Performance-based Fisher optimization for CMB-S4, 44cm vs 52cm aperture (w/ high-res/low-res 20 GHz)

 (Victor Buza)

This posting is in the style of the post-CDT optimization posting. Previously to that, there was also a similar optimization which lead to the definition of the noise spectra that in turn were used to define Data Challenge 2.0. In this posting I try to contrast the post-CDT optimization, which was performed under the assumption of 52cm apertures, to a version that considers a 44cm aperture, as requested by the technical council. This change of aperture results in larger beams (by 52/44=1.18), while all else is held the same. In addition I also compare moving the 20 GHz channel to a 6-m class telescope to the case where the 20GHz channel is on the small aperture telescopes. The specifications for the high-res 20 GHz channel are presented here.

1. Worked-out Example; Experiment Specification

Below, similar to Section 2 and 3 of the posting linked above, I present an application of this framework to an optimization grounded in achieved performance.

2. Parameter Constraints; \(\sigma_r\) performance

Figure 1:

(Top, Left) Optimal path indicating the total number of det-yrs, and the individual distribution of det-yrs at each point.
(Top, Right) Individual map depths for every channel, in \(\mu K\)-arcmin. Calculated from the accumulated weights in each channel, scaled from achieved performances.
(Bottom, Left) Ratio of the total effort that is spent on delensing, as a function of total effort, and the effective RMS lensing residual as a function of total effort.
(Bottom, Right) Resulting \(\sigma(r)\) constraints for each level of delensing.

2. Conclusions

For the CDT optimization, the end point was chosen to be 1.160M det-yrs, yielding a \(\sigma_r\) of \(6.5 \times 10^{-4}\) (for the optimal solution). For the same foreground assumptions: \(A_{sync}=3.8\) and \(\delta_{sync}=1\), to achieve the same constraint with a 44cm aperture instrument we need 1.215M det-yrs -- a 5% increase in effort. For the same level of effort as the CDT report, we achieve a \(\sigma_r\) that is \(6.7 \times 10^{-4}\) with the 44cm aperture instrument -- larger by ~3% than the 52cm one.

Switching the 20 GHz channel to a high-res channel, yields a \(\sigma_r\) that is 2% better than the low-res version (for the CDT configuration). Changing the aperture from 52cm to 44cm for the rest of the channels worsens the constraint by 2%, and requires ~3% more effort to get that back. We see here that the difference between 52cm and 44cm is smaller than the difference in the fully low-res instrument case.

A similar type of effect is seen in the other fiducial models with more or less sync, but similar decorrelation levels.

When switching to higher levels of decorrelation we observe a switch: the high-res version of the instrument now yields a \(\sigma_r\) that is ~3% worse than the low-res version (w/ 52cm for all channels except 20GHz). Changing the aperture from 52cm to 44cm worses the constraint by another 2%. To make up this cummulative 5% difference requires roughly 7.5% more effort.