103.3. Convert flux to magnitude with ADQL

For the Portal Aspect of the Rubin Science Platform at data.lsst.cloud.

Data Release: DP1

Last verified to run: 2025-06-28

Learning objective: Convert fluxes to magnitudes using a special ADQL function.

LSST data products: Object table

Credit: Originally developed by the Rubin Community Science Team. Please consider acknowledging them if this tutorial is used for the preparation of journal articles, software releases, or other tutorials.

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Magnitude conversions

While the Object table has columns for PSF and cModel magnitudes, all other photometry measures are provided only in fluxes (in nJy). These fluxes can be retrieved as AB magnitudes using the scisql_nanojanskyToAbMag() function, which applies the equation \(m = -2.5 log(f) + 31.4\), where \(m\) is magnitude and \(f\) is flux.

Warning! Fluxes measured in difference images can be negative. Negative fluxes should not be converted to magnitudes using this special ADQL function.

Note: for a demonstration of how to convert fluxes to magnitudes after a query is executed, see the tutorial on how to add a column to the results table.

1. Go to the DP1 Catalogs ADQL interface. Clear any past ADQL query statements from the query box.

2. Set up a query that returns magnitudes. The following query returns the r-band cModel and Sersic magnitudes, and their errors, for objects brighter than 25th magnitude in r-band. The query uses the AS functionality to name the newly-calculated magnitude columns. The query includes a spatial constraint: a circle with radius 0.2 degrees near the center of the ECDFS field.

SELECT r_cModelMag,
       scisql_nanojanskyToAbMag(r_sersicFlux) AS r_sersicMag,
       r_cModelMagErr,
       scisql_nanojanskyToAbMagSigma(r_sersicFlux, r_sersicFluxErr) AS r_sersicMagErr
FROM dp1.Object
WHERE CONTAINS(POINT('ICRS', coord_ra, coord_dec),
      CIRCLE('ICRS', 53.13, -28.1, 0.2))=1
AND r_cModelMag < 25
AND scisql_nanojanskyToAbMag(r_sersicFlux) < 25

3. View the results. Figure 1 shows the results interface, with the table containing only magnitudes and the default plot using the first two columns.

The table panel in the results tab shows the created magnitude columns.

Figure 1: The table panel in the results interface shows no flux columns, but magnitude and magnitude error columns.