Map-based asymmetric beam treatment: application of real-space beam patterns in CMB data analysis

Abstract

Real telescope beams are asymmetric, breaking the simple $B_{\ell }$ factorisation in harmonic space and coupling modes across $\ell$ and $m$. We develop a map-domain treatment in which beam convolution is a local, scan-dependent operation applied directly to the time-ordered data (TOD), accommodating the full beam shape and giving a direct handle on intensity-to-polarisation ($I\to P$) leakage. A noiseless validation against spherical-harmonic convolution reproduces the input temperature and $B$-mode power spectra up to $\ell \simeq 1000$, well beyond the range of primordial $B$-mode science. We outline how the operation maps onto a sparse linear BeamOperator for modular map-making.

arXiv comment2 pages, 1 figure. Contribution to the 2026 Cosmology session of the Rencontres de Moriond