JP-3102 follow-up: Catch source_pos nans from msa file (#8874)

changes/8874.assign_wcs.rst

@@ -0,0 +1 @@
+Catch NaN values in msa tables for source positions in slit and replace with slit center.

jwst/assign_wcs/nirspec.py

@@ -674,8 +674,9 @@ def get_open_msa_slits(prog_id, msa_file, msa_metadata_id, dither_position,
             if n_main_shutter == 1:
                 # Source is marked primary
                 source_id = slitlet['source_id']
-                source_xpos = slitlet['estimated_source_in_shutter_x']
-                source_ypos = slitlet['estimated_source_in_shutter_y']
+                source_xpos = np.nan_to_num(slitlet['estimated_source_in_shutter_x'], nan=0.5)
+                source_ypos = np.nan_to_num(slitlet['estimated_source_in_shutter_y'], nan=0.5)
+
                 log.info(f'Found fixed slit {slitlet_id} with source_id = {source_id}.')
 
                 # Get source info for this slitlet:
@@ -746,8 +747,8 @@ def get_open_msa_slits(prog_id, msa_file, msa_metadata_id, dither_position,
         elif n_main_shutter == 1:
             xcen, ycen, quadrant, source_xpos, source_ypos = [
                 (s['shutter_row'], s['shutter_column'], s['shutter_quadrant'],
-                 s['estimated_source_in_shutter_x'],
-                 s['estimated_source_in_shutter_y'])
+                 np.nan_to_num(s['estimated_source_in_shutter_x'], nan=0.5),
+                 np.nan_to_num(s['estimated_source_in_shutter_y'], nan=0.5))
                 for s in slitlet_rows if s['background'] == 'N'][0]
             shutter_id = xcen + (ycen - 1) * 365  # shutter numbers in MSA file are 1-indexed
 

jwst/assign_wcs/tests/test_nirspec.py

@@ -446,6 +446,38 @@ def test_msa_missing_source(tmp_path):
     _compare_slits(slitlet_info[1], ref_slit)
 
 
+def test_msa_nan_source_posn(tmp_path):
+    """
+    Test the get_open_msa_slits function with nan values for source position.
+    """
+    # modify an existing MSA file to remove source info
+    msaconfl = get_file_path('msa_fs_configuration.fits')
+    bad_confl = str(tmp_path / 'nan_msa_fs_configuration.fits')
+    shutil.copy(msaconfl, bad_confl)
+
+    with fits.open(bad_confl) as msa_hdu_list:
+        shutter_table = table.Table(msa_hdu_list['SHUTTER_INFO'].data)
+        shutter_table[-5:]['estimated_source_in_shutter_x'] = np.nan
+        msa_hdu_list['SHUTTER_INFO'] = fits.table_to_hdu(shutter_table)
+        msa_hdu_list[2].name = 'SHUTTER_INFO'
+        msa_hdu_list.writeto(bad_confl, overwrite=True)
+
+    prog_id = '1234'
+    msa_meta_id = 12
+    dither_position = 1
+
+    slitlet_info = nirspec.get_open_msa_slits(
+        prog_id, bad_confl, msa_meta_id, dither_position, slit_y_range=[-.5, .5])
+
+    # MSA slit: virtual source name assigned
+    ref_slit = trmodels.Slit(name='S200A1', shutter_id=0, dither_position=1, xcen=0, ycen=0,
+                             ymin=-0.5, ymax=0.5, quadrant=5, source_id=3, shutter_state='x',
+                             source_name='95065_3', source_alias='3', stellarity=1.0,
+                             source_xpos=0.0, source_ypos=-0.2290000021457672,
+                             source_ra=53.139904, source_dec=-27.805002)
+    _compare_slits(slitlet_info[1], ref_slit)
+
+
 open_shutters = [[24], [23, 24], [22, 23, 25, 27], [22, 23, 25, 27, 28]]
 main_shutter = [24, 23, 25, 28]
 result = ["x", "x1", "110x01", "110101x"]