From df83934eb9992e12c2632688a6750a8844ee62be Mon Sep 17 00:00:00 2001
From: Karl Kosack <karl.kosack@cea.fr>
Date: Mon, 18 May 2026 17:35:00 +0200
Subject: [PATCH 1/2] add altaz_to_icrs helper function

---
 src/ctapipe/coordinates/__init__.py |  2 ++
 src/ctapipe/coordinates/utils.py    | 21 +++++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/src/ctapipe/coordinates/__init__.py b/src/ctapipe/coordinates/__init__.py
index 7d6ed4ea864..2a3f2e6ad8a 100644
--- a/src/ctapipe/coordinates/__init__.py
+++ b/src/ctapipe/coordinates/__init__.py
@@ -22,6 +22,7 @@
 from .nominal_frame import NominalFrame
 from .telescope_frame import TelescopeFrame
 from .utils import (
+    altaz_to_icrs,
     altaz_to_nominal,
     altaz_to_righthanded_cartesian,
     get_point_on_shower_axis,
@@ -42,6 +43,7 @@
     "shower_impact_distance",
     "get_point_on_shower_axis",
     "altaz_to_nominal",
+    "altaz_to_icrs",
 ]
 
 
diff --git a/src/ctapipe/coordinates/utils.py b/src/ctapipe/coordinates/utils.py
index 52a993357ff..181727f170a 100644
--- a/src/ctapipe/coordinates/utils.py
+++ b/src/ctapipe/coordinates/utils.py
@@ -11,6 +11,7 @@
     "altaz_to_righthanded_cartesian",
     "get_point_on_shower_axis",
     "altaz_to_nominal",
+    "altaz_to_icrs",
 ]
 
 
@@ -103,3 +104,23 @@ def altaz_to_nominal(az, alt, pointing_az, pointing_alt) -> u.Quantity:
     return u.Quantity(
         np.column_stack((nominal_coord.fov_lon.deg, nominal_coord.fov_lat.deg)), u.deg
     )
+
+
+def altaz_to_icrs(az, alt, obstime, location) -> u.Quantity:
+    """
+    Compute nominal (FOV) coordinates from alt/az coordinates.
+
+    This can be used in a FeatureGenerator or ExpressionEngine to get a single
+    column with fov_lon, fov_lat coordinates.
+
+    Returns
+    -------
+    u.Quantity:
+       2D array of coordinates with 2 columns: ra, dec
+    """
+    event_coord = SkyCoord(
+        az=az, alt=alt, frame="altaz", obstime=obstime, location=location
+    )
+    icrs_coord = event_coord.transform_to("icrs")
+
+    return u.Quantity(np.column_stack((icrs_coord.ra.deg, icrs_coord.dec.deg)), u.deg)

From 7f5cb95a697c5859ed9585071f721324720dc7ba Mon Sep 17 00:00:00 2001
From: Karl Kosack <karl.kosack@cea.fr>
Date: Mon, 18 May 2026 17:35:20 +0200
Subject: [PATCH 2/2] add a featureset for dl2_to_dl3

---
 src/ctapipe/io/event_preprocessor.py | 57 ++++++++++++++++++++++++++--
 1 file changed, 54 insertions(+), 3 deletions(-)

diff --git a/src/ctapipe/io/event_preprocessor.py b/src/ctapipe/io/event_preprocessor.py
index d311035815d..faa23291379 100644
--- a/src/ctapipe/io/event_preprocessor.py
+++ b/src/ctapipe/io/event_preprocessor.py
@@ -2,7 +2,7 @@
 
 from astropy.coordinates import angular_separation
 
-from ..coordinates import altaz_to_nominal
+from ..coordinates import altaz_to_icrs, altaz_to_nominal
 from ..core import (
     Component,
     FeatureGenerator,
@@ -123,6 +123,45 @@ def _dl2_irf_config(preprocessor):
     }
 
 
+@FeatureSetRegistry.register("dl2_to_dl3")
+def _dl2_to_dl3_config(preprocessor: "EventPreprocessor"):
+    return {
+        "features_to_generate": [
+            ("ENERGY", f"{preprocessor.energy_reconstructor}_energy"),
+            ("ALT", f"{preprocessor.geometry_reconstructor}_alt"),
+            ("AZ", f"{preprocessor.geometry_reconstructor}_az"),
+            ("TIME", "time"),
+            ("EVENT_ID", "event_id"),
+            ("GAMMANESS", f"{preprocessor.gammaness_reconstructor}_prediction"),
+            (
+                "reco_fov_coord",
+                "altaz_to_nominal(AZ, ALT, subarray_pointing_lon, subarray_pointing_lat)",
+            ),
+            ("FOV_LON", "reco_fov_coord[:,0]"),
+            ("FOV_LAT", "reco_fov_coord[:,1]"),
+            (
+                "reco_icrs_coord",
+                "altaz_to_icrs(AZ, ALT, TIME, LOCATION)",
+            ),
+            ("RA", "reco_icrs_coord[:,0]"),
+            ("DEC", "reco_icrs_coord[:,1]"),
+        ],
+        "quality_criteria": [],
+        "output_features": [
+            "EVENT_ID",
+            "TIME",
+            "RA",
+            "DEC",
+            "ENERGY",
+            "ALT",
+            "AZ",
+            "FOV_LON",
+            "FOV_LAT",
+            "GAMMANESS",
+        ],
+    }
+
+
 class EventPreprocessor(Component):
     """
     Selects or generates features and filters tables of events.
@@ -198,14 +237,26 @@ def __init__(self, config=None, parent=None, **kwargs):
                 "of features in the configuration (DL2EventPreprocessor.features)."
             )
 
-    def __call__(self, table):
-        """Return new table with only the columns in features."""
+    def __call__(self, table, **other_attributes):
+        """
+        Return new table with only the columns in features.
+
+        Parameters
+        ----------
+        table: Table
+           Table to process
+        **other_attributes: Any
+           Other functions or objects that the FeatureGenerator should have
+           access to, in addition to the default ones.
+        """
 
         # generate new features, which includes renaming columns:
         generated = self.feature_generator(
             table,
             angular_separation=angular_separation,
             altaz_to_nominal=altaz_to_nominal,
+            altaz_to_icrs=altaz_to_icrs,
+            **other_attributes,
         )
 
         # apply event selection on the resulting table