Fix CTYPE parsing for WCS with SIP distortion extensions

The projection type was extracted from the last 3 characters of CTYPE,
which fails for SIP extensions (e.g., "RA---TAN-SIP" would give "SIP"
instead of "TAN").

Now extracts the projection from the fixed position [5:8] which works
for both standard CTYPE ("RA---TAN") and SIP-extended CTYPE
("RA---TAN-SIP").

Fixes #26

Author: astrofrog

pyavm/avm.py

@@ -585,9 +585,11 @@ def from_wcs(cls, wcs, shape=None):
         self.Spatial.Equinox = wcs.wcs.equinox
 
         # Projection
+        # CTYPE format is CCCC-PPP or CCCC-PPP-SIP for SIP distortions
+        # Extract projection from position 5:8 to handle both cases
 
-        proj1 = wcs.wcs.ctype[0][-3:]
-        proj2 = wcs.wcs.ctype[1][-3:]
+        proj1 = wcs.wcs.ctype[0][5:8]
+        proj2 = wcs.wcs.ctype[1][5:8]
         if proj1 == proj2:
             self.Spatial.CoordsystemProjection = decode_ascii(proj1)
         else:

pyavm/tests/test_avm.py

@@ -60,3 +60,21 @@ def test_controlled_vocabulary_dash_placeholder():
 
     avm.Spectral.Band = ["Optical", "-", "Infrared"]
     assert avm.Spectral.Band == ["Optical", "-", "Infrared"]
+
+
+def test_from_wcs_sip():
+    """Test that WCS with SIP distortion extensions are handled correctly.
+
+    This is a regression test for issue #26.
+    """
+    pytest.importorskip("astropy")
+    from astropy.wcs import WCS
+
+    wcs = WCS(naxis=2)
+    wcs.wcs.ctype = ["RA---TAN-SIP", "DEC--TAN-SIP"]
+    wcs.wcs.crpix = [100, 100]
+    wcs.wcs.crval = [180, 45]
+    wcs.wcs.cdelt = [-0.001, 0.001]
+
+    avm = AVM.from_wcs(wcs)
+    assert avm.Spatial.CoordsystemProjection == "TAN"