Index: /trunk/psastro/src/psastroMosaicGradients.c
===================================================================
--- /trunk/psastro/src/psastroMosaicGradients.c	(revision 19310)
+++ /trunk/psastro/src/psastroMosaicGradients.c	(revision 19311)
@@ -2,5 +2,5 @@
 static int nPass = 0;
 
-bool psastroMosaicGradients (pmFPA *fpa, psMetadata *recipe) {
+bool psastroMosaicDistortionFromGradients (pmFPA *fpa, psMetadata *recipe) {
 
     bool status;
@@ -9,4 +9,7 @@
     pmReadout *readout = NULL;
     psArray *gradients = NULL;
+
+    // Measure the gradient as a function of position.  This must be performed between the
+    // corrected ref->TP and the observed raw->FP, for which the distortion is a perturbation.
 
     pmFPAview *view = pmFPAviewAlloc (0);
@@ -59,4 +62,6 @@
     }
 
+    // Fit the gradient field and convert to the distortion terms.
+
     // allocate mosaic-level polynomial transformation and set masks needed by DVO
     int order = psMetadataLookupF32 (&status, recipe, "PSASTRO.MOSAIC.ORDER");
@@ -95,13 +100,7 @@
     }
 	
-    if (!psastroMosaicSetAstrom (fpa)) {
-	psError(PSASTRO_ERR_UNKNOWN, false, "failed to apply mosaic distortion terms\n");
-	psFree (gradients);
-	psFree (view);
-        return false;
-    }
-
     psFree (gradients);
     psFree (view);
     return true;
 }
+