limTOD

Time-Ordered Data simulator for single-dish (autocorrelation) radio intensity mapping — with a differentiable pure-JAX port.

📖 Documentation: https://limtod.readthedocs.io

🧭 Beam convention in one line

Multiplying a beam map by a sky map with no rotation — sum(beam * sky) — is the pointing lat_deg=90, el_deg=90, az_deg=0 (North Pole, zenith, azimuth 0), at lst_deg=0, selfrot=0.

Azimuth 0, not 180: at el=90 the boresight is the zenith whatever the azimuth, so azimuth only rolls the beam about the boresight and the identity is the zero roll. This is also the quickest way to verify the convention — see the one-test box in Theory & conventions.

limTOD simulates the time-ordered data (TOD) of a single-dish telescope scanning a HEALPix sky with an arbitrary (asymmetric) beam: the beam is rotated to each pointing in spherical-harmonic space and dotted with the sky, then combined with 1/f gain noise and white noise. The package also ships:

• Map-making — HPW_mapmaking (high-pass + Wiener) and GLS_mapmaking (full 1/f + white noise covariance, ported from hydra-tod's iterative GLS);
• limTOD.patchbeam — a MeerKLASS-optimal sky-TOD path that keeps narrow, finely-gridded beams on their native (l, m) grid and integrates disc-restricted sky patches (no harmonic rotation);
• limTOD.uvbeam — adapters for pyuvdata UVBeam objects, feeding measured/simulated beams into either path;
• limtod_jax — a pure-JAX, jit/vmap/grad-safe port of the sky→TOD chain, verified against the numpy implementation to ~1e-12 in float64. It powers the differentiable pipeline of replicant-telescope.

Latest changes: CHANGELOG.

Installation

pip install limTOD

The base install is deliberately lightweight (wheel-only: numpy, healpy, astropy, scipy, tqdm, mpmath — no compiler needed). Heavier dependencies are opt-in extras:

pip install "limTOD[full]"

From source: clone the repository and pip install -e ".[dev,full]" (runs the test suite over both the MPI-present and serial-fallback paths).

Quick start

Simulate multi-frequency TOD for a MeerKAT-like scan (sky model here needs [gdsm]; pass your own sky_func to go without):

from limTOD import TODSim, example_scan

simulator = TODSim(
    ant_latitude_deg=-30.7130, ant_longitude_deg=21.4430, ant_height_m=1054,
    beam_nside=256, sky_nside=256,
)
time_list, azimuth_list = example_scan()
tod, sky_tod, gain_noise = simulator.generate_TOD(
    freq_list=[950, 1000, 1050],          # MHz
    time_list=time_list,
    azimuth_deg_list=azimuth_list,
    elevation_deg=41.5,
)                                          # each (n_freq, n_time)

The same sky→TOD chain, differentiable in JAX ([jax] extra):

import jax; jax.config.update("jax_enable_x64", True)
import jax.numpy as jnp
import limtod_jax as ltj

sky_alm = ltj.map2alm_quad(sky_map, nside=nside, lmax=lmax)
psi, theta, phi = ltj.zyz_of_pointing(lst_deg, lat_deg, az_deg, el_deg, 0.0)
tod = ltj.generate_tod_sky(
    beam_alm, sky_alm, jnp.stack([psi, theta, phi], axis=-1), lmax=lmax,
)
grad = jax.grad(lambda b: ltj.generate_tod_sky(
    b, sky_alm, jnp.stack([psi, theta, phi], axis=-1), lmax=lmax).sum().real
)(beam_alm)                                # d(TOD sum)/d(beam alms)

Documentation

Full documentation: https://limtod.readthedocs.io

Worked notebooks: TOD simulation and map-making. Coordinate and beam-orientation conventions: Theory & conventions.

Citation

If you use limTOD in your research, please cite:

@ARTICLE{2026RASTI...5ag024Z,
       author = {{Zhang}, Zheng and {Bull}, Philip and {Santos}, Mario G. and {Nasirudin}, Ainulnabilah},
        title = "{Joint Bayesian calibration and map-making for intensity mapping experiments}",
      journal = {RAS Techniques and Instruments},
     keywords = {Data Methods, methods: data analysis, techniques: spectroscopic, radio lines: general, Instrumentation and Methods for Astrophysics},
         year = 2026,
        month = jan,
       volume = {5},
          eid = {rzag024},
        pages = {rzag024},
          doi = {10.1093/rasti/rzag024},
archivePrefix = {arXiv},
       eprint = {2509.10992},
 primaryClass = {astro-ph.IM},
       adsurl = {https://ui.adsabs.harvard.edu/abs/2026RASTI...5ag024Z},
      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}

License and authorship

MIT License — see LICENSE.

limTOD is developed and maintained by Zheng Zhang (University of Manchester), with help and advice from members of the MeerKLASS and RHINO collaborations — including Phil Bull, Piyanat Kittiwisit, Geoff Murphy, and Mario Santos.