feat: add direct ITRS observed transforms (swev-id: astropy__astropy-13398)

[casey-brooks]

Summary

• implement direct ITRS↔AltAz/HADec transforms with rotation helper, obstime validation, and refraction/unity warnings
• expose the transforms by importing the module in builtin_frames.__init__
• add regression tests covering round-trips, zenith alignment, unit-spherical behaviour, and comparison to the ICRS path

Testing

• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/h4qhxh7vwmxgy6w05g0xsf6r1bfi9vga-gcc-15.2.0-lib/lib:/nix/store/dvb9i3s69j29md1hpx3gpaqqds6530df-gcc-15.2.0-libgcc/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/python -m pytest astropy/coordinates/tests/test_itrs_observed_transforms.py
• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/h4qhxh7vwmxgy6w05g0xsf6r1bfi9vga-gcc-15.2.0-lib/lib:/nix/store/dvb9i3s69j29md1hpx3gpaqqds6530df-gcc-15.2.0-libgcc/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/python -m flake8 astropy/coordinates/builtin_frames/itrs_observed_transforms.py astropy/coordinates/tests/test_itrs_observed_transforms.py

Observed failure & reproduction

from astropy.coordinates import EarthLocation, AltAz
from astropy.time import Time
import astropy.units as u

loc = EarthLocation(lat=35*u.deg, lon=-111*u.deg, height=0*u.m)
obstime = Time('2024-01-01T00:00:00', scale='utc')

target_itrs = EarthLocation(lat=35*u.deg, lon=-111*u.deg, height=1000*u.m).get_itrs(obstime=obstime)

computed = target_itrs.transform_to(AltAz(obstime=obstime, location=loc, pressure=0*u.hPa))

assert abs((computed.alt - 90*u.deg).to_value(u.arcsec)) < 1, f"alt={computed.alt}"

Traceback (most recent call last):
  File "<stdin>", line 12, in <module>
AssertionError: alt=69.25429263945392 deg

Resolves #129.

[casey-brooks]

Local Validation

• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/h4qhxh7vwmxgy6w05g0xsf6r1bfi9vga-gcc-15.2.0-lib/lib:/nix/store/dvb9i3s69j29md1hpx3gpaqqds6530df-gcc-15.2.0-libgcc/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/python -m pytest astropy/coordinates/tests/test_itrs_observed_transforms.py → 5 passed, 0 failed
• SETUPTOOLS_USE_DISTUTILS=stdlib PYTHONPATH=. LD_LIBRARY_PATH=/nix/store/h4qhxh7vwmxgy6w05g0xsf6r1bfi9vga-gcc-15.2.0-lib/lib:/nix/store/dvb9i3s69j29md1hpx3gpaqqds6530df-gcc-15.2.0-libgcc/lib:/nix/store/n5lymg0y5x6i9wipkjrsi8aczv1nr4qc-zlib-1.3.1/lib .venv/bin/python -m flake8 astropy/coordinates/builtin_frames/itrs_observed_transforms.py astropy/coordinates/tests/test_itrs_observed_transforms.py → no issues

[noa-lucent]

Direct ITRS↔observed transforms look great overall, but the AltAz/HADec→ITRS path still needs to emit the same unit-spherical warning called for in the spec, and the test suite should cover that behaviour.

[noa-lucent]

Thanks for adding the observed→ITRS warning and coverage—the behaviour now matches the spec and the tests exercise both paths. I’m satisfied with the changes.