Index: trunk/psastro/src/psastroUtils.c
===================================================================
--- trunk/psastro/src/psastroUtils.c	(revision 10613)
+++ trunk/psastro/src/psastroUtils.c	(revision 10830)
@@ -1,4 +1,76 @@
 # include "psastro.h"
 # define RENORM 0
+
+// fix this to look up the value in the chip concepts
+static double getChipPixelScale (pmChip *chip) {
+    return 10.0;
+}
+
+// I have an FPA structure with multiple chips.  we have loaded or measured astrometry for each
+// chip with independent pixel/degree scaling values.  These will naturally compensate locally
+// somewhat for the telescope distortion.  to measure the telescope distortion, we need to
+// force the chips to have the same pixel scale and measure the difference from that solution.
+// Convert an FPA with disparate pixel scales to a common pixel scale (perhaps depending on the
+// chip -- eg, TC3)
+
+bool psastroMosaicCommonScale (pmFPA *fpa, psMetadata *recipe) {
+
+    // options : use the MIN or MAX chip as global reference or supplied pixel scales
+    // (microns/pixel), which may depend on the chip
+
+    float pixelScaleUse, pixelScale1,  pixelScale2,  pixelScale;
+
+    char *option = psMetadataLookupStr (NULL, recipe, "PSASTRO.COMMON.SCALE.OPTION");
+    if (option == NULL) {
+	psError(PSASTRO_ERR_DATA, false, "no choice set for common scale option\n");
+	return false;
+    }
+
+    bool useExternal = true;
+
+    // find the min or max scale chip
+    if (!strcasecmp (option, "MIN") || !strcasecmp (option, "MAX")) {
+
+	bool useMax = !strcasecmp (option, "MAX");
+	pixelScaleUse = (useMax) ? FLT_MIN : FLT_MAX;
+
+	for (int i = 0; i < fpa->chips->n; i++) {
+	    pmChip *chip = fpa->chips->data[i];
+	    
+	    pixelScale1 = hypot (chip->toFPA->x->coeff[1][0], chip->toFPA->x->coeff[0][1]);
+	    pixelScale2 = hypot (chip->toFPA->y->coeff[1][0], chip->toFPA->y->coeff[0][1]);
+	    pixelScale = 0.5*(pixelScale1 + pixelScale2);
+	    
+	    pixelScaleUse = (useMax) ? PS_MAX (pixelScale, pixelScaleUse) : PS_MIN (pixelScale, pixelScaleUse);
+	}
+	useExternal = false;
+    }
+
+    // rescale each chip by the reference scale
+    for (int i = 0; i < fpa->chips->n; i++) {
+	pmChip *chip = fpa->chips->data[i];
+
+	psPlaneTransform *toFPA = chip->toFPA;
+	psPlaneTransform *fromFPA = chip->fromFPA;
+	    
+	pixelScale1 = hypot (toFPA->x->coeff[1][0], toFPA->x->coeff[0][1]);
+	pixelScale2 = hypot (toFPA->y->coeff[1][0], toFPA->y->coeff[0][1]);
+
+	if (useExternal) {
+	    pixelScaleUse = getChipPixelScale (chip);
+	}
+
+	for (int i = 0; i <= toFPA->x->nX; i++) {
+	    for (int j = 0; j <= toFPA->x->nX; j++) {
+		toFPA->x->coeff[i][j] *= pixelScaleUse/pixelScale1;
+		toFPA->y->coeff[i][j] *= pixelScaleUse/pixelScale2;
+		fromFPA->x->coeff[i][j] *= pixelScale1/pixelScaleUse;
+		fromFPA->y->coeff[i][j] *= pixelScale2/pixelScaleUse;
+	    }
+	}
+
+    }
+    return true;
+}
 
 bool psastroUpdateChipToFPA (pmFPA *fpa, pmChip *chip, psArray *rawstars, psArray *refstars) {
@@ -13,5 +85,5 @@
 
         psPlaneTransformApply (raw->FP, chip->toFPA, raw->chip);
-        psPlaneDistortApply (raw->TP, fpa->toTPA, raw->FP, 0.0, 0.0);
+        psPlaneTransformApply (raw->TP, fpa->toTPA, raw->FP);
         psDeproject (raw->sky, raw->TP, fpa->toSky);
     }