Calibration of photometry from the Gemini Multi-Object Spectrograph on Gemini North

TL;DR

This paper establishes photometric calibration standards for the Gemini Multi-Object Spectrograph at Gemini North based on extensive observations from 2001 to 2003, providing accurate zero points and color terms for data prior to late 2004.

Contribution

It provides a comprehensive calibration for Gemini North GMOS photometry using a large dataset, including nightly zero points and color terms, improving accuracy for past and future observations.

Findings

Nightly zero points accurate to 0.02 mag or better

Calibration accuracy limited by sky concentration systematic errors

Achieves photometric accuracy of 0.035 to 0.05 mag

Abstract

All available observations of photometric standard stars obtained with the Gemini Multi-Object Spectrograph at Gemini North in the period from August 2001 to December 2003 have been used to establish the calibrations for photometry obtained with the instrument. The calibrations presented in this paper are based on significantly more photometric standard star observations than usually used by the individual users. Nightly photometric zero points as well as color terms are determined. The color terms are expected to be valid for all observations taken prior to UT 2004 November 21 at which time the Gemini North primary mirror was coated with silver instead of aluminum. While the nightly zero points are accurate to 0.02 mag or better (random errors), the accuracy of the calibrations is limited by systematic errors from so-called "sky concentration", an effect seen in all focal reducer instruments. We conclude that an accuracy of 0.035 to 0.05 mag can be achieved by using calibrations derived in this paper. The color terms are strongest for very red objects, e.g. for objects with (r'-z')=3.0 the resulting z' magnitudes will be ~0.35 mag too bright if the color term is ignored. The calibrations are of importance to the large Gemini user community with data obtained prior to UT 2004 November 21, as well as future users of achive data from this period in time.