Periodic Reporting for period 1 - CMBeam (Transforming cryogenic optics for cosmic microwave background experiments)
Reporting period: 2022-11-01 to 2025-04-30
The cosmic microwave background (CMB) has played a foundational role in the establishment of the standard model of cosmology. Future experiments mapping the microwave sky endeavour to shed light on the infant universe and resolve the neutrino hierarchy problem. This effort will be led by the flagship ground-based experiment of the decade, the Simons Observatory. Meanwhile, critical decisions are being made concerning the design of an upcoming next-generation CMB satellite mission and balloon-borne missions such as LiteBIRD and Taurus. The success of these experiments is dependent on our ability to increase detector sensitivities across a wide range of frequencies while controlling complicated instrumental systematic effects.
All cosmological results from CMB experiments are based on an extensive calibration process, which includes a correction for the optical response of our telescopes. Without this so-called beam correction, different experiments would generate wildly conflicting and incorrect cosmological results. The 10–100 nK polarized signals that we are now searching for in our data call on calibration campaigns to deliver unprecedented understanding of both the frequency and beam response of our telescopes, as even a minuscule optical non-ideality can create a spurious signal in our data. Up to this point, our community has been able to partially sidestep this issue by aggressively increasing the number of detectors fielded by ground-based experiments; an approach that is well justified since the sensitivity of these experiments is significantly limited by atmospheric noise.
Unfortunately, the ambitious increase in the number of deployed detectors fails to address a fundamental shortcoming in the current state of experiment design: our instrument models are oversimplified and we are unable to robustly predict the far-field beam response of our instruments with sufficient precision. This community-wide limitation is particularly problematic for future satellite missions that must make the most out of a very restricted instrument volume, while using technologies that are drastically different from older generations. Our community must therefore develop innovative methods to address this challenge. This is the motivation for the CMBeam project.
The primary goal of CMBeam is the development of advanced instrumentation and methodologies supporting microwave holography at cryogenic temperatures. Such efforts have the potential to transform our understanding of detector sensitivities and optical systematics, and in doing so, dramatically expand the scientific potential of experiments mapping the sky at mm-wavelengths.
These efforts, which are described in Theme 1 of the CMBeam project, will progress in parallel with continued development of a successful program in optical modelling (Theme 2), high-performance time-domain simulations (Theme 2), and technology development (Theme 3).
All cosmological results from CMB experiments are based on an extensive calibration process, which includes a correction for the optical response of our telescopes. Without this so-called beam correction, different experiments would generate wildly conflicting and incorrect cosmological results. The 10–100 nK polarized signals that we are now searching for in our data call on calibration campaigns to deliver unprecedented understanding of both the frequency and beam response of our telescopes, as even a minuscule optical non-ideality can create a spurious signal in our data. Up to this point, our community has been able to partially sidestep this issue by aggressively increasing the number of detectors fielded by ground-based experiments; an approach that is well justified since the sensitivity of these experiments is significantly limited by atmospheric noise.
Unfortunately, the ambitious increase in the number of deployed detectors fails to address a fundamental shortcoming in the current state of experiment design: our instrument models are oversimplified and we are unable to robustly predict the far-field beam response of our instruments with sufficient precision. This community-wide limitation is particularly problematic for future satellite missions that must make the most out of a very restricted instrument volume, while using technologies that are drastically different from older generations. Our community must therefore develop innovative methods to address this challenge. This is the motivation for the CMBeam project.
The primary goal of CMBeam is the development of advanced instrumentation and methodologies supporting microwave holography at cryogenic temperatures. Such efforts have the potential to transform our understanding of detector sensitivities and optical systematics, and in doing so, dramatically expand the scientific potential of experiments mapping the sky at mm-wavelengths.
These efforts, which are described in Theme 1 of the CMBeam project, will progress in parallel with continued development of a successful program in optical modelling (Theme 2), high-performance time-domain simulations (Theme 2), and technology development (Theme 3).
The primary emphasis of our research group has been the establishment of experimental infrastructure at the University of Iceland. Within the scope of that effort, our main effort as revolved around the design and construction of a novel optomechanical systems that enable high-fidelity cryogenic measurements of microwave components and telescope for experiments observing the cosmic microwave background. One of the first demonstrations of our infrastructure and methodology development is described in Balafendiev et al., 2024.
In addition, our work activities and main achievements include:
Theme 1: The construction of a 6.95 x 2.4 x 2.6 m anechoic chamber to facilitate holography measurements at both room and cryogenic temperatures.
Theme 1: The design, procurement, assembly, and testing of a custom phase-sensitive measurement equipment for the 75–110 and 220–330 GHz frequency range. Those custom systems have been found to compare well with off-the-shelf components, however, the custom systems enable high-fidelity measurements of various optical components at cryogenic temperatures. The successful use of these components at microwave temperatures is a key goal within Theme 1.
Theme 1: Gascard et al. (2024) presents the design, simulation, and validation of CryoSim, a parameterized cryostat model for generic cryostats that can be used for optical testing in cosmic microwave background (CMB) experiments. CryoSim integrates mechanical and thermal analyses, enabling rapid optimization of cryostat configurations.
Theme 2: Beam calibration of the Simons Observatory Small Aperture Telescopes which saw first light in year 2023. The first scientific result of that work is described in Dachlythra et al., 2024.
Theme 2: The advancing of beamconv time-domain simulations. Adler et al. (2024) discusses the simulation of systematic errors in the Taurus experiment, a balloon-borne mission designed to measure cosmic microwave background (CMB) polarization, particularly focusing on large-scale E-mode polarization and the optical depth to reionization.
Theme 2: Monelli et al. (2023) uses beamconv to generate time-domain simulations for a generic CMB satellite mission scanning the sky with a non-ideal half-wave plate to study how those non-idealities impact our ability to study parity violating physics through observables in the CMB angular power spectra.
In addition, our work activities and main achievements include:
Theme 1: The construction of a 6.95 x 2.4 x 2.6 m anechoic chamber to facilitate holography measurements at both room and cryogenic temperatures.
Theme 1: The design, procurement, assembly, and testing of a custom phase-sensitive measurement equipment for the 75–110 and 220–330 GHz frequency range. Those custom systems have been found to compare well with off-the-shelf components, however, the custom systems enable high-fidelity measurements of various optical components at cryogenic temperatures. The successful use of these components at microwave temperatures is a key goal within Theme 1.
Theme 1: Gascard et al. (2024) presents the design, simulation, and validation of CryoSim, a parameterized cryostat model for generic cryostats that can be used for optical testing in cosmic microwave background (CMB) experiments. CryoSim integrates mechanical and thermal analyses, enabling rapid optimization of cryostat configurations.
Theme 2: Beam calibration of the Simons Observatory Small Aperture Telescopes which saw first light in year 2023. The first scientific result of that work is described in Dachlythra et al., 2024.
Theme 2: The advancing of beamconv time-domain simulations. Adler et al. (2024) discusses the simulation of systematic errors in the Taurus experiment, a balloon-borne mission designed to measure cosmic microwave background (CMB) polarization, particularly focusing on large-scale E-mode polarization and the optical depth to reionization.
Theme 2: Monelli et al. (2023) uses beamconv to generate time-domain simulations for a generic CMB satellite mission scanning the sky with a non-ideal half-wave plate to study how those non-idealities impact our ability to study parity violating physics through observables in the CMB angular power spectra.
The primary novel methodology that our team has developed involves the use of a 6-axis robot for holographic measurements at millimetre wavelengths. As part of this, we have developed an open-source code that interfaces the robot control software with a Rohde & Schwarz vector network analyser. The code includes a graphical user interface and software backend that supports the calculations of key optical parameters. Technological implications are still being explored, but this work has demonstrated tremendous potential for rapid microwave characterization of various components used in industry, telecommunications, or astrophysics. Initial aspects of this work are summarized in Balafendiev et al. (2024). This work is at the core of Theme 1 of the CMBeam project.