#553 update direction frame determination

[dmehring]

No description provided.

[Copilot]

Pull request overview

Updates CASA image writing to determine the direction coordinate system from either reference_direction.attrs.equinox or reference_direction.attrs.frame, and adds regression tests for the new behavior.

Changes:

• Update CASA direction coordinate metadata generation to fall back to frame when equinox is missing and raise a clear error when neither is present.
• Add unit tests validating CASA writes when equinox is absent and when both equinox and frame are missing.
• Introduce a small test helper to build a minimal SKY image dataset for these coordinate-write tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/xradio/image/_util/_casacore/xds_to_casacore.py	Adds frame fallback (and an explicit error) for determining CASA direction system during write.
tests/unit/image/test_image.py	Adds new regression tests and a helper dataset builder for CASA direction-frame determination.

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 2 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rc/xradio/image/_util/_casacore/xds_to_casacore.py	85.71%	2 Missing ⚠️

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[dmehring]

This fixes the ICRS issue illustrated by this data set and in general improves the xradio/astropy -> casacore direction frame mapping.

This cannot resolve the actual ticket for which this ticket was opened as the data_groups data_var mismatch happened upstream so that needs to be traced to its root cause before it can be addressed. The workaround is as shown in the ticket; add the necessary sky mapping to the base data group by hand.