Instrument Overview and Observing Modes:

GMOS (Gemini Multi-Object Spectrographs) North and South are the medium-resolution optical spectrographs at Gemini. Long-slit, multi-object, and integral field unit spectroscopy modes are offered, as well as imaging with narrow/broad band filters. The Gemini GMOS page provides a comprehensive overview including references to the design. The North and South detectors consist of three 2k x 4k pixel CCDs each. The plate scale is roughly 0.08 arcsecond per pixel. Additional information on GMOS can be found in the GMOS Cookbook.

  • Red-Sensitive CCDs manufactured by Hamamatsu Photonics were installed at GMOS-S in June 2014 and at GMOS-N in February 2017; additional information regarding these CCDs is available on the GMOS Detector Array webpages.
  • The multi-object spectroscopy (MOS) mode requires the use of a dedicated software provided by the observatory. Follow this link for the current GMMPS MOS mask software and documentation.
  • Important: Before reducing GMOS data, you should check its Status and Availability webpage for information and warnings that can affect your data.


Proposal Preparation:

Planning observations for GMOS can be challenging because of the large number of available instrument configurations and observing modes. Try our new GMOS Observation Planning Guide. This document contains guides to instrument configuration and baseline calibrations, as well as Phase 1 and Phase 2 checklists.


Data Reduction:

General information on GMOS data reduction

Links to data reduction procedures can be found on the Gemini web site Getting Started page.  There is additional information available at the Data Format and Reduction link on the GMOS web page.   Discussions of GMOS reduction procedures also can be found in a presentation by R. Carrasco and  a presentation by R. McDermid in the Proceedings from the 2010 Tucson Gemini Data Workshop.  There is also a presentation by R. Schiavon and a presentation by R. Carrasco that were presented at the 2011 South American Gemini Data Workshop.   The proceedings web sites contain links to demonstration data. 

A comprehensive data reduction cookbook has been written by Dick Shaw. This document contains an overall discussion of GMOS and details the reduction of all available modes. Other references are included.

GMOS-S users: The Hamamatsu CCDs installed at GMOS-S in June 2014 have different basic parameters than those of the GMOS-N CCDs.  As a result an update was made to Gemini IRAF.  To use Gemini IRAF with GMOS-S GMOS data taken after June 2014 you must have the current version of Gemini IRAF.  If an old version of IRAF is used IRAF produces a terse (and not helpful) error message about the GMOS-S name. 

Imaging Data reduction

If you are a new user of GMOS note that GMOS data consists of a header and three extensions with the data.  For even a quick look at the images we recommend the GMOS package in the Gemini section of IRAF.    NOAO IRAF will not recognize multi-extension FITS.  R. Carrasco presented a tutorial on GMOS image reduction at the 2011 South American Gemini Data Workshop. More experienced users can access the Advanced Data Reduction Tips Tutorial for GMOS (Imaging only).

Long Slit Data reduction

Gemini IRAF presents a list of Gemini-IRAF tasks for reducing long slit data.  Cookbooks and manuals of long-slit data reduction are useful to get the basic concepts.  The Gemini links provide descriptions of the IRAF commands. We recommend the use of P. Massey et al. (1992) 's guide to reducing slit spectra with IRAF as a good, if somewhat dated, introduction.  An other overview is given in the presentation by Tom Matheson on Low-Resolution Optical Spectroscopy presented at the 2010 NOAO Gemini Data Reduction Workshop.  R. Carasco reviews the necessary steps in his 2011 presentation.  Another useful guide is a tutorial presented by R. Schiavon at the 2011 South American Gemini Data Workshop.

Feed back by users points to a few general warnings:

  • The Gemini webpages do not include the new CCD gain and read-noise tables (EEV).
  •  The biases provided by Gemini through the data archive may not take into account over-scanning.
  •  The arc lines found at are not clearly labeled for blended features.

MOS Data reduction

Analyzing MOS data involves the extraction of multi-slit spectra, and then it proceeds as in a long-slit spectral analysis for each slitlet spectrum. The IRAF tasks are those given above for longslit. MOS reductions are discussed by R. Schiavon (2011).  Stanghellini et al (2014 A&A 567, A88, Sec 2) describe in detail the steps required for MOS data reduction.

Nod and Shuffle Data Reduction

A good general introduction to Nod & Shuffle is found on the GOS Nod & Shuffle web page and the links. The best data reduction descriptions can be found in the 2010 Gemini Data Workshop presentations by K. Roth and R. Carrasco. This is especially true of Roth's description of handling DTA-X offsets (for N & S).  Note that you need to input these as *Y* values with the opposite sign. This is not made prominent on the web page.  Nod & Shuffle is also reviewed briefly by R. Schiavon (2011).

IFU Data Reduction

The Introduction to GMOS IFU data reduction tutorial, presented at the San Francisco conference, is now available. Another main reference for IFU data reduction can be found at the gmosinfoifu.html  web page. Most of the information is  found as .cl scripts. These include and from 2002.  Note that these scripts may include some typos, and have not been updated in a while.

The 2011 South American Gemini Data Workshop presentation by R Carrasco (2011) for the IFU is recomended although it does not contain a comprehensive review of all issues regarding IFU data analysis.

An IDL data reduction package for IFU data, kungifu,  written by Bolton and Burles can be found on Adam Bolton's website.  The package has been applied to GMOS IFU data.


Frequently Asked Questions:

Some questions and their solutions can be found on the FAQ page. Many of these are helpdesk questions that have been previously submitted and answered.


Updated on December 16, 2021, 7:47 am