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The GMOS (Gemini Multi-Object Spectrograph) project at the ATC

GMOS instrument without enclosure
GMOS instrument without enclosure and electronic cabinets


Both Gemini 8-metre telescopes in Hawaii and Chile are now equipped with multi-object spectrographs (GMOS-North and -South) operating in the optical region of the spectrum. GMOS-N and -S are versatile instruments designed to fully exploit the excellent images that the telescopes produce. They are the workhorse instruments at the Gemini Observatories. GMOS is an international collaborative project involving the UK Astronomy Technology Centre the Dominion Astrophysical Observatory of Canada and the University of Durham.


GMOS-South is the latest addition to Gemini's suite of instruments, following successful commissioning in 2002/03. UK ATC has thus played a key role in 3 of the 4 facility-class instruments operational at Gemini - GMOS-S, GMOS-N and Michelle.

Commissioning progressed smoothly, with the instrument operating straight from the box (or from the 24 packing crates it took to transport it!)

Chris Tierney, GMOS Systems Engineer at UK ATC, oversees the installation on GMOS-S on the Gemini telescope on Cerro Pachon.

Kei Szeto (DAO, Canada), Morag Hastie (UK ATC) and Luis Godoy (Gemini) stand proudly beside GMOS-S, successfully mounted on Gemini-South in Chile.

Among the images and spectra acquired during commissioning of GMOS-S, one image is particularly compelling: this image reveals remarkable details, previously only seen from space, of the Hickson Compact Group 87 (HCG87). HCG87 is a diverse group of galaxies located about 400 million light years away in the direction of the constellation Capricornus.

Remarkable first-light image, obtained with GMOS-N at the Gemini-North telescope on Hawaii's Mauna Kea. The image is of the large galaxy in Pisces called NGC 628 (or Messier 74) has been called the Perfect Spiral Galaxy due to its nearly ideal form, which is clearly revealed in this image.



Wavelength range  0.36-1.1 microns (with a design capability to 1.8 microns)
Field of View  5.5 x 5.5 arcmin 
Detector  array of 3 CCDs: each 4608x2048 13.5um pixels
Scale  0.07 arcsec/pixel 
Filters  Two wheels with 11 slots + 1 clear aperture each. Initial filter set is SDSS g' r' i' z'. 
Gratings  Grating turret holds 3 interchangeable gratings and one mirror. Initially 6 gratings for 2 GMOSs
Resolving power  R up to 10,000 with 0.25 arcsec slits 
Slit Masks  Up to 600 slits per mask, minimum slit width = 0.2 arcsec 
Straight or curved slits possible 
Maximum slit length = 5.5 arcmin 
Integral Field Unit  0.2 arcsec sampling over 50 square arcsec field


Since the GMOS spectrographs are common-user instruments, there are many scientific drivers, eg:


One of the remarkable features of GMOS is the fine image scale, needed to exploit the excellent images provided by the telescopes. GMOS includes an integrated on-instrument wavefront sensor to provide the necessary correction signals for the articulated tip-tilt secondary mirror. Recent observations with the newly commissioned GMOS-S on Cerro Pachon in Chile achieved natural-seeing resolution of nearly 0.3 arcsec in the r filter.


The optical design uses refractive optics and reflection gratings. The image quality is such that 50% of the light is enclosed within a diameter of ~0.1 arcsec over a field ~7 arcmin in diameter. Sol-Gel very broad-band anti-reflection coatings are being utilised on the GMOS optics.

GMOS optical path
GMOS Optical Path

The optical system includes an integrated atmospheric dispersion compensator and a corrector to improve the image quality in both imaging and spectroscopic modes.

Remotely deployable masks are used for both multiple-aperture and single-aperture spectroscopy. Because of the great accuracy required in the location of the slits, all masks are designed with the aid of direct images taken with GMOS. Masks will be made on a facility near to the northern telescope. With a 5.5 x 5.5 arcmin2 field of view, GMOS should be able to locate hundreds of objects in a single mask, though typical densities for most projects are more like 50.

It is possible to make accurate slits as narrow as 0.25 arcsec in order to exploit the best seeing conditions and to achieve the highest spectral resolution. The combination of large field, excellent image quality and the ability to arrange the slits in multiple tiers implies a maximum multiplex gain of several hundred.

Accurate target acquisition is critical to the instrumental performance. The simplest and most reliable method is to image the sky through masks which include small holes through which reference stars can be viewed. The mask-sky displacement is then used to offset the telescope and rotate the field to ensure accurate registration.

GMOS provides a large range of dispersion options via a four-position grating turret which includes a plane mirror for imaging.

Filter wheel and Grating turret
Filter Wheel and Grating Turret

It is a key requirement that the tilt of each grating be maintained precisely, even when moved out of the light path, so that the spectroscopic setup is not disturbed during target acquisition. Instrument flexure is minimised to permit measurement of radial velocities to an accuracy of 1-2 km/s with hourly on-target wavelength calibration. For GMOS-S, based on Cerro Pachon in Chile where temperature fluctuations can be rapid, a flexure model incorporating the effects of both temperature and inclination was derived in the ATC laboratory using a cold room and flexure rig.


The mechanical system has been designed to minimise tilts between optical components so that the only effect of flexure is a slow motion of the image on the slit of the detector. This means that flexure can be compensated simply by moving the detector. A novel cryogenic translation and focus mechanism has been designed for this purpose. The translation mechanism provides two orthogonal axes of fine motion for the CCD mount. The resolution of the translation mechanism is 0.4mm per motor step and the total travel is ±150mm. The principle of operation of a single axis of the translation mechanism is shown below.
CCD Translation stage CCD Translation stage
CCD Translation Unit

A central box is supported from a surrounding frame by two pivoted lever arms. The arms are linked by a cross-piece pivoted to their outer ends. The levers have a ratio of 5:1. The cross-piece is moved by a leadscrew and nut, driven by an external stepping motor. The centre of mass of the box lies on the line joining the two pivots on the box. The centre of mass of the box plus levers and cross-piece lies on the line joining the two fulcra on the frame. Meeting these two conditions minimises the twisting of the pivots due to the changing gravity vector. Backlash in the nut is removed by using springs (not shown) to pull the cross-piece to one end of its travel. The second axis of translation is provided by a similar mechanism which is built as an integral unit around the inner one. The outer frame of this forms part of the vacuum vessel and carries the attachment points by which the whole CCD unit is joined to the rest of GMOS.

CCD assembly
CCD Assembly



Last modified by:
Rob Ivison
12 Sept 2003.