GMOS Image of M8. Photo credit: Gemini/J. Arias & R. Barba.

GMOS Data Reduction Cookbook

The Gemini Observatory provides a variety of facility-class instruments for use by the professional astronomy community, including the twin Gemini Multi-Object Spectrographs (GMOS). These imaging spectrographs offer useful sensitivity in the optical from the near-UV to the near-IR, at high spatial resolution and low to moderate spectral resolution. GMOS-North was the first instrument to be delivered, in 2001 July, and GMOS-South followed in 2002 December; data from the commissioning periods to the present are offered in the Gemini Observatory Archive.

Raw data for these instruments are archived and made available for public use, but no calibrated, science-ready products are produced. This Cookbook is intended as a guide to data reduction and calibration for PIs and Archive users of data from either of the GMOS instruments. The descriptions, recipes, and scripts offered in this Cookbook will provide the necessary information for deriving scientifically viable (but not necessarily optimal) spectra and images from GMOS. However, users should be aware that the ultimate utility of the data for any specific scientific goal depends strongly upon a number of external factors, including:

  • the environmental conditions that prevailed at the time of the observations (see the Gemini data quality assessment process),
  • the performance of the instrument,
  • the observing procedures used to obtain the data,
  • the scientific objectives of the original observing program.


The GMOS instruments are very well documented on the Gemini/GMOS website and in published papers (see Literature References). But it is not necessary to understand every detail of the instrument design and operation to reduce your data; links to relevant portions of the instrument literature will appear throughout this manual.

Data reduction begins with installing and configuring the data reduction software environment, searching for and retrieving data, and previewing and organizing the data for processing.

Details of processing science data are presented in a set of tutorials, which reduce data obtained directly from the Archive for each GMOS operating mode.

Some of the techniques or resources used in the course of data reduction, such as wavelength calibration, spectrum extraction, or calibration artifacts, are described in more detail.

The following chapter provides many links to on-line resources, software tools, and literature references that may be very helpful for GMOS data reduction.

Finally, a glossary of technical terms and a description of how this manual was prepared appear in the final two chapters.