Large-scale power loss in ground-based CMB mapmaking

CMB mapmaking relies on a data model to solve for the sky map, and this process is vulnerable to bias if the data model cannot capture the full behavior of the signal. We demonstrate that this bias is not just limited to small-scale effects in high-contrast regions of the sky, but can manifest as $\mathcal{O}(1)$ power loss on large scales in the map under conditions and assumptions realistic for ground-based CMB telescopes. This bias is invisible to simulation-based tests that do not explicitly model them, making it easy to miss. We investigate the common cases of sub-pixel errors and gain errors in more detail and demonstrate that subpixel bias can be eliminated using bilinear pointing matrices. Finally, we provide simple methods for testing for the presence of large-scale model error bias in the general case.

The published version of this paper has been changed on arXiv to fix an error in the toy example in section 3.1. This did not impact any of the results.