UMICH-2015: Instrumentation II break-out session 2

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Goals: There are three goals for this session: (1) to identify requirements for cold lenses and polarization modulators; (2) to sketch the status of current technologies; and (3) to identify what work needs to go on to ready the technologies needed for CMB-S4.

Cold Optics


Current optical designs using cold lenses include: BICEP/ Keck style refractors using lenses up to ~60 cm in diameter; POLARBEAR/Simons and SPT-3G style lens-fed telescopes using large diameter (>45 cm) lenses; and the ACTPol lens-fed telescope that uses multiple optics tubes each comprised of three smaller lenses (< 45 cm) to feed multiple detector arrays. The Simons Array, SPT-3G and AdvACT are all going to use multichroic detectors with ratio bandwidth up to 3:1. The full S-4 instrument is likely to cover 20-300 GHz. At this point all three designs are candidates for CMB-S4.

Questions for discussion:

  1. Do these designs encompass all the design space we need to consider to set requirements on cold dielectric optics?
  2. What are the tradeoffs between the POLARBEAR / SPT-3G approach for large telescopes versus the ACTPol approach?
  1. What are the requirements?
  • Bandwidth requirement?
  • 3:1 is natural based on the multichroic polarimeters, but 5:1 might be desirable if multiple types of detectors are behind a single set of lenses. Debate:
  • Emissivity requirement?
  • Mapping speed is roughly linear in optical efficiency. How much can we tolerate? This depends on the optical design.
  • Control of loading
  • How do we specify the requirements on polarization transfer and its frequency dependance?
  • What simulations do we need to answer this? What do different experiments show on this front?
  • What are the requirements for the control of reflections?
  • this is related to the optical efficiency, ghosting, and cross-polarization. It is design dependent. What do different experiments show? What do we do to answer this?
  • Scattering?
  • what are the requirements on scattering?
  • Other requirements?

Existing Technologies: Required Work and Studies

What are the existing technologies and where do they stand?

Polarization Modulators

Bold Statement: we need polarization modulators to maximize the return from low ell. Debate.

Demonstrated Performance of Deployed Modulators



BICEP2 pair diff

Quantifying Requirements

  • requirements for reflections
  • loss of signal
  • modulation of the atmosphere
  • polarization dependance
  • ghosting
  • requirements for emission
  • loading
  • polarization dependent emission
  • warm vs cold modulator?
  • where does the modulator fit into the optical design
  • what is the drive mechanism
  • how big do the modulators have to be

Required Work and Studies

  • large sapphire HWPs [slide]
  • metamaterial silicon [slide]
  • metamaterial on platic
  • VPMs [slide]
  • Cryogenic continuously rotating HWP / Mechanical (slide)
  • other ideas


  • Define an analytic framework and tools for what systematic effects to consider in evaluating optical designs, such as emission, transmission, scattering, birefringence, beam squint, etc.
  • Measure the loss tangent of alumina down to 4 K (well-measured at 100 K and pretty good), and perhaps other materials under consideration.
  • Make shareable (or at least comparable) tools for quantifying the performance of an entire optics train, including emission/loading, transmission/efficiency and bandwidth limitations.
  • Tom, Shaul, Akito to make a punchlist of what effects to consider when comparing methods of polarization modulation, including pair differencing, such as bandwidth limitations, I--Q improvements, stability, etc.
  • Make machinery for doing simulations for modulators, including effects of different atmospheric conditions at different sites, different scan strategies,vs what $1/f$ knee is achievable.
  • In the next 6 months provide data from the variety of new modulation techniques being tested now (including eg metamaterial HWPs, VPMs, multi-layer HWPs>)