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.
- 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?
- loaded epoxy coatings and alumina and silicon lenses
- metamaterial coatings on silicon lenses
- Other approaches?
Bold Statement: we need polarization modulators to maximize the return from low ell. Debate.