302.2. Stellar multi-color photometry

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

Data Release: DP2

Last verified to run: 2026-11-05

Learning objective: Explore stellar color-magnitude and color-color diagrams in DP2.

LSST data products: Object

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. DOI: 10.11578/rubin/dc.20250909.20

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.

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1. Execute an ADQL query for stars in the NGC 6822 field. Log in to the Portal Aspect, select the “DP1 & DP2 Catalogs” tab, and click “Edit ADQL” at upper right. Enter the following ADQL statement, set the row limit to 100,000, and click “Search” at lower left. The query retrieves coordinates, PSF photometry in gri, and E(B-V) values by Schlegel, Finkbeiner & Davis (1998) within a 0.5-degree radius centered on NGC 6822 (RA = 296.24, Dec = -14.8). It restricts the results to point sources (refExtendedness = 0) with a PSF flux signal-to-noise ratio greater than 5 in gri. This dwarf galaxy lies ~500 kpc away, just beyond the Milky Way’s halo, and is resolved into individual stars by Rubin, making it a good target for demonstrating star-by-star Milky Way foreground dust removal.

SELECT coord_ra, coord_dec, g_psfMag, r_psfMag, i_psfMag, ebv
      FROM dp2.Object
      WHERE CONTAINS(POINT('ICRS', coord_ra, coord_dec),
            CIRCLE('ICRS', 296.24, -14.8, 0.5)) = 1
      AND refExtendedness = 0
      AND g_psfFlux/g_psfFluxErr > 5
      AND r_psfFlux/r_psfFluxErr > 5
      AND i_psfFlux/i_psfFluxErr > 5

The query should return 70,838 rows.

2. Apply dust correction. Use the extinction coefficients for the Rubin filters for dust correction: \(R_g = 3.66\), \(R_r = 2.70\), and \(R_i = 2.05\). These coefficients are derived from the CCM89 extinction law (Cardelli, Clayton, & Mathis 1989), assuming a flat spectral energy distribution and a default total-to-selective extinction ratio of \(R_V = 3.1\), representative of the diffuse Milky Way interstellar medium. To add columns for the dust-corrected magnitudes, click the “Add a column” icon in the table toolbar, name the new column g0, and enter the expression g_psfMag - 3.66 * ebv. Repeat this for r0 with r_psfMag - 2.70 * ebv, and for i0 with i_psfMag - 2.05 * ebv.

3. Plot the *E(g-i)* color excess. The color excess E(g-i) quantifies interstellar reddening by measuring how much redder a star appears than its intrinsic, unreddened color: \(E(g-i) = (g-i)_{\mathrm{obs}} - (g-i)_0\). To visualize the E(g-i) distribution, click the “+” icon in the “Active Chart” panel. Choose “Histogram” as the plot type, enter (g_psfMag - i_psfMag) - (g0 - i0) into the expression field, set the number of bins to 100, set the x-axis label to “E(g-i) = (g - i) - (g0 - i0)”, and click “OK” to generate the plot. Delete the default chart of RA and Dec coordinates.

Figure 1: Histogram of the E(g-i) color excess. Because the observed magnitudes cancel, \(E(g-i) = (R_g - R_i)\,E(B-V)\), so the histogram is simply a rescaling of the E(B-V) values at the stars’ positions and does not depend on the stellar photometry. Its purpose is to show the typical reddening toward individual stars and its spread across the field.

4. Plot the CCD and CMD. Plot the color-color and color-magnitude diagrams (CCD and CMD, respectively) using the dust-corrected magnitudes g0, r0, and i0. Click the “+” icon to add a chart, and choose “Heatmap” as the plot type. Set the x-axis to g0-i0 and the y-axis to r0-i0, using 300 bins for both axes. Finally, set the range for both axes from -2 to 4, and click OK to display the plot. The resulting plot should look like Figure 2. Repeat the same steps for the CMD, but set the y-axis to i0 and its range from 14 to 25 with the “reverse” option enabled.

Figure 2: The CCD (left) and CMD (right) of stars in the NGC 6822 field, constructed with dust-corrected magnitudes g0, r0, and i0. Stars form a well-defined locus in the CCD. In the CMD, distinct stellar populations are visible: the blue plume at -0.5 < g0-i0 < 0 corresponds to upper main-sequence stars belonging to NGC 6822; the densest feature at 0 < g0-i0 < 2 and 20 < i0 < 24 traces the NGC 6822 red-giant sequence; and the vertical structure near g0-i0 ~ 0.9 is composed of Milky Way foreground main-sequence turnoff stars.