CMB-S4 design simulation tool

Generate CMB-S4 simulated maps of foregrounds/atmosphere and noise based on the configuration of the experiment

s4_design_sim_tool is a library, a command-line tool and a web interface to properly combine and weight pre-executed maps from time-domain and map-domain simulation based on input parameters and the instrument configuration (e.g. location of telescopes, distribution in frequency of the tubes).

The software is available in the CMB-S4/s4_design_sim_tool repository on Github, and can be installed locally with PyPI, it currently needs to run at NERSC to access the input maps:

pip install s4_design_sim_tool

the web interface is currently unavailable.

Releases and changelog

See the CHANGELOG

Configuration options

The simulation configuration is defined by a TOML text file, see for example the TOML configuration for the CMB-S4 baseline design: s4_design.toml

Sky emission

The first section of the configuration files defines which input components should be considered, it is possible to choose a weight between 0 and 1 for all components, for example we can simulate residual foregrounds after cleaning or partial de-lensing, and we can choose the tensor-to-scalar ratio r. For the case of partial de-lensing, consider that lensing is a non-linear and this is a very rough approximation, still it could be useful in same cases, for example low-ell BB modes.

Input maps are already top-hat bandpass integrated, beam-smoothed, and ran through a filter-and-bin mapmaking algorithm in time-domain, they are combined based on the configuration file and are not influenced by the experiment configuration. For more details, see the input maps section below and the Jupyter notebook with the implementation.

Experiment

The second section defines the design of the instrument, it is possible to customize the number and location of SAT and LAT telescope and for each of them modify what tube are mounting, keeping the constraint of 3 tubes for SAT and 19 for LAT.

Scaling of atmospheric and instrument noise is performed with these assumption:

• scale the 10-day simulations to 1 year considering the observing efficiency
• scale by the detector-years for noise and telescope-years for atmosphere
• for both emissions also consider the overall expected observing efficiency, for noise also consider the sensitivity factor

Therefore 2 tubes on the same telescope have the same atmospheric noise of 1 tube, to reduce noise from the atmosphere we need to distribute tubes across multiple telescopes. For instrument noise instead, it doesn't matter their distribution across telescopes, just their number.

For more details, see the input maps section below and the Jupyter notebooks with the implementation for the atmosphere and noise.

Splits

The tool supports loading up to 8 splits, which are suitable to simulate 1 full mission map and 7 yearly maps (or 7 interleaved splits). In this case, the tool will generate first a full mission map and then the number of splits requested, loading different realizations of atmosphere and noise and weighting them properly.

Input maps

Sky signal

• Full-sky Nside 4096 (LAT) and Nside 512 (SAT)
• Galactic, extragalactic and CMB
• Bandpass integrated with tophat bandpasses from s4sim
• Smoothed with gaussian beams

See the 202102_design_tool_input map based simulations.

Then the maps were used as inputs for a time-domain simulation with TOAST to simulate the effect of a filter-and-bin mapmaking with the CMB-S4 scanning strategy both for Pole and Chile.

Noise maps

Noise was simulated for one tube in each telescope. We observed according to a 10-day schedule without Sun or Moon avoidance. For Chile, the schedules already emulate the maximum observing efficiency. For simplicity, the Pole schedules only include one full scan of the respective patch per day. As a result, the Pole observing efficiencies are 46.29% (SAT) and 37.23% (LAT). These efficiencies must be accounted for by downweighting the Pole noise and atmospheric maps with their scheduled efficiences.

These factors should be corrected for the noise and atmosphere maps: map_out = map_in * sqrt(efficiency).

Also for expedience, we downsampled the densest focal planes to reduce the overall detector counts. These factors should be corrected for in the noise maps but not in the atmospheric maps: map_out = map_in / sqrt(thinfp).

More information about the noise and atmosphere simulations are available at:

• https://cmb-s4.org/wiki/index.php/Reference_design_simulation_tool