Photometric Standard catalogs

Observations of photometric standard stars are included in the baseline calibrations for GMOS. These observations allow to calibrate the data acquired into a magnitude or flux scales.

Like the SDSS, the magnitudes in the GMOS filters are on the AB system, 

m_AB = -2.5 log f_nu,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. (1996) also give transformations between AB magnitudes and magnitudes in conventional systems.

For GMOS baseline calibrations photometric standard stars are chosen from the sources listed in the following table. The magnitudes for Landolt (1992) standard stars are transformed to AB magnitudes using the transformation equations given in Fukugita et al. (1996)*. Accurate empirical transformations between the GMOS instrumental magnitudes and the Johnson-Kron-Cousins system are not available, however these may be established as part of GMOS engineering in a near future. For GMOS-N and GMOS-S, baseline calibration photometric standard stars are observed in a regular basis. For Gemini North, the main sources are the Landolt (1992) and Smith at al. (2002) standard star catalogs, while for Gemini South, the Landolt (1992) standard star fields are used as baseline calibration. Starting from 2008B semester, standard stars from Landolt (1992) are not longer observed at Gemini South. The observations are replaced by the photometric standard star from Smith et al. (2007).

* The u'g'r'i'z' magnitudes for Landolt (1992) standard stars are part of the GMOS calibration files included in the Gemini IRAF package.

Photometric Zero Points

As part of GMOS monitoring, images of standard star field are reduced with the GMOS reduction software and photometric zero points for the u'g'r'i'z' GMOS filters are derived. For images taken under photometric conditions, the approximate standard calibration is derived as follows:

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

In this equation, the median atmospheric extinction coefficients (k) for Mauna Kea and Cerro Pachon are 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 5-10 percent for nights with atmospheric extinction close to the median for Mauna Kea and Cerro Pachon.

Last update: August 29, 2008; Rodrigo Carrasco
Previous version: November 29, 2001; Inger Jørgensen