Index: trunk/psastro/src/psastroMosaicGradients.c
===================================================================
--- trunk/psastro/src/psastroMosaicGradients.c	(revision 39926)
+++ trunk/psastro/src/psastroMosaicGradients.c	(revision 41895)
@@ -84,6 +84,8 @@
         return false;
     }
+
+    // forward transformations (chip->fpa->tpa->sky) use Ordinary polynomials
     psFree (fpa->toTPA);
-    fpa->toTPA = psPlaneTransformAlloc (order, order);
+    fpa->toTPA = psPlaneTransformAlloc (order, order, PS_POLYNOMIAL_ORD);
     for (int i = 0; i <= fpa->toTPA->x->nX; i++) {
         for (int j = 0; j <= fpa->toTPA->x->nY; j++) {