diff --git a/scopesim/effects/spectral_trace_list.py b/scopesim/effects/spectral_trace_list.py
index 45120f9f..480d546d 100644
--- a/scopesim/effects/spectral_trace_list.py
+++ b/scopesim/effects/spectral_trace_list.py
@@ -121,13 +121,14 @@ def __init__(self, **kwargs):
             "x_colname": "x",
             "y_colname": "y",
             "s_colname": "s",
+            "offset_x": 0,     # [mm] in detector plane
+            "offset_y": 0,     # [mm] in detector plane
             "wave_colname": "wavelength",
             "center_on_wave_mid": False,
             "dwave": 0.002,  # [um] for finding the best fit dispersion
             "invalid_value": None,  # for dodgy trace file values
         }
         self.meta.update(params)
-
         # Parameters that are specific to the subclass
         self.meta.update(self._class_params)
         self.meta.update(kwargs)
@@ -239,6 +240,18 @@ def apply_to(self, obj, **kwargs):
                 logger.info("Making cube")
                 obj.cube = obj.make_hdu()
 
+            # Check whether an offset slit is used. If so, recompute spectral traces.
+            offset_x = obj.cube.header["CRVAL1D"]
+            offset_y = obj.cube.header["CRVAL2D"]
+            if (offset_x != self.meta["offset_x"] or
+                offset_y != self.meta["offset_y"]):
+                logger.debug("Recomputing spectral traces for offset (%.1g, %.1g)",
+                             offset_x, offset_y)
+                self.meta["offset_x"] = offset_x
+                self.meta["offset_y"] = offset_y
+                self.make_spectral_traces()
+                self.update_meta()
+
             spt = self.spectral_traces[obj.trace_id]
             obj.hdu = spt.map_spectra_to_focal_plane(obj)
 
diff --git a/scopesim/effects/spectral_trace_list_utils.py b/scopesim/effects/spectral_trace_list_utils.py
index 44b4f5a5..28111b49 100644
--- a/scopesim/effects/spectral_trace_list_utils.py
+++ b/scopesim/effects/spectral_trace_list_utils.py
@@ -49,6 +49,8 @@ class SpectralTrace:
         "x_colname": "x",
         "y_colname": "y",
         "s_colname": "s",
+        "offset_x": 0,
+        "offset_y": 0,
         "wave_colname": "wavelength",
         "dwave": 0.002,
         "aperture_id": 0,
@@ -119,9 +121,12 @@ def compute_interpolation_functions(self):
         Focal plane coordinates are `x` and `y`, in mm. Slit coordinates are
         `xi` (spatial coordinate along the slit, in arcsec) and `lam`
         (wavelength, in um).
+
+        The interpolation functions include a shift in the focal-plane
+        coordinates, determined from the CRVAL of the source FOV.
         """
-        x_arr = self.table[self.meta["x_colname"]]
-        y_arr = self.table[self.meta["y_colname"]]
+        x_arr = self.table[self.meta["x_colname"]] + self.meta["offset_x"]
+        y_arr = self.table[self.meta["y_colname"]] + self.meta["offset_y"]
         xi_arr = self.table[self.meta["s_colname"]]
         lam_arr = self.table[self.meta["wave_colname"]]