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The Precursor Small Aperture Telescope (PreSAT) CMB polarimeter

Matthew A. Petroff, Zeeshan Ahmed, James J. Bock, Marion Dierickx, Sofia Fatigoni, David C. Goldfinger, Paul K. Grimes, Shawn W. Henderson, Kirit S. Karkare, John M. Kovac, Hien T. Nguyen, Scott N. Paine, Anna R. Polish, Clement Pryke, Thibault Romand, Benjamin L. Schmitt, Abigail G. Vieregg

TL;DR

The paper presents the Precursor Small Aperture Telescope (PreSAT) as an early, integrated testbed to validate prototype CMB-S4 components within a Bicep Array receiver. It outlines plans for a 95/155 GHz dichroic detector stack cooled to 100 mK with an ADR, a prototype CMB-S4 optics stack, and a revised readout system, all aimed at identifying and mitigating systematics that could limit B-mode searches. The work emphasizes full-stack laboratory testing, rigorous cryogenic and magnetic shielding evaluation, and field deployment at the South Pole to correlate lab results with on-sky performance, thereby reducing risk for the eventual CMB-S4 deployment. The initiative seeks to answer key questions about beam systematics, forebaffle coupling, data quality, and overall instrument efficiency to enable robust, high-sensitivity measurements of primordial gravitational waves, quantified by $r$.

Abstract

The search for the polarized imprint of primordial gravitational waves in the cosmic microwave background (CMB) as direct evidence of cosmic inflation requires exquisite sensitivity and control over systematics. The next-generation CMB-S4 project intends to improve upon current-generation experiments by deploying a significantly greater number of highly-sensitive detectors, combined with refined instrument components based on designs from field-proven instruments. The Precursor Small Aperture Telescope (PreSAT) is envisioned as an early step to this next generation, which will test prototype CMB-S4 components and technologies within an existing BICEP Array receiver, with the aim of enabling full-stack laboratory testing and early risk retirement, along with direct correlation of laboratory component-level performance measurements with deployed system performance. The instrument will utilize new 95/155GHz dichroic dual-linear-polarization prototype detectors developed for CMB-S4, cooled to 100mK via the installation of an adiabatic demagnetization refrigerator, along with a prototype readout chain and prototype optics manufactured with wide-band anti-reflection coatings. The experience gained by integrating, deploying, and calibrating PreSAT will also help inform planning for CMB-S4 small aperture telescope commissioning, calibration, and operations well in advance of the fabrication of CMB-S4 production hardware.

The Precursor Small Aperture Telescope (PreSAT) CMB polarimeter

TL;DR

The paper presents the Precursor Small Aperture Telescope (PreSAT) as an early, integrated testbed to validate prototype CMB-S4 components within a Bicep Array receiver. It outlines plans for a 95/155 GHz dichroic detector stack cooled to 100 mK with an ADR, a prototype CMB-S4 optics stack, and a revised readout system, all aimed at identifying and mitigating systematics that could limit B-mode searches. The work emphasizes full-stack laboratory testing, rigorous cryogenic and magnetic shielding evaluation, and field deployment at the South Pole to correlate lab results with on-sky performance, thereby reducing risk for the eventual CMB-S4 deployment. The initiative seeks to answer key questions about beam systematics, forebaffle coupling, data quality, and overall instrument efficiency to enable robust, high-sensitivity measurements of primordial gravitational waves, quantified by .

Abstract

The search for the polarized imprint of primordial gravitational waves in the cosmic microwave background (CMB) as direct evidence of cosmic inflation requires exquisite sensitivity and control over systematics. The next-generation CMB-S4 project intends to improve upon current-generation experiments by deploying a significantly greater number of highly-sensitive detectors, combined with refined instrument components based on designs from field-proven instruments. The Precursor Small Aperture Telescope (PreSAT) is envisioned as an early step to this next generation, which will test prototype CMB-S4 components and technologies within an existing BICEP Array receiver, with the aim of enabling full-stack laboratory testing and early risk retirement, along with direct correlation of laboratory component-level performance measurements with deployed system performance. The instrument will utilize new 95/155GHz dichroic dual-linear-polarization prototype detectors developed for CMB-S4, cooled to 100mK via the installation of an adiabatic demagnetization refrigerator, along with a prototype readout chain and prototype optics manufactured with wide-band anti-reflection coatings. The experience gained by integrating, deploying, and calibrating PreSAT will also help inform planning for CMB-S4 small aperture telescope commissioning, calibration, and operations well in advance of the fabrication of CMB-S4 production hardware.
Paper Structure (6 sections, 4 figures)

This paper contains 6 sections, 4 figures.

Table of Contents

  1. Introduction
  2. Optics
  3. Cryogenics
  4. Magnetic shielding and pickup
  5. Field Deployment
  6. Conclusions

Figures (4)

  • Figure 1: Experiment overview. Comparison of a Bicep Array (BA) receiver to a mockup of the PreSAT instrument. PreSAT will resemble a BA receiver but will utilize a 100mK adiabatic demagnetization refrigerator (backed by the BA sorption fridge) and feature CMB-S4 prototype optics, detectors, and time-division multiplexing (TDM) readout, among other potential changes.
  • Figure 2: Ray trace of a potential optics design incorporating, from left to right, two high-density polyethylene (HDPE) lenses and a curved focal surface with a radius of 1400mm. Flat filter elements are not included in this diagram.
  • Figure 3: Holography setup installed on a Keck Array receiver. To the left is the movable source on an x-y translation stage, while the fixed source is on the right.
  • Figure 4: Magnetic testing setup. Left: Helmholtz coils installed on PreSAT receiver. Right: Magnetometers (in solid circles) installed inside cryostat. A Bicep3 detector module (in dashed circle) and a CMB-S4 SQ1 test module (in dotted circle) are installed in the focal plane. The niobium cup is visible around the left and top edges.