MER Photometry Cookbook

Introduction

The MER Pipeline performs four kinds of photometric measurements:

Sources are detected in the VIS detection mosaic and in a NIR-stack detection mosaic (the VIS detection has the priority). The VIS_DET column in the MER final catalog provides the information about the provenance of each source.

The scope of this cookbook is to give the user a quick overview on how to use the photometric information stored in the MER final catalog.

Main information from the MER final catalog

The MER final catalog provides the following information:

  • FLUX_DETECTION_TOTAL: flux measured within a Kron aperture in the detection mosaic (VIS mosaic or NIR-stack mosaic, as indicated by the VIS_DET flag). It corresponds to the good old FLUX_AUTO in SExtractor.

  • FLUX SEGMENTATION: flux within the segmented pixels of a source (FLUX_ISO in SExtractor).

  • FLUX_<band>_nFWHM_APER: these are fluxes measured in fixed circular apertures on images PSF-matched to the one with the worst resolution (typically EXT u). The apertures have a diameter of n FWHM of the worst band PSF, with n=1,2,3,4, and they are computed individually source by source (because the PSF varies across the FoV). The FWHM column lists the value in arcseconds used for each source.

  • FLUX_<band>_TEMPLFIT: fluxes from template-fitting run on the given band.

  • FLUX_VIS_TO_<band>_TEMPLFIT: fluxes from template-fitting run on the VIS image smoothed to the resolution of the given band. This is only useful to compute colors from template-fitting.

  • FLUX_<band>_SERSIC: fluxes from model-fitting (single Sersic models) obtained using SourceXtractor++.

  • FLUX_VIS_PSF: fluxes from PSF-fitting on VIS only.

For all these flux estimates, the corresponding uncertainties are given in the columns named FLUXERR_*. All fluxes in the MER final catalog are in \({\mu}Jy\).

Photometry flags

The reference photometic measurement is given by the FLUX_DETECTION_TOTAL column. It contains the flux measured within a Kron aperture in the detection mosaic. To assess the reliability of FLUX_DETECTION_TOTAL, we provide a binary flag, DET_QUALITY_FLAG, that encodes the following information:

Bit set

Integer

Descriptiom

1

1

Source contaminated by close neighbors or has bad pixels

2

2

Source blended with another one (must comply with PARENT_ID ≠ -1)

3

4

Source saturated

4

8

Source close to a border

8

128

Source within the VIS bright star mask

9

256

Source within the NIR bright star mask

10

512

Source within an extended object area

11

1024

Source skipped by the deblending algorithm due to large pixel size

Note that DET_QUALITY_FLAG contains more information than the single-band flags, FLAG_<band> (e.g FLAG_VIS, FLAG_Y, FLAG_G_EXT_DECAM), that only use bits from 1 to 4:

Bit set

Integer

Descriptiom

1

1

Source contaminated by closer neighbors or has bad pixels

2

2

Source blended with another one (must comply with PARENT_ID ≠ -1)

3

4

Source saturated

4

8

Source close to a border

How to get the photometry you need?

Colors

Colors can be obtained in three ways:

  • Using aperture photometry: e.g.

    (1)\[\textrm{H} - \textrm{g} = -2.5 \times \log_{10}\left(\frac{\textrm{FLUX_H_nFWHM_APER}}{\textrm{FLUX_g_nFWHM_APER}}\right)\]

    This is a color obtained with fixed apertures on PSF-matched images.

  • Using template fitting, with VIS only: e.g.

    (2)\[\textrm{H} - \textrm{VIS} = -2.5 \times \log_{10}\left(\frac{\textrm{FLUX_H_TEMPLFIT}}{\textrm{FLUX_VIS_TO_H_TEMPLFIT}}\right)\]

    This is a color obtained with VIS smoothed to the relevant band.

  • Using model fitting: e.g.

    (3)\[\textrm{g} - \textrm{J} = -2.5 \times \log_{10}\left(\frac{\textrm{FLUX_g_SERSIC}}{\textrm{FLUX_J_SERSIC}}\right)\]

    This is a color between two total fluxes estimated via model fitting.

Total fluxes

In the detection band, FLUX_DETECTION_TOTAL is the best estimate of the total flux of the source. This is the VIS Kron flux for the VIS detected sources, i.e. VIS_DET = 1 and a (non-physical) NIR-stack flux for NIR detected sources.

In all other bands, there is the possibility is to scale the detection flux with a color term:

  • Using aperture photometry: total fluxes are obtained by scaling FLUX_DETECTION_TOTAL with the aperture color between the relevant band and the detection band (both PSF-matched to the worst resolution); e.g.

    (4)\[F_{\textrm{H, tot}} = \textrm{FLUX_DETECTION_TOTAL} \times \frac{\textrm{FLUX_H_nFWHM_APER}}{\textrm{FLUX_VIS_nFWHM_APER}}\]

    if the source is detected in VIS (i.e. has VIS_DET = 1), and

    (5)\[F_{\textrm{H, tot}} = \textrm{FLUX_DETECTION_TOTAL} \times \frac{\textrm{FLUX_H_nFWHM_APER}}{\textrm{FLUX_NIR_STACK_nFWHM_APER}}\]

    if the source is detected in the NIR-stack (i.e. has VIS_DET = 0).

  • Using template fitting: by color-correcting the detection flux. In this case the relevant column is VIS_TO_<band> (because here VIS is smoothed to the relevant band); e.g.

    (6)\[F_{\textrm{H, tot}} = \textrm{FLUX_DETECTION_TOTAL} \times \frac{\textrm{FLUX_H_TEMPLFIT}}{\textrm{FLUX_VIS_TO_H_TEMPLFIT}}\]

Alternatively, FLUX_<band>_TEMPLFIT can be taken directly as a total flux, especially for EXT bands.

Finally, also FLUX_<band>_SERSIC columns provide total flux estimates from the model fitting.

Magnitudes

As fluxes are in \({\mu}Jy\), the Zero Point to get AB magnitudes out of (above) fluxes is ZP = 23.9 in all bands:

(7)\[m_{\textrm{AB}} = -2.5 \times \log_{10}\left(\textrm{flux}_{{\mu}Jy}\right) + 23.9\]