Instrument Overview and Observing Modes:

GMOS is the medium-resolution optical spectrograph at Gemini. Long-slit, multi-object, and integral field unit spectroscopy, and imaging modes are offered. Nod and shuffle is also available. The North and South instruments are nearly identical, however, as of 2015 different CCDs are used (see more below).

The Gemini GMOS pages provide a comprehensive overview including references to the design.  The detector consists of three CCD's each with an approximate 2 K by 4K pixel layout.  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; more information regarding these CCDs is available on the GMOS-S Array (Hamamatsu) webpages. The GMOS-N array features the deep depletion devices from e2v: more information about it can be found on the GMOS-N Array (e2v DD) webpages.


The current GMMPS MOS mask documentation can be found at

Before reducing GMOS-S data you should check the Status and Availability webpage for details on the Hamamatsu project and, especially, warnings issued February  26, 2015 and October 3 and November 3, 2016.

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.

Data Reduction:

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 GMOS modes. Numerous references are included.  

Advanced Data Reduction Tips Tutorial for GMOS (Imaging only) is now available.

The Introduction to GMOS IFU data reduction tutorial, presented at the San Francisco                                       conference, is now available.


There are a number of articles that can be found on-line or in the literature on various aspects of GMOS data reduction.  This material is linked here.


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 contains guides to instrument configuration and baseline calibrations as well as phase 1 and phase 2 checklists.  



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Last updated or reviewed December 26, 2018.