JP-3426: Check WFSS background mask against input dq array (#8070)

Co-authored-by: Howard Bushouse <[email protected]>

CHANGES.rst

@@ -1,6 +1,12 @@
 1.12.6 (unreleased)
 ===================
 
+background
+----------
+
+- Ensure that WFSS background mask does not leave only
+  pixels with DO_NOT_USE flagged. [#8070]
+
 extract_1d
 ----------
 

jwst/background/background_sub.py

@@ -3,6 +3,7 @@
 import numpy as np
 
 from stdatamodels.jwst import datamodels
+from stdatamodels.jwst.datamodels.dqflags import pixel
 
 from . import subtract_images
 from ..assign_wcs.util import create_grism_bbox
@@ -295,7 +296,12 @@ def subtract_wfss_bkg(input_model, bkg_filename, wl_range_name, mmag_extract=Non
     # i.e. in regions we can use as background.
     if got_catalog:
         bkg_mask = mask_from_source_cat(input_model, wl_range_name, mmag_extract)
-        if bkg_mask.sum() < 100:
+        # Ensure mask has 100 pixels and that those pixels correspond to valid
+        # pixels using model DQ array
+        if np.count_nonzero(input_model.dq[bkg_mask]
+                            ^ pixel['DO_NOT_USE']
+                            & pixel['DO_NOT_USE']
+                            ) < 100:
             log.warning("Not enough background pixels to work with.")
             log.warning("Step will be SKIPPED.")
             return None