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DM-53234: Fix typo in equation for magnitude limit #1184

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Merged
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Nov 18, 2025
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DM-53234: Fix typo in equation for magnitude limit #1184

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3 changes: 2 additions & 1 deletion python/lsst/pipe/tasks/computeExposureSummaryStats.py
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Original file line number Diff line number Diff line change
Expand Up @@ -950,7 +950,8 @@ def compute_magnitude_limit(
# Total background counts derived from Eq. 45 of LSE-40
background = (skyBg/gain + sigma_inst**2) * neff
# Source counts to achieve the desired SNR (from quadratic formula)
source = (snr**2)/(2*gain) + np.sqrt((snr**4)/(4*gain) + snr**2 * background)
# Note typo in gain exponent in Eq. 45 of LSE-40 (DM-53234)
source = (snr**2)/(2*gain) + np.sqrt((snr**4)/(4*gain**2) + snr**2 * background)

# Convert to a magnitude using the zeropoint.
# Note: Zeropoint currently includes exposure time
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25 changes: 17 additions & 8 deletions tests/test_computeExposureSummaryStats.py
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Expand Up @@ -36,6 +36,15 @@
from lsst.pipe.tasks.computeExposureSummaryStats import ComputeExposureSummaryStatsTask
from lsst.pipe.tasks.computeExposureSummaryStats import compute_magnitude_limit

# Values from syseng_throughputs notebook assuming 30s exposure
# consisting of 2x15s snaps each with readnoise of 9e-
fwhm_eff_fid = {'g': 0.87, 'r': 0.83, 'i': 0.80}
skycounts_fid = {'g': 463.634122, 'r': 988.626863, 'i': 1588.280513}
zeropoint_fid = {'g': 28.508375, 'r': 28.360838, 'i': 28.171396}
readnoise_fid = {'g': 12.73, 'r': 12.73, 'i': 12.73}
# Output magnitude limit from syseng_throughputs notebook
m5_fid = {'g': 24.90, 'r': 24.48, 'i': 24.10}


class ComputeExposureSummaryTestCase(lsst.utils.tests.TestCase):

Expand Down Expand Up @@ -167,17 +176,11 @@ def testComputeMagnitudeLimit(self):
"""Test the magnitude limit calculation using fiducials from SMTN-002
and syseng_throughputs."""

# Values from syseng_throughputs notebook assuming 30s exposure
# consisting of 2x15s snaps each with readnoise of 9e-
fwhm_eff_fid = {'g': 0.87, 'r': 0.83, 'i': 0.80}
skycounts_fid = {'g': 463.634122, 'r': 988.626863, 'i': 1588.280513}
zeropoint_fid = {'g': 28.508375, 'r': 28.360838, 'i': 28.171396}
readnoise_fid = {'g': 12.73, 'r': 12.73, 'i': 12.73}
# Assumed values from SMTN-002
snr = 5
gain = 1.0
# Output magnitude limit from syseng_throughputs notebook
m5_fid = {'g': 24.90, 'r': 24.48, 'i': 24.10}
# For testing non-unity gain
gain_test = 2.0

for band in ['g', 'r', 'i']:
# Translate into DM quantities
Expand All @@ -190,6 +193,12 @@ def testComputeMagnitudeLimit(self):
m5 = compute_magnitude_limit(psfArea, skyBg, zeroPoint, readNoise, gain, snr)
self.assertFloatsAlmostEqual(m5, m5_fid[band], atol=1e-2)

# Changing the gain consistently should not change M5
m5_gain = compute_magnitude_limit(psfArea, skyBg/gain_test,
zeroPoint - 2.5*np.log10(gain_test),
readNoise/gain_test, gain_test, snr)
self.assertFloatsAlmostEqual(m5_gain, m5, atol=1e-6)

# Check that input NaN lead to output NaN
nan = float('nan')
m5 = compute_magnitude_limit(nan, skyBg, zeroPoint, readNoise, gain, snr)
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