DM-52677: Add functors for trailLengthErr and trailAngleErr

python/lsst/pipe/tasks/functors.py

@@ -28,6 +28,7 @@
            "ComputePixelScale", "ConvertPixelToArcseconds",
            "ConvertPixelSqToArcsecondsSq",
            "ConvertDetectorAngleToPositionAngle",
+           "ConvertDetectorAngleErrToPositionAngleErr",
            "ReferenceBand", "Photometry",
            "NanoJansky", "NanoJanskyErr", "LocalPhotometry", "LocalNanojansky",
            "LocalNanojanskyErr", "LocalDipoleMeanFlux",
@@ -1394,6 +1395,46 @@ def getPositionAngleFromDetectorAngle(self, theta, cd11, cd12, cd21, cd22):
         # Position angle of vector from (RA1, Dec1) to (RA2, Dec2)
         return np.rad2deg(self.computePositionAngle(ra1, dec1, ra2, dec2))
 
+    def getPositionAngleErrFromDetectorAngleErr(self, theta, theta_err, cd11,
+                                                cd12, cd21, cd22):
+        """Compute position angle error (E of N) from detector angle error.
+
+        Parameters
+        ----------
+        theta : `float`
+            detector angle [radian]
+        theta_err : `float`
+            detector angle err [radian]
+        cd11 : `float`
+            [1, 1] element of the local Wcs affine transform.
+        cd12 : `float`
+            [1, 2] element of the local Wcs affine transform.
+        cd21 : `float`
+            [2, 1] element of the local Wcs affine transform.
+        cd22 : `float`
+            [2, 2] element of the local Wcs affine transform.
+
+        Returns
+        -------
+        Position Angle Error: `~pandas.Series`
+            Position angle error in degrees
+
+        Notes
+        -----
+        The error is estimated by evaluating the position angle at
+        ``theta ± theta_err`` and taking half the absolute difference.
+        """
+        pa_plus = self.getPositionAngleFromDetectorAngle(
+            theta + theta_err, cd11, cd12, cd21, cd22
+        )
+        pa_minus = self.getPositionAngleFromDetectorAngle(
+            theta - theta_err, cd11, cd12, cd21, cd22
+        )
+        # Wrap difference into (-180, 180] to handle the ±180 deg boundary.
+        delta = pa_plus - pa_minus
+        delta = (delta + 180) % 360 - 180
+        return np.abs(delta) / 2
+
 
 class ComputePixelScale(LocalWcs):
     """Compute the local pixel scale from the stored CDMatrix.
@@ -1554,6 +1595,53 @@ def _func(self, df):
         )
 
 
+class ConvertDetectorAngleErrToPositionAngleErr(LocalWcs):
+    """Compute a position angle error from a detector angle error and the
+    stored CDMatrix.
+
+    Returns
+    -------
+    position angle error : degrees
+    """
+
+    name = "PositionAngleErr"
+
+    def __init__(
+        self,
+        theta_col,
+        theta_err_col,
+        colCD_1_1,
+        colCD_1_2,
+        colCD_2_1,
+        colCD_2_2,
+        **kwargs
+    ):
+        self.theta_col = theta_col
+        self.theta_err_col = theta_err_col
+        super().__init__(colCD_1_1, colCD_1_2, colCD_2_1, colCD_2_2, **kwargs)
+
+    @property
+    def columns(self):
+        return [
+            self.theta_col,
+            self.theta_err_col,
+            self.colCD_1_1,
+            self.colCD_1_2,
+            self.colCD_2_1,
+            self.colCD_2_2
+        ]
+
+    def _func(self, df):
+        return self.getPositionAngleErrFromDetectorAngleErr(
+            df[self.theta_col],
+            df[self.theta_err_col],
+            df[self.colCD_1_1],
+            df[self.colCD_1_2],
+            df[self.colCD_2_1],
+            df[self.colCD_2_2]
+        )
+
+
 class ReferenceBand(Functor):
     """Return the band used to seed multiband forced photometry.
 
@@ -1992,14 +2080,7 @@ def _func(self, df):
 
 
 class MomentsBase(Functor):
-    """Base class for functors that use shape moments and localWCS
-
-    Attributes
-    ----------
-    is_covariance : bool
-        Whether the shape columns are terms of a covariance matrix. If False,
-        they will be assumed to be terms of a correlation matrix instead.
-    """
+    """Base class for functors that use shape moments and localWCS"""
 
     is_covariance: bool = True
 

tests/test_functors.py

@@ -53,8 +53,8 @@
                                       MomentsG1Sky, MomentsG2Sky, MomentsTraceSky,
                                       CorrelationIuuSky, CorrelationIvvSky, CorrelationIuvSky,
                                       SemimajorAxisFromCorrelation, SemiminorAxisFromCorrelation,
-                                      PositionAngleFromCorrelation
-                                      )
+                                      PositionAngleFromCorrelation,
+                                      ConvertDetectorAngleErrToPositionAngleErr)
 
 ROOT = os.path.abspath(os.path.dirname(__file__))
 
@@ -860,6 +860,171 @@ def testConvertPixelToArcseconds(self):
                                             atol=1e-16,
                                             rtol=1e-16))
 
+    def testConvertDetectorAngleErrToPositionAngleErr(self):
+        """Test conversion of position angle err in detector degrees to
+        position angle err on sky.
+
+        Requires a similar setup to testConvertDetectorAngleToPositionAngle.
+        """
+        dipoleSep = 10
+        ixx = 10
+        testPixelDeltas = np.random.uniform(-100, 100, size=(self.nRecords, 2))
+
+        for dec in np.linspace(-80, 80, 10):
+            for theta in (-35, 0, 90):
+                for x, y in zip(
+                        np.random.uniform(2 * 1109.99981456774, size=10),
+                        np.random.uniform(2 * 560.018167811613, size=10)):
+                    wcs = self._makeWcs(dec=dec, theta=theta)
+                    cd_matrix = wcs.getCdMatrix()
+
+                    self.dataDict["dipoleSep"] = np.full(self.nRecords,
+                                                         dipoleSep)
+                    self.dataDict["ixx"] = np.full(self.nRecords, ixx)
+                    self.dataDict["slot_Centroid_x"] = np.full(self.nRecords,
+                                                               x)
+                    self.dataDict["slot_Centroid_y"] = np.full(self.nRecords,
+                                                               y)
+                    self.dataDict["someCentroid_x"] = x + testPixelDeltas[:, 0]
+                    self.dataDict["someCentroid_y"] = y + testPixelDeltas[:, 1]
+                    self.dataDict["orientation"] = np.arctan2(
+                        self.dataDict["someCentroid_y"] - self.dataDict[
+                            "slot_Centroid_y"],
+                        self.dataDict["someCentroid_x"] - self.dataDict[
+                            "slot_Centroid_x"],
+                    )
+                    self.dataDict["orientation_err"] = np.abs(np.arctan2(
+                        self.dataDict["someCentroid_y"] - self.dataDict[
+                            "slot_Centroid_y"],
+                        self.dataDict["someCentroid_x"] - self.dataDict[
+                            "slot_Centroid_x"],
+                    )) * 0.001
+
+                    self.dataDict["base_LocalWcs_CDMatrix_1_1"] = np.full(
+                        self.nRecords,
+                        cd_matrix[0, 0])
+                    self.dataDict["base_LocalWcs_CDMatrix_1_2"] = np.full(
+                        self.nRecords,
+                        cd_matrix[0, 1])
+                    self.dataDict["base_LocalWcs_CDMatrix_2_1"] = np.full(
+                        self.nRecords,
+                        cd_matrix[1, 0])
+                    self.dataDict["base_LocalWcs_CDMatrix_2_2"] = np.full(
+                        self.nRecords,
+                        cd_matrix[1, 1])
+                    df = self.getMultiIndexDataFrame(self.dataDict)
+
+                    # Test detector angle err to position angle err conversion
+                    func = ConvertDetectorAngleErrToPositionAngleErr(
+                        "orientation",
+                        "orientation_err",
+                        "base_LocalWcs_CDMatrix_1_1",
+                        "base_LocalWcs_CDMatrix_1_2",
+                        "base_LocalWcs_CDMatrix_2_1",
+                        "base_LocalWcs_CDMatrix_2_2"
+                    )
+                    val = self._funcVal(func, df)
+
+                    # Numerical derivative of the same PA function so a
+                    # compariosn can be made
+                    step = self.dataDict["orientation_err"]  # radians. Bigger? Smaller?
+                    pa_plus_deg = func.getPositionAngleFromDetectorAngle(
+                        df[("meas", "g", "orientation")] + step,
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_1_1")],
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_1_2")],
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_2_1")],
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_2_2")],
+                    )
+                    pa_minus_deg = func.getPositionAngleFromDetectorAngle(
+                        df[("meas", "g", "orientation")] - step,
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_1_1")],
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_1_2")],
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_2_1")],
+                        df[("meas", "g", "base_LocalWcs_CDMatrix_2_2")],
+                    )
+
+                    pa_plus = np.deg2rad(pa_plus_deg.to_numpy())
+                    pa_minus = np.deg2rad(pa_minus_deg.to_numpy())
+
+                    # From example: smallest signed angular difference in
+                    # (-pi, +pi]
+                    dpa = np.angle(np.exp(1j * (pa_plus - pa_minus)))
+                    dpa_dtheta = dpa / (2.0 * step)
+
+                    theta_err = df[("meas", "g", "orientation_err")].to_numpy()
+                    expected_pa_err_deg = np.rad2deg(
+                        np.abs(dpa_dtheta) * theta_err)
+
+                    np.testing.assert_allclose(
+                        val.to_numpy(),
+                        expected_pa_err_deg,
+                        rtol=0,
+                        atol=1e-6,
+                    )
+
+    def testPositionAngleErrPureRotationWcs(self):
+        """PA error for a pure scaling WCS is exactly rad2deg(theta_err).
+
+        For CD = diag(s, -s) the sky direction of a pixel unit vector at
+        angle theta is PA(theta) = theta + pi/2 (flat-sky), so
+        |dPA/dtheta| = 1 everywhere and the propagated error is simply
+        np.rad2deg(theta_err) regardless of theta.  This gives an
+        independent analytical expectation that does not depend on the
+        implementation under test.
+        """
+        s = 5e-5  # degrees/pixel, representative of LSST
+        cd11, cd12, cd21, cd22 = s, 0.0, 0.0, -s
+        local_wcs = LocalWcs("cd11", "cd12", "cd21", "cd22")
+
+        thetas = pd.Series([0.0, np.pi / 4, np.pi / 3, -np.pi / 6, 1.2, -0.8])
+        theta_err = 0.01  # radians, small enough that linearity holds precisely
+        n = len(thetas)
+        cd11_s = pd.Series(np.full(n, cd11))
+        cd12_s = pd.Series(np.full(n, cd12))
+        cd21_s = pd.Series(np.full(n, cd21))
+        cd22_s = pd.Series(np.full(n, cd22))
+
+        pa_err = local_wcs.getPositionAngleErrFromDetectorAngleErr(
+            thetas, theta_err, cd11_s, cd12_s, cd21_s, cd22_s
+        )
+
+        expected_pa_err_deg = np.rad2deg(theta_err)
+        np.testing.assert_allclose(
+            pa_err.to_numpy(), expected_pa_err_deg, rtol=1e-4, atol=0
+        )
+
+    def testPositionAngleErrWrapAround(self):
+        """PA error is correct when PA crosses the ±180 deg boundary.
+
+        For CD = diag(s, -s) and theta = pi/2, PA(theta) = pi deg exactly
+        (the wrap boundary).  With a small theta_err:
+          pa_plus  = PA(pi/2 + theta_err) wraps to  -180 + rad2deg(theta_err)
+          pa_minus = PA(pi/2 - theta_err)          = +180 - rad2deg(theta_err)
+
+        Naive subtraction gives ~-360 deg → apparent error ~180 deg (wrong).
+        The wrap-corrected implementation should return rad2deg(theta_err).
+        """
+        s = 5e-5  # degrees/pixel
+        cd11, cd12, cd21, cd22 = s, 0.0, 0.0, -s
+        local_wcs = LocalWcs("cd11", "cd12", "cd21", "cd22")
+
+        theta_center = np.pi / 2   # PA(theta_center) = 180 deg → wrap boundary
+        theta_err = 0.01  # radians
+
+        cd_s = lambda v: pd.Series([v])  # noqa: E731
+        pa_err = local_wcs.getPositionAngleErrFromDetectorAngleErr(
+            pd.Series([theta_center]),
+            theta_err,
+            cd_s(cd11), cd_s(cd12), cd_s(cd21), cd_s(cd22),
+        )
+
+        expected_pa_err_deg = np.rad2deg(theta_err)
+        # Correct result is ~0.57 deg; naive (unwrapped) result would be ~180 deg.
+        np.testing.assert_allclose(
+            pa_err.to_numpy(), expected_pa_err_deg, rtol=1e-4, atol=0
+        )
+        self.assertLess(float(pa_err.iloc[0]), 1.0)
+
     def _makeWcs(self, dec=53.1595451514076, theta=0):
         """Create a wcs from real CFHT values, rotated by an optional theta.