Small Aperture Telescopes - Parallel

J. M. Kovac, A. Kusaka

A. Vieregg, S. Paine, P. Grimes, C. Pryke

J. Saba, B. L. Schmitt, T. Norton, K. Karkare

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Tuesday March 9, 3:30-5:00 ET

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Parallel Agenda

SAT Baseline Design Discussion:

• Baseline Design Overview and Drivers - John Kovac
• Optics - Abby Vieregg / Paul Grimes / Scott Paine
• Cryostats - Akito Kusaka / Joe Saba
• Mount - Clem Pryke
• Groundshields and Exterior Baffles - Ben Schmitt
• Calibration - Kirit Karkare

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SAT Zemax Lens Design Updates - Fred Matsuda / Tony Stark

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SAT Calibration Plan Updates - Kirit Karkare

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Baseline Design Overview & Drivers

John Kovac

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

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• Science Requirement (SR1) driving SAT design:
• r < 0.001 at 95%, or detect r > 0.003 at 5σ confidence (from PLR)

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• This means achieving < 10 nK for:
• foreground separation
• raw sensitivity
• systematic control

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…All are made harder at degree scales by

1/f noise & red-spectrum confusion signals

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Why are SATs required?

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DSR (arXiv:1907.04473), Fig 6

• Intrinsic advantages: efficient to integrate/test/deploy many detectors; stability of cryogenic optics; aperture filling calibrators; aperture filling modulation; superior sidelobe control and shielding
• ONLY proven approach for deep r measurement
• SAT pBD builds on proven experience: BICEP-style cryogenic refractors, while incorporating new technologies (e.g. dichroic horns, dilution fridges, and (for Chile) SO-derived HWPs) where they promise low risk & improved performance margin.

(DSR Sec. 4.4)

10 nK

SAT Overview / Intro to WBS

CMB-S4 Collaboration Workshop - March 8th-12th 2021

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SAT Design Drivers

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat System, Optics Tubes, Integration & Test

Telescope Mount & Ground Shield

30/40 GHz

85/145 GHz

95/155 GHz

220/270 GHz

SAT Overview

CMB-S4 Collaboration Workshop - March 8th-12th 2021

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SAT Overview (the project view)

CMB-S4 Collaboration Workshop - March 8th-12th 2021

1.07 SAT Requirements (now tracked in Jama)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Preliminary Baseline Design Summary

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• Preliminary Baseline Design for SATs
• 6 mounts + groundshields at Pole
• 6 cryostats, each with 3 optics tubes (18 total)
• Optics design heritage from BA and SO
• Option of HWPs, allowing additional use in Chile
• North American Integration and Test

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Receiver Design Overview

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• SAT design will draw directly on design heritage from BICEP3, BICEP Array, and Simons Observatory Small Aperture Telescope Receivers

CMB-S4 Collaboration Workshop - March 8th-12th 2021

SAT Design Overview

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• “In considering changes for the baseline design compared to the small-aperture telescopes that have achieved previous deep r measurements, we have incorporated new technologies—e.g. dichroic detectors, dilution refrigerators, and (if small-aperture telescopes are deployed to Chile) cryogenic half-wave plate modulators—where there is a consensus that they promise improved performance while adding little technical risk.”
• In design choices we attempt to distinguish
• engineering issues: those that can be fully developed and demonstrated in the lab to retire risk
• science issues: those whose impact on successfully meeting the measurement and science requirements must be judged with comparison to direct experience of making deep B-mode maps.
• Most aspects of cryostat design are primarily engineering issues because we are confident our design choices can be fully validated in the lab.
• Examples of science issues include beam and sidelobe optical performance, polarization modulation approach, ground pickup and shielding, and other systematic effects where instrumental and environmental couplings are complex enough to require field validation for any fundamental change of approach. For design choices that impact these issues we have endeavored to stay close to and to build upon proven experience, guided by comparative testing.

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Optics

Abby Vieregg, Paul Grimes, Scott Paine

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cold Optics

• Critical component for reaching target σ(r)
• Need to control polarization systematics and optical efficiency

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• While there is lots of heritage to draw on, this is a major R&D item for SATs and laying out baseline configuration
• Mass production capability is an added requirement to a S4-ready technology.

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• On our timeline: assessment for identification of baseline and alternative optics choices

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• Many components in common with LAT cold optics

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cold Optics Heritage

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BICEP3

Simons Observatory

HDPE Window

IR Filters

Half-wave Plate

Aperture Stop

LPE Filters

Silicon Lens 1

Silicon Lens 2

Silicon Lens 1

LPE Filters + Focal Plane

Alumina AR Example (Illinois)

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Silicon AR Example (Michigan/Chicago)

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AR coating is a major R&D item.

Technology shared with LAT.

Optics design based on matured study.

CMB-S4 Collaboration Workshop - March 8th-12th 2021

SAT lens design

• ZEMAX candidate baseline designs advanced by Fred Matsuda, Tony Stark
• Recent development: slightly curved focal surface (r = 2.4 m) dramatically improves performance of two-lens designs

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HDPE

Alumina

Silicon

490 mm

490 mm

422 mm

422 mm

490 mm

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Baseline choices for lens materials

• Silicon is the lowest-risk choice for the highest-frequency bands.
• The dielectric loss is known to be low for high-purity silicon.
• Broadband AR-coatings have been demonstrated.
• HDPE for 30/40, 85/145, 95/155 GHz bands.
• There is significant experimental heritage with HDPE and alumina, but in both cases the dielectric loss is poorly constrained at higher frequencies.
• HDPE is the safest baseline choice (cost, AR-coating, marginally better loss and IR filtering).
• Trade for the HF band: low loss and fewer detectors, vs. more pixels and (potentially) higher loading.
• Strong motivation to improve cold loss measurements of all 3 materials.
• All designs include a 10 mm alumina filter, highlighting a continued need for alumina AR-coating solutions.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Dielectric Loss (better data needed!)

• CMB-S4’s HDPE baseline 40% thinner than shown
• CMB-S4’s alumina and silicon lenses are ~25% thinner
• Baseline alumina IR filter (10mm) is low risk: should be < 7% loss at 300 GHz

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Marion Dierickx

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Baseline optics stack

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Thermal loading

• IR model of baseline design by Lingzhen Zeng.
• DR has 25 mW / 400 uW cooling power at the 1K / 100 mK stages.
• Option to include a 1K nylon filter to increase margin on 100 mK stage.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

RT-MLI and low-pass edge (LPE) Filters

• Zotefoam scattering filters
• Thermal and LPE filters fabricated at Cardiff
• Up to 675 mm diameter thermal filters available
• Up to 500 mm diameter LPE filters available

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Carole Tucker

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Keith Thompson

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Anti-Reflection Coating of Alumina

• Various technologies deployed or in development: Laminate (plastic and epoxy), epoxy, thermal spray, laser, metamaterial

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Laminate Epoxy coating

(Thompson/Dierickx)

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Epoxy coating

(Charlie Hill)

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Laminate Plastic coating

(Joaquin Vieira)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Anti-Reflection Coating of Alumina

• Various technologies in development: Laminate (plastic and epoxy), epoxy, thermal spray, laser, metamaterial

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Thermal Spray coating

(Oliver Jeong)

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Laser Ablation-based coating

(Tomotake Matsumura)

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Metamaterial coating

(Joey Golec)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Anti-Reflection Coating of Silicon

• Metamaterial AR coating of silicon lenses at U. Michigan
• Technology deployed for Adv ACTPol
• Up to 46 cm diameter lens fabrication in process for Simons Observatory SAT

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Joey Golec

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryogenic Baffles - Sidelobe Control

• Deep cryogenic baffles with pyramidal-shaped absorbers made from carbon-loaded plastic material in 1K optics tube for sidelobe suppression

• HR-10-coated baffle rings in 4K optics tube suppress sidelobes measured in previous BICEP receivers

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Simons Observatory

BICEP Array

Images from Kenji Kiuchi

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Option of Cryogenic Half-Wave Plate

• 50 cm optical diameter cryogenic (45K) half-wave plate system
• 3-layer sapphire with AR coatings (similar to alumina)
• Currently in development for Simons Observatory SAT
• HWP in CMB-S4 baseline, but baseline SAT cryostat is compatible
• Would require stopping down aperture from 56 → 44cm, with systematics tradeoffs

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PB2/Simons Array

SO (draws from PB2/Simons Array heritage)

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostats

Akito Kusaka, Joe Saba

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostats

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Design based on proven heritage, and maturity rapidly improving toward cryostat prototyping after CD-1.

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Dilution Refrigerator to be used.

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Receiver Cross-Section

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Dilution Refrigerator

Pulse Tube

Receiver Tube

(optics tube and focal plane)

Bus Cryostat

CMB-S4 Collaboration Workshop - March 8th-12th 2021

SAT Cryostats Draw on Successful Heritage

Example:

Cryogenic Bus Assembly is based on BICEP Array Heritage

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Matrix Comparison of DR Commercial Firm Outreach

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Overview of DR Gas Handling System (Janis Example)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat design progress

In the process of advancing the design maturity.

Several key interfaces w/ other WBS and other subcomponents are identified.

• Readout : Hermetic flanges and wirings are unlikely to be the constraining factor. -Room for 72 warm readout modules underneath each optics tube
• Optics Tube spacing :
• Warm baffling wants the tubes be spaced apart.
• The existing mount design constrains the maximum cryostat OD and optics tube OD.
• Optics Tube OD : interfacing to the lens diameter, radiation shield, cryo baffling.
• Detector modules : the focal plane packing density (gap between modules) is key interface to the optics, the optics tube diameter, and the detector module WBS.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat design progress

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SAT WBS Internal Interfaces N^2

✓

✓

✓

✓

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat design progress

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Interface: OT diameter/spacing vs. mount vs. warm baffle

Interface: Readout vs. optics tube diameter

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat design progress

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Optics tube / radiation shield vs. lens OD vs. focal plane

Focal plane diameter vs. module spacing

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat design progress

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2400 mm radius FP

Nb magnetic shielding

Low pass edge filter (if required)

Fred Matsuda’s Zemax ray traces (HDPE design for LF & MF)

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat Prototype

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A prototype is the crucial step toward CD-2, validating several aspects of the design:

• Suite 1, thermal modeling (incl. cooldown time) and validation.
• Suite 2, integration procedure, magnetic shielding, and RF shielding and pick-up.
• Suite 3 (new), test pre-production detector modules with readout electronics.

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Implementation

• When in the project, duration?
• Design completion post CD-1. Fab and suite 1 completion takes ~1 year.
• Suite 2 takes ~8 months.
• Suite 3 is under planning, and dependencies with other WBS to be resolved.
• What does it depend on? (ie, what needs to be done before this)
• Sufficient maturity of the cryostat design including assembly order and integration strategy.
• Suite 3 requires detector/module/readout pre-production availability.
• What else depends on prototyping? (ie, what will be unlocked by completion of each suite of the validation described above)
• Suite 1 completion unlocks the start of the cryostat fabrication.
• Suite 2 and 3 currently does not unlock anything in the current P6, but they likely to unlock some of the later-produced components and assembly strategy (incl. grounding etc.).

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Cryostat No. Hemisphere Integration & Test

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Keck array: parallel integration

Simons Observatory: 4 cryostats integrated in parallel

The field and the team has experience in integrating multiple cryostats in parallel.

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Mount

Clem Pryke

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Baseline Design/Requirements - Mounts

• Baseline Design:
• Draws on BICEP Array mount heritage
• BICEP Array mount successfully deployed between Nov 2019 - Jan 2020, and now operating at the Amundsen-Scott South Pole Station

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• S4-SAT Mount Requirements:
• Needs to accommodate a single three-tube SAT receiver, rather than four individual BA-type receivers (design updates to continue to allow rear-loading underway)
• SAT receiver must include mounting points for strut interfaces to the mount structure (similar to BICEP Array)
• Mount must also accommodate the DR gas handling system and provide sufficient mounting volumes for warm readout interfaces.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Heritage: BICEP Array Mount Integration and Deployment

BICEP Array Mount @ UMN (Aug 2020)

BICEP Array Mount @ Pole (Jan 2020)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

SAT Mount Parameters

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

SAT Mount Overview

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DR Gas Handling System

Flexible Environmental Enclosure

Interface to SAT Tower (Sites)

SAT Receiver

Receiver - Mount Interface Struts

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Receiver Integration with SAT Reference Mount

Ex: Mount-Receiver Strut Interface from BICEP Array

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SAT Receiver in this configuration held rigidly at the top and bottom of cryostat assembly by struts connected to mount azimuth assembly.

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Receiver Integration with SAT Reference Mount

COLINEARITY OF SAT RECEIVER

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• Colinearity is not a major issue as each SAT optics tube has its own pointing model. The main pointing requirements are on the rigidity (flexure < 1 arcmin) and repeatability (variation < 20 arcsec) of each SAT tube’s pointing.

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• Because of the large SAT beam sizes, these requirements are fairly relaxed compared to most telescopes, are met by this reference mount design.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

DR Gas Handling System Integration with SAT Reference Mount

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SAT Receiver and “unpackaged” DR Gas Handling System installed on telescope mount.

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Telescope Mount on Tower

More to come in South Pole Sites session Thursday

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Ground Shields and Exterior Baffles

Ben Schmitt

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Ground Shield & Exterior Baffling

• Controlling far-sidelobe response to ~300 K ground is critical - to constrain r we are attempting to measure nK-level fluctuations!
• SATs have typically used multiple levels of shielding to prevent far sidelobes from coupling to the ground/Galaxy

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• “Double diffraction” criterion: ground radiation must diffract twice before entering any optics tube window

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• For 3-tube SAT receiver, studied the sizes of various shields needed to enforce the “double-diffraction” criterion
• Forebaffle: co-moving with Az / El / Boresight, can be absorptive or reflective
• Ground shield: fixed, reflective

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Ground Shield

Under double-diffraction criterion, at 50 degrees minimum elevation, we find that the SAT receiver can be shielded with:

• Forebaffle: 1.75 m tall, 0.8 m radius
• Ground Shield: 5.9 m tall, 12.4 m radius

Smallest achievable ground shield size for a 2-shield scenario (given the maximum forebaffle

size allowed by the SAT receiver design)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Ground Shield

Cylindrical Warm Forebaffles: integrated part of optics design, key element of systematics control.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Calibration

Kirit Karkare

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(more discussion later)

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Calibration Apparatus

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Based on experience from previous generations of SATs, design specialized hardware to

• Validate SAT performance during commissioning
• Do responsivity, beam shapes, etc. look reasonable?
• Measure instrument parameters to well-defined precision, in lab and in situ
• Bandpasses, beam shapes, polarization angles…
• Probe potential instrumental systematics relevant to the r measurement
• T->P, E->B, sidelobe pickup…

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Calibration should be built into the SAT design and schedule!

• Mounting points (in lab and in situ), cranes...
• Should understand measurement SNR to plan calibration campaigns

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Calibration Apparatus

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~200 m

Far-field measurements using a redirecting flat mirror and source on mast

Thermal chopper

24” aperture

Far-field flat mirror

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Calibration Apparatus

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Amplified broad spectrum noise source

Rotating polarized source (referenced to gravity)

Far sidelobe measurements

FTS measurements

(multi-axis optical coupling)

CMB-S4 Collaboration Workshop - March 8th-12th 2021

Path to CD-1 (this year’s priorities)

Ongoing and upcoming key activities to advance design.

• Cryostat design
• Hybrid design in progress, burning down engineering risks (this type of cryostat has not been deployed before)
• By CD-1, conceptual design mature, ready for prototyping
• Cryostat prototyping
• This R&D will burn down risks in cryogenics and assembly process.
• Cold Optics
• Addresses technical risks in material losses, scattering, absorption, and AR technologies, as well as the production throughput of the AR coating (Alumina, HDPE and Silicon)
• By CD-1, process risks retired & conceptual design mature, ready for prototyping
• Optics Prototyping
• Beam and sidelobe are the top performance risks in the project. This prototyping R&D will burn these risks.

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Conclusions

• Thanks to lots of collaboration input, SAT design maturity has advanced
• We have a good understanding of key/driving interfaces, especially dimensional ones
• Allows parallel design development of cryostat, optics, shields, mount, calibrators

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• PBDR text will continue to be refined
• We solicit suggestions from the collaboration
• (We need to add appropriate references to heritage work! Please help us.)
• SAT WG meetings every other Monday
• Great chance to contribute to technical design choices

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Rest of this SAT parallel will be dedicated to:

• SAT Zemax Lens Design Updates - Fred Matsuda / Tony Stark
• SAT Calibration Plan Updates - Kirit Karkare

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Backup Slides

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CMB-S4 Collaboration Workshop - March 8th-12th 2021

Schematic of SAT Interfaces with other L2's

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CMB-S4 Collaboration Workshop - March 8th-12th 2021