UMICH-2015: Instrumentation II break-out session 2
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.
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:
- Do these designs encompass all the design space we need to consider to set requirements on cold dielectric optics?
- What are the tradeoffs between the POLARBEAR / SPT-3G approach for large telescopes versus the ACTPol approach?
- 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?
- what are the requirements on scattering?
- Other requirements?
Existing Technologies: Required Work and Studies
What are the existing technologies and where do they stand?
Bold Statement: we need polarization modulators to maximize the return from low ell. Debate.
Demonstrated Performance of Deployed Modulators
- requirements for reflections
- loss of signal
- modulation of the atmosphere
- polarization dependance
- requirements for emission
- 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
- 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>)