Replace test_field_distortion.py

tests/test_field_distortion.py

@@ -238,7 +238,7 @@ def field_distortion_model(
             # if iccd!=1: continue
             iccdKey = "ccd" + str("0%d" % (iccd))[-2:]
             # open the ditortion data
-            ipsfDatn = cat_dir + "ccd%d_%s.dat" % (iccd, iwaveKey)
+            ipsfDatn = os.path.join(cat_dir, "ccd%d_%s.dat" % (iccd, iwaveKey))
             ipsfDat = Table.read(ipsfDatn, format="ascii")
 
             ixwList, iywList = [], []