Optical Photometric Standard Stars

Observations of photometric standard stars will be included in the baseline calibrations for GMOS. These observations allow the calibration of data acquired into a magnitude or flux scale.

mAB = -2.5 log fnu,eff - 48.60 ,

where f_nu,eff is the effective flux in erg/cm²/s/Hz. A description of the SDSS photometric system is presented in Fukugita et al. (1996, AJ, 111, 1748). Fukugita et al. also give transformations between AB magnitudes and magnitudes in conventional systems.

Secondary standard stars for the SDSS phototometric system are not yet available. Thus, for the baseline calibrations for GMOS, photometric standard stars will be chosen from Landolt (1992, AJ, 104, 340). Accurate empirical transformations between the GMOS instrumental magnitudes and the Johnson-Kron-Cousins system may be established as part of GMOS engineering.

GMOS images reduced with the GMOS reduction software which is part of the Gemini IRAF package will have counts in units of e-/pixel. For images taken under photometric conditions, the approximate standard calibration is as follows:

m_std(AB) = m_zero - 2.5 log10 (N[-e]/exptime) - k (airmass-1.)

The median atmospheric extinction coefficients (k) for Mauna Kea may be used. The exposure time (exptime) is contained in the keyword EXPTIME in the primary header unit of the images. The airmass is contained in the keyword AIRMASS in the primary header unit. The magnitude zero points are given in the table below, together with the assumed atmospheric extinction coefficients for the filters. These calibrations result in AB magnitudes accurate to 5-8 percent for nights with atmospheric extinction close to the median for Mauna Kea.

Last update November 29, 2001; Inger Jørgensen