Fast partial-sky spherical harmonic transforms

TL;DR

The paper introduces a fast, efficient algorithm for partial-sky spherical harmonic transforms that significantly accelerates computations for high-resolution, small sky patches, benefiting cosmological observations like CMB lensing and gravitational waves.

Contribution

It presents a novel algorithm based on the Fourier-sphere method that handles arbitrary locations efficiently, outperforming traditional methods in speed and flexibility for small sky regions.

Findings

Achieves 3 to 10 times speed-up on large sky patches.

Eliminates disadvantages of arbitrary pixelisations compared to traditional methods.

Enables efficient repeated SHTs on limited sky regions for cosmological studies.

Abstract

We discuss in some details a novel algorithm for performing partial-sky spherical harmonic transforms (SHT), building on the Fourier-sphere method of Reinecke et al (2023) handling efficiently high numbers of arbitrary locations on the sphere. Our main motivations are Cosmic Microwave Background lensing from the South Pole Telescope, and the South Pole Observatory program targeting primordial gravitational waves from inflation, requiring high-resolution, numerically intensive work on small sky fractions. We achieve speed-up factors ranging from 3 to 10 on SPT-3G main field and BICEP3 deep footprint, and much more on smaller patches. More generally, the algorithm eliminates in our case study the usual disadvantages of arbitrary pixelisations in comparison to isolatitude pixelisations or flat-sky approximations, making it ideal for ambitious workflows that require repeated SHTs on limited sky regions.