Photometric Calibration with Stellar and White Dwarf Loci

Description of the calibration method

The reference photometric calibration of J-PLUS DR1 is based on the Stellar Locus Regresion (SLR) technique. The SLR calibration has two main drawbacks. First, the Milky Way extinction is not accounted for in the stellar locus estimation, as described here. This implies that inhomogeneities in the survey photometry due to differential dust reddening are absorbed by the zero points, and the calibration is therefore not referred to the top of the atmosphere but at some intermediate location of the MW halo. This complicates the interpretation of the data and the proper de-reddening of J-PLUS magnitudes for galactic and extragalactic studies. Second, the data used to estimate the reference stellar locus relies on other calibration methods, and thus the SLR inherits any flux bias from the primary calibration source.

Because of these issues, we have developed a new calibration methodology based on the stellar and white dwarf loci (SWDL). The method uses Gaia and Pan-STARRS ancillary data as reference, permits a consistent flux calibration of the 12 J-PLUS filters in all the surveyed area, can be applied in non photometric nights, no previous calibration of the medium bands is needed, and includes the effect of MW dust in the estimation of the loci.

The SWDL method and its performance is presented in López-Sanjuan et al. 2019c. The comparison between the calibration precision on J-PLUS DR1 achieved with SLR and SWDL is shown in Figure 1. We net improvement in the precision is a factor of two.

SWDL photometric calibration

Figure 1. Final calibration uncertainty in J-PLUS DR1. The black pentagons show the accuracy achieved with the SWDL calibration procedure. The red dots show the accuracy of the SLR methodology used as reference in J-PLUS DR1. The dark (light) grey area marks a precision of 10 mmag (20 mmag).

Instructions to update the photometry

We provide all the needed information to update the J-PLUS DR1 photometry from SLR to SWDL. We disregard to directly update the magnitudes and fluxes in the database to keep a neat control of the data version and to ensure reproducibility.

We store the SLR to SWDL transformation in the new column zpt_swdl_calc. There exist several practical cases (magnitudes and fluxes; single mode and dual mode catalogues), so we present the needed ADQL queries for each case in the following. Extra query conditions should be added to the reported queries.

Single mode magnitudes

The magnitudes in the table jplus.MagABSingleObj can be updated as

SELECT mag.tile_id, mag.number, mag.MAG_ISO + mag.zpt_swdl_calc as mag_iso_swdl
FROM jplus.MagABSingleObj mag
WHERE mag.filter_id = jplus::uJAVA

There is one important enumeration on this query. The reference to the other J-PLUS filters can be found here in the enumeration called 'jplus'.

Single mode fluxes

We can obtain the updated fluxes as

SELECT flux.tile_id, flux.number, flux.FLUX_ISO * flux.zpt_swdl_calc as fnu_iso_swdl
FROM jplus.FNuSingleObj flux
WHERE flux.filter_id = jplus::uJAVA


SELECT flux.tile_id, flux.number, flux.FLUX_ISO * flux.zpt_swdl_calc as flambda_iso_swdl
FROM jplus.FLambdaSingleObj flux
WHERE flux.filter_id = jplus::uJAVA

Dual mode magnitudes and fluxes

Because of the dual mode catalogues structure, in this case the column zpt_swdl_calc is an array that encapsulates the change in the photometry for the twelve filters of each source. We also developed functions to deal with arrays operations, so the magnitudes can be updated as

SELECT mag.tile_id, mag.number, ARRAY_ADD(mag.MAG_ISO, mag.zpt_swdl_calc) as mag_iso_swdl
FROM jplus.MagABDualObj mag
, while the fluxes as
SELECT flux.tile_id, flux.number, ARRAY_MULT(flux.FLUX_ISO, flux.zpt_swdl_calc) as fnu_iso_swdl
FROM jplus.FNuDualObj flux
SELECT flux.tile_id, flux.number, ARRAY_MULT(flux.FLUX_ISO, flux.zpt_swdl_calc) as flambda_iso_swdl
FROM jplus.FLambdaDualObj flux

Update of downloaded catalogues

The users can also update the DR1 photometry in their own computer. For that, one should

  • Download the desired magnitudes or fluxes plus the corresponding field zpt_swdl_calc.
  • For each source and filter, update the magnitudes as m_new = m_old + zpt_swdl_calc, and the fluxes as flux_new = flux_old * zpt_swdl_calc.