diff --git a/python/lsst/summit/utils/guiders/detection.py b/python/lsst/summit/utils/guiders/detection.py
index 4f17b3c2..feb34c83 100644
--- a/python/lsst/summit/utils/guiders/detection.py
+++ b/python/lsst/summit/utils/guiders/detection.py
@@ -373,8 +373,13 @@ def measureStarOnStamp(
     star.runAperturePhotometry(dataBkgSub, apertureRadius, gain=gain, bkgStd=bkgStd)
 
     # 4)  Add centroid and shape in amplifier roi coordinates
-    star.xroi += cutout.xmin_original
-    star.yroi += cutout.ymin_original
+    # GalSim uses 1-based indexing where (1,1) is the
+    # center of pixel [0,0].Matplotlib's imshow with
+    # extent=(0,w,0,h) places pixel centers at half-integers.
+    # To align: subtract 0.5 to convert GalSim coords to
+    # matplotlib pixel-center coords.
+    star.xroi += cutout.xmin_original - 0.5
+    star.yroi += cutout.ymin_original - 0.5
     return star
 
 
diff --git a/python/lsst/summit/utils/guiders/metrics.py b/python/lsst/summit/utils/guiders/metrics.py
index e57d6078..ec41e9dd 100644
--- a/python/lsst/summit/utils/guiders/metrics.py
+++ b/python/lsst/summit/utils/guiders/metrics.py
@@ -297,7 +297,7 @@ def computeTrendMetrics(
     global_std = float(mad_std(y))
 
     fitter = RobustFitter()
-    fit_res = fitter.fit(x, y)
+    fit_res = fitter.fit(x, y, residualThreshold=3.0)
 
     return GuiderDriftResult(
         fit=fit_res,