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  1. Data Management
  2. DM-16598

Add PhotoCalib.calibrateImage() option to compute variance without calib err term

    Details

    • Type: Story
    • Status: Done
    • Resolution: Done
    • Fix Version/s: None
    • Component/s: afw
    • Templates:
    • Story Points:
      2
    • Sprint:
      AP S19-1
    • Team:
      Alert Production

      Description

      Currently PhotoCalib.calibrateImage() computes the variance plane as:

      fluxVar = instFlux.variance * scaleVar +  instFlux.variance * scale**2 + instFlux.image**2 * scaleVar
      

      where scale and scaleVar are my own names for the scale factor that the photocalib applies and the measured uncertainty on that photocalib.

      This equation assumes that the scale factor variance and the image variance are independent, and this isn't obviously true.

      Whether this is the correct variance equation is irrelevant, however. It is a change to how the variance plane in the zeropoint-normalized warps is currently computed, and I suspect it is the primary source of the photometry differences we're seeing in coadds produced with jointcal. While we are testing the effects of using jointcal vs. meas_mosaic's calibration on coadd (DM-16596), it is important that we are able to test these two changes (jointcal's calibrations and the variance plane equation change) separately.

      Implementation: This could be done with an extra argument like photoCalib.calibrateImage(Image, addCalibVar=False or with a state switch like Calib's Calib.setThrowOnNegativeFlux(False).

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                • Assignee:
                  Parejkoj John Parejko
                  Reporter:
                  yusra Yusra AlSayyad
                  Reviewers:
                  Yusra AlSayyad
                  Watchers:
                  Jim Bosch, John Parejko, John Swinbank, Lauren MacArthur, Yusra AlSayyad
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                  • Created:
                    Updated:
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