JP-3612: Fix MOS background slit source position (#8461)

Co-authored-by: Nadia Dencheva <nadia.dencheva@gmail.com>
spacetelescope · May 8, 2024 · c9e8275 · c9e8275
1 parent 854f84b
commit c9e8275
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Showing 3 changed files with 6 additions and 3 deletions.
diff --git a/CHANGES.rst b/CHANGES.rst
@@ -15,6 +15,9 @@ assign_wcs
   of pixel row rather than a rotation of the pixel coordinates.  The practical impact is
   to ensure that iso-lambda is along pixel rows after this change. [#8411]
 
+- Move the assigned source position for dedicated NIRSpec MOS background slits from the
+  lower left corner of the slit to the middle of the slit. [#8461]
+
 associations
 ------------
 

diff --git a/jwst/assign_wcs/nirspec.py b/jwst/assign_wcs/nirspec.py
@@ -662,8 +662,8 @@ def get_open_msa_slits(msa_file, msa_metadata_id, dither_position,
             quadrant = slitlets_sid[0]['shutter_quadrant']
             ycen = j
             xcen = slitlets_sid[0]['shutter_row']  # grab the first as they are all the same
-            source_xpos = 0.0
-            source_ypos = 0.0
+            source_xpos = 0.5
+            source_ypos = 0.5
             source_id = _get_bkg_source_id(bkg_counter, max_source_id)
             log.info(f'Slitlet_id {slitlet_id} is background only; assigned source_id = {source_id}')
             bkg_counter += 1

diff --git a/jwst/assign_wcs/tests/test_nirspec.py b/jwst/assign_wcs/tests/test_nirspec.py
@@ -311,7 +311,7 @@ def test_msa_configuration_all_background():
     slitlet_info = nirspec.get_open_msa_slits(msaconfl, msa_meta_id, dither_position,
                                               slit_y_range=[-.5, .5])
     ref_slit = trmodels.Slit(57, 8281, 1, 251, 23, -2.15, 2.15, 4, 0, '1x1', 'background_57', 'bkg_57',
-                             0, -0.5, -0.5)
+                             0, 0.0, 0.0)
     _compare_slits(slitlet_info[0], ref_slit)