the associated optics will have a combined surface accuracy of better than 20 microns rms to allow precision operation in the submillimeter atmospheric windows. The telescope will be surrounded with a large reflecting ground screen to reduce sensitivity to thermal emission from the ground and local interference. The optics of the telescope will support a square degree field of view at 2mm wavelength and will feed a new 1000-element micro-lithographed planar bolometric array with superconducting transition-edge sensors and frequency-multiplexed readouts. The first key project will be to conduct a survey over approximately 4000 degrees for galaxy clusters using the Sunyaev-Zel'dovich Effect. This survey should find many thousands of clusters with a mass selection criteria that is remarkably uniform with redshift. Armed with redshifts obtained from optical and infrared follow-up observations, it is expected that the survey will enable significant constraints to be placed on the equation of state of the dark energy.

Figures (13)

• Figure 1: (Left) rear view of the SPT at elevation $0^{\circ}$, and (right) side view at elevation $45^{\circ}$ with the outer ground shield.
• Figure 2: (Left) optical configuration of the SPT. Solid lines are the principal and marginal rays for the field center. Dashed and dot-dashed lines are for the edges of a $1^{\circ}$ diameter field. (Right) spot diagrams. Circles show the Airy disc at $\lambda=2$ mm.
• Figure 3: A cutaway view of the cold secondary and 10 K baffle optical configuration. Radiation from the primary enters through a foam vacuum window from the upper left, passing through an IR blocking filter that reduces radiative heat input on the 10 K system. The baffle is formed by two metal cones coated with a millimeter-wave absorber, with black annular rings attached to the cone wall near the 4 K lens and focal plane. For a Gaussian beam with 6 dB edge taper and a baffle emissivity of 0.5, ray-tracing of this design shows that less than 1% of the spillover power eventually exits the cryostat window.
• Figure 4: Montage image of a single 55-element TES spider bolometer wedge to show how an array of six identical wedges would look. The complete prototype array will have 330 bolometers and be 12 cm in diameter.
• Figure 5: Close up of a 55 element bolometer wedge. The sensors are constructed with an Al/Ti proximity effect sandwich. Webs are metalized with gold for microwave absorption. Suspended spider-web absorbers are fabricated from 1 $\rm{\mu m}$ thick silicon nitride. The membrane is released from the front side using a gaseous xenon diflouride etch. Bolometers are 5 mm diameter with 0.5 mm long legs. Wiring layer is superconducting aluminum. This array was fabricated in the U.C. Berkeley microfabrication facility.