DM-45815: Add utility functions to access gain and readnoise

python/lsst/ip/isr/isrFunctions.py

@@ -45,6 +45,8 @@
     "trimToMatchCalibBBox",
     "updateVariance",
     "widenSaturationTrails",
+    "getExposureGains",
+    "getExposureReadNoises",
 ]
 
 import math
@@ -1222,3 +1224,67 @@ def countMaskedPixels(maskedIm, maskPlane):
     maskBit = maskedIm.mask.getPlaneBitMask(maskPlane)
     nPix = numpy.where(numpy.bitwise_and(maskedIm.mask.array, maskBit))[0].flatten().size
     return nPix
+
+
+def getExposureGains(exposure):
+    """Get the per-amplifier gains used for this exposure.
+
+    Parameters
+    ----------
+    exposure : `lsst.afw.image.Exposure`
+        The exposure to find gains for.
+
+    Returns
+    -------
+    gains : `dict` [`str` `float`]
+        Dictionary of gain values, keyed by amplifier name.
+        Returns empty dict when detector is None.
+    """
+    det = exposure.getDetector()
+    if det is None:
+        return dict()
+
+    metadata = exposure.getMetadata()
+    gains = {}
+    for amp in det:
+        ampName = amp.getName()
+        # The key may use the new LSST ISR or the old LSST prefix
+        if (key1 := f"LSST ISR GAIN {ampName}") in metadata:
+            gains[ampName] = metadata[key1]
+        elif (key2 := f"LSST GAIN {ampName}") in metadata:
+            gains[ampName] = metadata[key2]
+        else:
+            gains[ampName] = amp.getGain()
+    return gains
+
+
+def getExposureReadNoises(exposure):
+    """Get the per-amplifier read noise used for this exposure.
+
+    Parameters
+    ----------
+    exposure : `lsst.afw.image.Exposure`
+        The exposure to find read noise for.
+
+    Returns
+    -------
+    readnoises : `dict` [`str` `float`]
+        Dictionary of read noise values, keyed by amplifier name.
+        Returns empty dict when detector is None.
+    """
+    det = exposure.getDetector()
+    if det is None:
+        return dict()
+
+    metadata = exposure.getMetadata()
+    readnoises = {}
+    for amp in det:
+        ampName = amp.getName()
+        # The key may use the new LSST ISR or the old LSST prefix
+        if (key1 := f"LSST ISR READNOISE {ampName}") in metadata:
+            readnoises[ampName] = metadata[key1]
+        elif (key2 := f"LSST READNOISE {ampName}") in metadata:
+            readnoises[ampName] = metadata[key2]
+        else:
+            readnoises[ampName] = amp.getReadNoise()
+    return readnoises

tests/test_isrFunctions.py

@@ -344,6 +344,48 @@ def test_countMaskedPixels(self):
         mi.mask[noDataBox] ^= NODATABIT  # XOR to reset what we did
         self.assertEqual(ipIsr.countMaskedPixels(mi, 'NO_DATA'), 0)
 
+    def test_getExposureGains(self):
+        exposure = self.inputExp.clone()
+        metadata = exposure.metadata
+        amps = exposure.getDetector().getAmplifiers()
+
+        # Check the default values
+        values = ipIsr.getExposureGains(exposure).values()
+        self.assertEqual(list(values), len(amps)*[1.0])
+
+        # Set values using old ISR keys
+        for amp in amps:
+            metadata[f"LSST GAIN {amp.getName()}"] = 2.0
+        values = ipIsr.getExposureGains(exposure).values()
+        self.assertEqual(list(values), len(amps)*[2.0])
+
+        # Set values using new ISR keys
+        for amp in amps:
+            metadata[f"LSST ISR GAIN {amp.getName()}"] = 3.0
+        values = ipIsr.getExposureGains(exposure).values()
+        self.assertEqual(list(values), len(amps)*[3.0])
+
+    def test_getExposureReadNoises(self):
+        exposure = self.inputExp.clone()
+        metadata = exposure.metadata
+        amps = exposure.getDetector().getAmplifiers()
+
+        # Check the default values
+        values = ipIsr.getExposureReadNoises(exposure).values()
+        self.assertEqual(list(values), len(amps)*[10.0])
+
+        # Set values using old ISR keys
+        for amp in amps:
+            metadata[f"LSST READNOISE {amp.getName()}"] = 2.0
+        values = ipIsr.getExposureReadNoises(exposure).values()
+        self.assertEqual(list(values), len(amps)*[2.0])
+
+        # Set values using new ISR keys
+        for amp in amps:
+            metadata[f"LSST ISR READNOISE {amp.getName()}"] = 3.0
+        values = ipIsr.getExposureReadNoises(exposure).values()
+        self.assertEqual(list(values), len(amps)*[3.0])
+
 
 class MemoryTester(lsst.utils.tests.MemoryTestCase):
     pass