English

Fast partial-sky spherical harmonic transforms

Cosmology and Nongalactic Astrophysics 2026-03-20 v2 Instrumentation and Methods for Astrophysics

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.

Keywords

Cite

@article{arxiv.2603.17166,
  title  = {Fast partial-sky spherical harmonic transforms},
  author = {Julien Carron and Martin Reinecke},
  journal= {arXiv preprint arXiv:2603.17166},
  year   = {2026}
}

Comments

10 pages, 5 figures