Affected Repositories

• PyAutoArray (primary)

Branch Survey

Suggested branch: feature/sparse-interferometer-unequal-sigma-guard

Implementation Steps

1. autoarray/dataset/interferometer/dataset.py — at the top of apply_sparse_operator, if not np.allclose(self.noise_map.real, self.noise_map.imag): raise exc.DatasetException(...) with a message stating the W~ operator is built from the real-part sigma only and requires sigma_real == sigma_imag per visibility; suggest not calling apply_sparse_operator (dense mapping path) or equalising the sigmas.
2. Same file / inversion_interferometer_util.py — docstring notes on the precondition.
3. test_autoarray/dataset/interferometer/test_dataset.py — test__apply_sparse_operator__unequal_real_imag_noise_raises and an equal-but-non-uniform sigma control that succeeds.
4. PR body: short assessment of the two-operator generalisation (real- and imag-weighted precision operators combined in apply_operator).

Key Files

• autoarray/dataset/interferometer/dataset.py — guard + docstring
• test_autoarray/dataset/interferometer/test_dataset.py — tests

Sparse interferometer W~ path silently assumes equal real/imag noise sigma

Type: bug
Target: PyAutoArray
Repos:

• PyAutoArray
• workspaces
  Difficulty: medium
  Autonomy: supervised
  Priority: normal
  Status: formalised

Sparse interferometer W~ path silently assumes equal real/imag noise sigma

Type: bug
Target: PyAutoArray
Repos:

• PyAutoArray
  Difficulty: medium
  Autonomy: supervised
  Priority: normal

Problem

InterferometerSparseOperator builds the real-space operator W~ = Re(F^H W F) from a single
NUFFT precision operator. That reduction is only exact when every visibility has equal real and
imaginary noise sigma. With sigma_real != sigma_imag the sparse InversionInterferometerSparse
curvature matrix disagrees with the dense InversionInterferometerMapping path even on the
single-mapper route: measured 5e-16 relative with equal sigmas vs 5e-10 to 3e-2 with unequal
sigmas (geometry dependent). Pre-existing, not introduced by #500. SimulatorInterferometer and
real datasets satisfy the equality, so it is latent — but nothing guards it.

Fix

1. Raise a clear InversionException (or in Interferometer.apply_sparse_operator) when the noise
   map has unequal real/imag sigma for any visibility, with a unit test. Do this first.
2. Assess extending the operator to the general case (two precision operators, real- and
   imag-weighted, combined in apply_operator) — only if the cost is modest; otherwise the guard stands.

Context

Found 2026-08-28 while shipping #499 / #500. The misc/jax_assertions/fit_interferometer_sparse_operator.py parity
scripts in both test workspaces document the limitation in their
docstrings and deliberately use equal-sigma noise maps; they need no change.