diff --git a/src/ctapipe/io/__init__.py b/src/ctapipe/io/__init__.py
index dbf555a7bd6..7974f1ffaaa 100644
--- a/src/ctapipe/io/__init__.py
+++ b/src/ctapipe/io/__init__.py
@@ -21,7 +21,7 @@
 from .interpolation import (
     Interpolator,
     PointingInterpolator,
-    GainInterpolator,
+    FlatFieldInterpolator,
     PedestalInterpolator,
 )
 
@@ -45,5 +45,5 @@
     "Interpolator",
     "PointingInterpolator",
     "PedestalInterpolator",
-    "GainInterpolator",
+    "FlatFieldInterpolator",
 ]
diff --git a/src/ctapipe/io/interpolation.py b/src/ctapipe/io/interpolation.py
index dc0953e3892..e82d96b2e9b 100644
--- a/src/ctapipe/io/interpolation.py
+++ b/src/ctapipe/io/interpolation.py
@@ -165,7 +165,7 @@ class PointingInterpolator(Interpolator):
     """
 
     telescope_data_group = "/dl0/monitoring/telescope/pointing"
-    required_columns = {"time", "azimuth", "altitude"}
+    required_columns = frozenset(["time", "azimuth", "altitude"])
     expected_units = {"azimuth": u.rad, "altitude": u.rad}
 
     def __call__(self, tel_id, time):
@@ -230,16 +230,16 @@ def add_table(self, tel_id, input_table):
         self._interpolators[tel_id]["alt"] = interp1d(mjd, alt, **self.interp_options)
 
 
-class GainInterpolator(Interpolator):
+class FlatFieldInterpolator(Interpolator):
     """
-    Interpolator for relative gain data
+    Interpolator for flatfield data
     """
 
     telescope_data_group = "dl1/calibration/gain"  # TBD
 
     def __call__(self, tel_id, time):
         """
-        Interpolate pedestal or gain for a given time and tel_id.
+        Interpolate flatfield data for a given time and tel_id.
 
         Parameters
         ----------
@@ -250,14 +250,14 @@ def __call__(self, tel_id, time):
 
         Returns
         -------
-        gain : array [float]
-            interpolated relative gain
+        ffield : array [float]
+            interpolated flatfield data
         """
 
         self._check_interpolators(tel_id)
 
-        gain = self._interpolators[tel_id]["gain"](time)
-        return gain
+        ffield = self._interpolators[tel_id]["gain"](time)
+        return ffield
 
     def add_table(self, tel_id, input_table):
         """
@@ -270,10 +270,10 @@ def add_table(self, tel_id, input_table):
         input_table : astropy.table.Table
             Table of pointing values, expected columns
             are ``time`` as ``Time`` column and "gain"
-            for the relative gain data
+            for the flatfield data
         """
 
-        self.required_columns = {"time", "gain"}
+        self.required_columns = frozenset(["time", "gain"])  # TBD
 
         self._check_tables(input_table)
 
@@ -329,7 +329,7 @@ def add_table(self, tel_id, input_table):
             for the pedestal data
         """
 
-        self.required_columns = {"time", "pedestal"}
+        self.required_columns = frozenset(["time", "pedestal"])  # TBD
 
         self._check_tables(input_table)
 
diff --git a/src/ctapipe/io/tests/test_interpolator.py b/src/ctapipe/io/tests/test_interpolator.py
index 1279dc5c784..4d41ceeff0b 100644
--- a/src/ctapipe/io/tests/test_interpolator.py
+++ b/src/ctapipe/io/tests/test_interpolator.py
@@ -6,7 +6,7 @@
 from astropy.time import Time
 
 from ctapipe.io.interpolation import (
-    GainInterpolator,
+    FlatFieldInterpolator,
     PedestalInterpolator,
     PointingInterpolator,
 )
@@ -108,7 +108,7 @@ def test_bounds():
         },
     )
 
-    table_gain = Table(
+    table_flatfield = Table(
         {
             "time": np.arange(0.0, 10.1, 2.0),
             "gain": np.reshape(np.random.normal(1.0, 1.0, 1850 * 6), (6, 1850)),
@@ -117,10 +117,10 @@ def test_bounds():
 
     interpolator_pointing = PointingInterpolator()
     interpolator_pedestal = PedestalInterpolator()
-    interpolator_gain = GainInterpolator()
+    interpolator_flatfield = FlatFieldInterpolator()
     interpolator_pointing.add_table(1, table_pointing)
     interpolator_pedestal.add_table(1, table_pedestal)
-    interpolator_gain.add_table(1, table_gain)
+    interpolator_flatfield.add_table(1, table_flatfield)
 
     error_message = "below the interpolation range"
 
@@ -131,7 +131,7 @@ def test_bounds():
         interpolator_pedestal(tel_id=1, time=-0.1)
 
     with pytest.raises(ValueError, match=error_message):
-        interpolator_gain(tel_id=1, time=-0.1)
+        interpolator_flatfield(tel_id=1, time=-0.1)
 
     with pytest.raises(ValueError, match="above the interpolation range"):
         interpolator_pointing(tel_id=1, time=t0 + 10.2 * u.s)
@@ -142,21 +142,21 @@ def test_bounds():
 
     pedestal = interpolator_pedestal(tel_id=1, time=1.0)
     assert all(pedestal == table_pedestal["pedestal"][0])
-    gain = interpolator_gain(tel_id=1, time=1.0)
-    assert all(gain == table_gain["gain"][0])
+    flatfield = interpolator_flatfield(tel_id=1, time=1.0)
+    assert all(flatfield == table_flatfield["gain"][0])
     with pytest.raises(KeyError):
         interpolator_pointing(tel_id=2, time=t0 + 1 * u.s)
     with pytest.raises(KeyError):
         interpolator_pedestal(tel_id=2, time=1.0)
     with pytest.raises(KeyError):
-        interpolator_gain(tel_id=2, time=1.0)
+        interpolator_flatfield(tel_id=2, time=1.0)
 
     interpolator_pointing = PointingInterpolator(bounds_error=False)
     interpolator_pedestal = PedestalInterpolator(bounds_error=False)
-    interpolator_gain = GainInterpolator(bounds_error=False)
+    interpolator_flatfield = FlatFieldInterpolator(bounds_error=False)
     interpolator_pointing.add_table(1, table_pointing)
     interpolator_pedestal.add_table(1, table_pedestal)
-    interpolator_gain.add_table(1, table_gain)
+    interpolator_flatfield.add_table(1, table_flatfield)
 
     for dt in (-0.1, 10.1) * u.s:
         alt, az = interpolator_pointing(tel_id=1, time=t0 + dt)
@@ -164,7 +164,7 @@ def test_bounds():
         assert np.isnan(az.value)
 
     assert all(np.isnan(interpolator_pedestal(tel_id=1, time=-0.1)))
-    assert all(np.isnan(interpolator_gain(tel_id=1, time=-0.1)))
+    assert all(np.isnan(interpolator_flatfield(tel_id=1, time=-0.1)))
 
     interpolator_pointing = PointingInterpolator(bounds_error=False, extrapolate=True)
     interpolator_pointing.add_table(1, table_pointing)