SNLS and SDSS-SN joined Photometric Calibration

Table of Contents

1 Overview

This work is part of the SDSS+SNLS joint analysis. It provides photometric calibration standards for the SNLS and SDSS surpernova surveys. The paper associated with products presented here can be found here (see in particular Appendix E).

We release 4 products:

  • Catalogs of natural AB magnitudes for stars selected as tertiary standards.
  • MegaCam instrument transmission functions at 1.25 airmass.
  • Covariance matrices for the estimated calibration error.

All products are delivered as a single tarball.

In addition, we release the material required to update the calibration according to new releases of the HST CALSPEC spectrophotometry.

  • Measured magnitudes of the primary standards
  • Covariance matrices of primary standard measurements

2 Description of calibration products

2.1 Calibrated catalogs of tertiary standard stars in the science fields

Those catalogs deliver natural AB magnitudes for stars selected as tertiary standards for the SNLS and SDSS supernova surveys. The catalogs deliver MegaCam \(u_{M} g_{M} r_{M} i_{M} i2_{M}\) and \(z_{M}\) AB magnitudes. The SNLS cover the 4 deep fields of the CFHTLS:

  • catalogs/D1_release_ab_v3_2.ascii
  • catalogs/D2_release_ab_v3_2.ascii
  • catalogs/D3_release_ab_v3_2.ascii
  • catalogs/D4_release_ab_v3_2.ascii

The release of the catalogs is associated with a position dependant model for MegaPrime instrument response (see below MegaCam filters).

The SDSS SN survey cover a \(2.5^\circ\times120^\circ\) degree region centered on the celestial equator and designated as stripe 82. The calog deliver AB magnitudes in the 2.5m SDSS photometric system \(ugriz\).

  • catalogs/S82_release_ab_v3_2.ascii

The associated model for the system response of the SDSS 2.5-meter is delivered in Doi et al. (2010). We use the average filter model.

2.2 MegaCam filters

The model for the transmission curves incorporate the following contributions:

  • CCD quantum efficiency curves
  • Position-dependent filter transmission curves at several radii
  • Transmission curves for the wide-field corrector optics
  • Reflectivity of the primary mirror
  • Average transmission curve of the atmosphere above Mauna-Kea at an airmass of 1.25, as published in Buton et al. (2012).

Each component is provided as a tabulated function in the MegaCam_v3.2 directory. Respectively, the corresponding files are:

  • QE_camera_high_res_model.dat
  • {u,g,r,i,y,z}*.list for the \(u_{M} g_{M} r_{M} i_{M} i2_{M}\) and \(z_{M}\) filters
  • CFHT_MegaPrime_Transmission.dat
  • CFHT_Primary_Transmission.dat
  • SNIFS_extinction_buton2012_with_tl_X1_25.dat

The effective filter is the product of the five components. Filter transmission curves must be interpolated a given position.

2.3 Covariance matrix

The covariance matrix of the \(u_{M} g_{M} r_{M} i_{M} i2_{M} z_{M} u g r i z\) calibration offsets is delivered as a fits image in the file:

covariance/ab_offset_cov_mat_full.fits

2.4 MegaCam photometric flatfields

We do not release the flat-field photometric corrections, as they are specific to the photometry method used in the calibration paper.

3 Updating the calibration

Calibration offsets for the tertiary catalogs have been computed by comparing measured magnitudes of a few CALSPEC primary spectrophotometric standards to synthetic magnitudes. Offsets computed with the release 003 of the CALPSEC spectra have been applied to the catalogs.

Any revision of the HST calibration can be propagated to the SNLS and SDSS tertiary standards by recomputing calibration offsets. For this purpose we release the measured magnitudes of the CALPSEC standards in the tertiary catalog magnitude system, as well as the covariance matrix of those measurements:

  • CALSPEC_v3.2/hst_mmags.ascii
  • covariance/cov_meas.fits

Date: 2012-12-13 17:42:26 CET

Author: Marc Betoule

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