BoloCalc: a sensitivity calculator for the design of Simons Observatory

Abstract

The Simons Observatory (SO) is an upcoming experiment that will study temperature and polarization fluctuations in the cosmic microwave background (CMB) from the Atacama Desert in Chile. SO will field both a large aperture telescope (LAT) and an array of small aperture telescopes (SATs) that will observe in six bands with center frequencies spanning from 27 to 270 GHz. Key considerations during the SO design phase are vast, including the number of cameras per telescope, focal plane magnification and pixel density, in-band optical power and camera throughput, detector parameter tolerances, and scan strategy optimization. To inform the SO design in a rapid, organized, and traceable manner, we have created a Python-based sensitivity calculator with several state-of-the-art features, including detector-to-detector optical white-noise correlations, a handling of simulated and measured bandpasses, and propagation of low-level parameter uncertainties to uncertainty in on-sky noise performance. We discuss the mathematics of the sensitivity calculation, the calculator's object-oriented structure and key features, how it has informed the design of SO, and how it can enhance instrument design in the broader CMB community, particularly for CMB-S4. Seven cameras are planned for initial deployment within the LAT receiver.† The LAT is designed with a larger FOV that can accommodate up to nineteen cameras, allowing for future LAT receiver upgrades.

Systematic uncertainties in the Simons Observatory: optical effects and sensitivity considerations

“…The figure of merit used is the fraction of power reaching the sky after N bounces for each model configuration. The mapping speed of the experiment is a steep function of the spillover fraction as discussed in Hill et al 2018 32 (section 4.4). Based in these mapping speed studies, we have established that the SO sensitivity goal corresponds to a 98% spillover fraction while the threshold for operation corresponds to 95%, we define a baseline level at 96.5%.…”

The Simons Observatory: modeling optical systematics in the Large Aperture Telescope

“…On top of this, mitigation of stray light is critical -the SO mapping speed is limited in part by thermal loading from the atmosphere as well as loading due to spillover on warm optics. As an example, a one-percent increase in the amount of optical throughput that spills past the warm reflectors (on to 300 K) will reduce the 150-GHz mapping speed by roughly 20 % [20]. A considerable effort in reducing warm spillover is therefore warranted.…”

“…For the model that includes the receiver baffle, there is a significant increase seen in the amount of power that makes it to the sky between 2 and 3 jumps. It is estimated that this baffle can boost the fraction of light making it to the sky by as much as 0.5% of the total input optical power (depending on the field location), which would have an impact of roughly 10-15 % in mapping speed at MF frequencies [20], at the expense of having a circularly symmetric sidelobe at 7-8 degrees from the main beam. Figure 7 summarizes the results of our ray tracing simulations for the conical baffle.…”

Cold optical design for the large aperture Simons' Observatory telescope

“…At some frequencies, 2% of spillover to 300K can reduce the mapping speed by a factor of 1.5. 10 Finally, the optics have to be designed with cost, modularity, future expandability, and effective use of the focal plane in mind.…”