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: 2025-06-16

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.

Get Support: Everyone is encouraged to ask questions or raise issues in the Support Category of the Rubin Community Forum. Rubin staff will respond to all questions posted there.

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

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1. Go to the DP0.2 catalog ADQL interface. Navigate to the Portal’s DP0.2 Catalogs tab and switch to the ADQL interface.

2. To return magnitudes use the scisql_nanojanskyToAbMag() function. This function will convert a flux value with the units of nanoJanskys (nJy) into an AB magnitude. The conversion is \(m = -2.5 log(f) + 31.4\), where \(m\) is magnitude and \(f\) is flux.

SELECT coord_ra,coord_dec,refExtendedness,
       scisql_nanojanskyToAbMag(u_cModelFlux) AS u_cModelMag,
       scisql_nanojanskyToAbMag(g_cModelFlux) AS g_cModelMag,
       scisql_nanojanskyToAbMag(r_cModelFlux) AS r_cModelMag,
       scisql_nanojanskyToAbMag(i_cModelFlux) AS i_cModelMag,
       scisql_nanojanskyToAbMag(z_cModelFlux) AS z_cModelMag,
       scisql_nanojanskyToAbMag(y_cModelFlux) AS y_cModelMag
FROM dp1_v29.Object
WHERE CONTAINS(POINT('ICRS', coord_ra, coord_dec),
      CIRCLE('ICRS', 53.13, -28.1, 0.167))=1
      AND (refExtendedness =1
           AND u_cModelFlux >360 AND g_cModelFlux >360
           AND r_cModelFlux >360 AND i_cModelFlux >360
           AND z_cModelFlux >360 AND y_cModelFlux >360)

3. To return magnitude errors use the scisql_nanojanskyToAbMagSigma() function. This funtion will convert the corresponding flux errors, in nJy, into AB magnitude errors.

SELECT coord_ra, coord_dec, refExtendedness,
       scisql_nanojanskyToAbMag(u_cModelFlux) AS u_cModelMag,
       scisql_nanojanskyToAbMagSigma(u_cModelFlux, u_cModelFluxErr) AS u_cModelMagErr,
       scisql_nanojanskyToAbMag(g_cModelFlux) AS g_cModelMag,
       scisql_nanojanskyToAbMagSigma(g_cModelFlux, g_cModelFluxErr) AS g_cModelMagErr,
       scisql_nanojanskyToAbMag(r_cModelFlux) AS r_cModelMag,
       scisql_nanojanskyToAbMagSigma(r_cModelFlux, r_cModelFluxErr) AS r_cModelMagErr,
       scisql_nanojanskyToAbMag(i_cModelFlux) AS i_cModelMag,
       scisql_nanojanskyToAbMagSigma(i_cModelFlux, i_cModelFluxErr) AS i_cModelMagErr,
       scisql_nanojanskyToAbMag(z_cModelFlux) AS z_cModelMag,
       scisql_nanojanskyToAbMagSigma(z_cModelFlux, z_cModelFluxErr) AS z_cModelMagErr,
       scisql_nanojanskyToAbMag(y_cModelFlux) AS y_cModelMag,
       scisql_nanojanskyToAbMagSigma(y_cModelFlux, y_cModelFluxErr) AS y_cModelMagErr
FROM dp1_v29.Object
WHERE CONTAINS(POINT('ICRS', coord_ra, coord_dec),
      CIRCLE('ICRS', 53.13, -28.1, 0.167)) =1
      AND (refExtendedness =1
          AND u_cModelFlux >360 AND g_cModelFlux >360
          AND r_cModelFlux >360 AND i_cModelFlux >360
          AND z_cModelFlux >360 AND y_cModelFlux >360)

4. Notice the query constraints on coordinate, extendedness, and flux are set in order to retrieve a sample of extended sources (galaxies) brighter than 25th magnitude.

5. View the results. Figure 1 shows the table panel in the Results tab for the query above. Notice that there are no flux columns because they were retrieved AS magnitudes, and the columns have been renamed as specified in the ADQL statement.

Figure 1: The table panel in the Results tab for the query above shows no flux columns, but magnitude and magnitude error columns.

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.