SETTING UBVRI PHOTOMETRIC ZERO-POINTS USING SLOAN DIGITAL SKY SURVEY ugriz MAGNITUDES

Abstract

We discuss the use of Sloan Digital Sky Survey (SDSS) ugriz point-spread function (PSF) photometry for setting the zero points of UBVRI CCD images. From a comparison with the Landolt (1992, AJ, 104, 340) standards and our own photometry we find that there is a fairly abrupt change in B, V, R, and I zero points around g, r, i ∼ 14.5, and in the U zero point at u ∼ 16. These changes correspond to where there is significant interpolation due to saturation in the SDSS PSF fluxes. There also seems to be another, much smaller systematic effect for stars with g, r 19.5. The latter effect is consistent with a small Malmquist bias. Because of the difficulties with PSF fluxes of brighter stars, we recommend that comparisons of ugriz and UBVRI photometry should only be made for unsaturated stars with g, r, and i in the range 14.5-19.5, and u in the range 16-19.5. We give a prescription for setting the UBVRI zero points for CCD images, and general equations for transforming from ugriz to UBVRI.

Updated 34-band Photometry for the SINGS/KINGFISH Samples of Nearby Galaxies

“…We adopted the Tonry et al (2012) quadratic filter transformations for stars between Pan-STARRS1g PS1 r PS1 i PS1 z PS1 and BVR C I C , although very similar results are obtained when using linear transformations between SDSS and Johnson/Cousins filters (Jester et al 2005;Lupton et al 2005;Jordi et al 2006;Chonis & Gaskell 2008;Tonry et al 2012). The only significant outliers for any of these various stellar transformations are the Chonis &Gaskell (2008) V andLupton et al (2005) R C linear transformations, both of which yield calibrations that ultimately result in galaxy fluxes that are 25%-30% (∼0.25-0.30 mag) brighter compared to when using other published transformations.…”

“…The only significant outliers for any of these various stellar transformations are the Chonis &Gaskell (2008) V andLupton et al (2005) R C linear transformations, both of which yield calibrations that ultimately result in galaxy fluxes that are 25%-30% (∼0.25-0.30 mag) brighter compared to when using other published transformations. The photometric calibrations are derived in practice from the errorweighted differences between the instrumental BVR C I C fluxes and the measured PS1 fluxes (transformed to BVR C I C ) for the suite of suitable foreground stars identified for each galaxy.…”

The Least-Luminous Galaxy: Spectroscopy of the Milky Way Satellite Segue 1

“…The method compares the observed spectrum to a grid of synthetic spectra covering a range of effective temperature, surface gravity, and composition. We estimate effective temperature and surface gravity for each star based on the Johnson-Cousins V I magnitude which we determine by transforming the SDSS gri magnitudes (Chonis & Gaskell 2008). The results are unaffected by using alternative photometric methods to determine these parameters.…”

The Alhambra Photometric System

“…We can see that the zero point estimated with the comparison of spectra and SEDs (see Table 13) is a good fit to the point distribution in general, although also some of the stars with brighter u magnitude move systematically away from the central value. This anomalous result appears also when other photometric systems (e.g., UBVRI) are compared to the SDSS system, and these transformations are then used to determine the zero point in the UBVRI system from data of the SDSS survey (as shown in Chonis & Gaskell 2008). Those authors proposed some possible explanations to account for the observed systematic difference.…”