Setting UBVRI Photometric Zero-Points Using Sloan Digital Sky Survey ugriz Magnitudes

TL;DR

This paper presents a method to calibrate UBVRI photometric zero points using SDSS ugriz data, identifying systematic effects and providing transformation equations for accurate photometry.

Contribution

It introduces a new approach for setting UBVRI zero points based on SDSS ugriz photometry, addressing saturation effects and systematic biases.

Findings

Zero point changes occur at specific magnitude thresholds due to saturation.

Comparison is reliable only within certain magnitude ranges (14.5-19.5).

Provides equations for transforming ugriz to UBVRI photometry.

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) standards and our own photometry we find that there is a fairly abrupt change in B, V, R, & 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.