photomancy

Divination from light.

photomancy is a JAX-native engine for Bayesian inference and value of information over astrophysical scenes observed by direct imaging. It is part of the Habitable Worlds Observatory simulation suite: orbix builds the geometry, skyscapes builds the scene, and photomancy divines the scene back from the data, producing posteriors over the scene parameters, the Bayesian evidence for model comparison, and the expected information gain of a candidate next observation.

The engine is forward-model agnostic. A fit is a logdensity over a partitioned scene PyTree, assembled from three plug-ins, a forward model, a likelihood, and a prior, and run through a uniform backend that returns one posterior. The same inference machinery serves orbit fitting today and disk, atmosphere, and image-domain fitting as those forward models come online, so an improvement to a sampler or to the information-gain calculation reaches every domain at once.

Installation

pip install photomancy

photomancy requires Python 3.11 or newer and is in early development, so the API may still change. For a source checkout with the test and documentation extras, clone the repository and run pip install -e ".[test,docs]".

Quick start

A domain supplies three plug-ins over a scene, and the engine turns them into a fit:

from photomancy import LaplaceBackend, build_scene_logdensity

# forward:    scene -> predicted data
# likelihood: predicted -> scalar log-likelihood
# prior:      an AbstractPrior over the fitted leaves
logdensity, z0, unravel = build_scene_logdensity(
    scene, forward_model=forward, likelihood=likelihood, prior=prior
)

posterior = LaplaceBackend().run(logdensity, z0)
samples = posterior.sample(key, 1000)  # (1000, d), flat parameter space
log_evidence = posterior.evidence

The orbit, disk, and atmosphere domains package these plug-ins for their own data, so a typical fit calls a domain helper rather than wiring the three pieces by hand.

What it does

• Posteriors over scene parameters behind one interface (sample, log_prob, evidence), whichever backend produced them.
• Bayesian evidence and Bayes factors for model comparison and detection, answering questions such as whether an orbit needs eccentricity or whether a spectral feature is present.
• Multimodal posteriors as evidence-weighted mixtures, so period aliases and mirror ambiguities are carried honestly rather than collapsed onto one answer.
• Sequential updating, where a posterior becomes the prior for the next observation through to_prior, so information accumulates across a campaign.
• Expected information gain over candidate observations, turning a fit into a recommendation for where to look next.

Backends

Every backend exposes one method, run(logdensity, init, key) -> Posterior, and sees only the flat logdensity, never the scene or the forward model, so a new sampler becomes useful to every domain at once.

Documentation

The full documentation lives in docs/ and builds with Sphinx once the docs extra is installed:

sphinx-build -b html docs docs/_build/html

It covers the architecture and design principles, a mathematical treatment of the fit, the evidence, and the information gain, and worked examples for a visual walkthrough on an abstract problem and for orbit fitting. If the JAX and Bayesian vocabulary is unfamiliar, the glossary defines the terms the rest of the documentation relies on.

Status

Orbit fitting is implemented across radial-velocity, astrometry, and imaging data, and disk fitting rides the same engine. Atmospheric retrieval is in progress, and image-domain fitting against a coronagraph forward is the next major target. On the backend side, a stochastic variational backend with a normalizing-flow guide is planned.

License

photomancy is released under the MIT license, see LICENSE.