Incorrect magnitude zeropoint after calibration

My colleague @rfahed and I are using the LSST Science Pipelines to perform Single Visit Processing of LSST sims produced by the Euclid Science Ground Segment.
Since our star catalogue are in units Jansky, we to convert to nJy using: convertReferenceCatalog script

attached is the config file: (3.5 KB)

For our LSST sims, the mag zeropoint values are around ~28.3 in g-band.
However, the magnitude zeropoint we get after processing is

>>> mag_zp=calexp.getPhotoCalib().instFluxToMagnitude(1)
>>> mag_zp

other info:
weekly: w_2022_29
instrument: LSSTCam-imSim

any idea what’s going wrong with the conversion to AB mag?

Just to check: are you following these instructions? How to generate an LSST reference catalog — LSST Science Pipelines

The file you give above has mag_column_list containing things like TU_FNU_U_LSST: are those magnitude columns, or flux columns? The reference catalog converter assumes those are AB magnitudes, but you say above that your catalogs have units of janksy.

Thanks for the prompt reply.


Actually, my colleague noticed this, and was puzzled as indeed TU_FNU_U_LSST are flux columns. Looking at the Configuration fields for the ConvertReferenceCatalogTask; it seems only AB mags are accepted. is that correct?

Yes, the basic converter only accepts AB mags to generate the flux fields. You can subclass ConvertRefcatManager and override the _getFluxes method. See ConvertGaiaManager for an example. This is noted in section 2 of the “How to” doc I linked.

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Thanks! We didn’t read as carefully the instructions as we thought. :blush:

Now we get the result we expect:

(lsst-scipipe-4.0.1) [jpollack@cca010 172117]$ fitsheader calexp_LSSTCam-imSim_g_g_sim_1_4_172117_R22_S11_output_20221003T134125Z.fits | grep 'ZERO'
MAGZERO =     28.2791046770789
HIERARCH MAGZERO_RMS = 0.000303732138799485