What is the intended meaning of fluxMag0Err?

Data Management
#1

What is the intent of fluxMag0Err returned as the second parameter from a call to calib.getFluxMag0() ?

Is it
A) The Poisson shot noise uncertainty for a magnitude 0 source?
or
B) The uncertainty in the determination of fluxMag0?

The documentation, e.g.,
http://doxygen.lsst.codes/stack/doxygen/x_masterDoxyDoc/classlsst_1_1afw_1_1image_1_1_calib.html#a37e84a8e28a09a2f9e6792c709ee15b3

says
“Return the flux, and error in flux, of a zero-magnitude object.”

which implies it’s some shot-noise “extrapolation”

The flux uncertainty of a detection is a function of the photoelectrons in the signal and the noise, but also PSF modeling, aperture corrections, BF effect, etc.

I.e. for a detection that is significantly above sky background but below BF kernel and uncorrected non-linearity the Poisson noise from the photoelectrons attributed to the source is likely very close to the total error. This perhaps applies to 2-3 mag range. Is fluxMag0Err an extrapolation from this region?

#2

The uncertainty in fluxMag0 (you can read that sentence as saying so, but I agree that it can be read in other ways). But I think that @parejkoj has been reworking this interface.

#3

Calib will be replaced with PhotoCalib in the next few weeks. PhotoCalib is defined in line with the Science Requirements Document photometric calibration definitions (section 3.3.4), where the calibration is multiplied with the instrumental flux to get the source flux. fluxMag0 and fluxMag0Err will be going away.

Are the PhotoCalib docs more informative regarding the calibration error?