Publications of the Astronomical Society of Australia Publications of the Astronomical Society of Australia Society
Publications of the Astronomical Society of Australia

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

Inger Jørgensen
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- Author Affiliations

Gemini Observatory, 670 N. A‘ohoku Pl., Hilo, HI 96720, USA. Email:

Publications of the Astronomical Society of Australia 26(1) 17-30
Submitted: 2 May 2008  Accepted: 5 November 2008   Published: 2 April 2009


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 aluminium. 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.

Keywords: techniques: photometric — methods: data analysis


The anomymous referee is thanked for useful comments on this paper. John Thorstensen is thanked for supplying the improved coordinates for the Landolt standard stars. Special thanks to the Gemini engineering staff for supplying mirror reflectivity measurements, and the many student interns at Gemini North who contributed to the processing of the bias images and flat fields. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministerio da Ciencia e Tecnologia (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). This paper is based on all available standard star observations obtained for all GMOS-N imaging programs executed on Gemini North in the period from August 2001 to December 2003. This research has made use of the USNOFS Image and Catalogue Archive operated by the United States Naval Observatory, Flagstaff Station (


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