GMOS Introduction

The two Gemini Multi-Object Spectrographs (GMOS), one on each telescope, provide 0.36-1.10 µm long-slit and multi-slit spectroscopy and imaging over a 5.5 arcmin field of view. Each GMOS is also equipped with an Integral Field Unit (IFU) making it possible to obtain spectra simultaneously of an area of about 35 square arcsec with a sampling of 0.2 arcsec. The Nod and Shuffle mode, enabling superior sky subtraction, is available for long-slit and multi-slit spectroscopy with both GMOSs. Nod and Shuffle with the IFU is only available at GMOS-S. The GMOSs were built by a collaboration between the UK (Astronomical Technology Centre at ROE and University of Durham) and Canada (HIA). GMOS-N was delivered in July 2001 with GMOS-S following in December 2002. There is a table reviewing the GMOSs' capabilities at the bottom of this page.

Click for larger image	The GMOS optical train is presented in this schematic. One of six available gratings or the imaging mirror is located at the single fold in the optical path. Three 2048x4608 CCDs with 13.5 µm pixels are used as the detector, providing 0.073" pixels. The wavelength range of the optical design extends from 0.36 to 1.10 µm. The CCDs in GMOS North are optimized for the red end of the optical wavelength region, while the CCDs in GMOS South provide better blue and UV sensitivity. It is worth noting however that the primary and secondary optics at both Gemini telescopes have been silver coated, which limits the throughput in the blue for both GMOSs.
Click for larger image	The forward slit environment of GMOS is fairly complex, since it houses a large number of masks (up to 18 at one time) and the Integral Field Unit in a "jukebox" changer, an on-instrument wavefront sensor used for guiding, and includes the capability to hold an atmospheric dispersion corrector (no current plans for installation). The spectral resolutions available range from about 670 to 4400 with a slit width of 0.5". Wider slits give proportionally lower resolutions.
Click for larger image	A central truss system is used to suspend the optical elements as well as the CCD dewar from the Instrument Support Structure (ISS). A grating turret with micropositioners behind each grating is used to switch between gratings remotely. Note that although there are six gratings available for both GMOSs, only three can be mounted in the instrument at one time (plus the imaging mirror). Finally two filter wheels mounted on a separate structure and measuring ~1 m in diameter house up to 24 filters for various broad-band and narrow-band imaging and spectrum order sorting functions. The available set of GMOS North filters and GMOS South filters are slightly different.

An overview of the GMOSs' capabilities is given below. For more detailed information on GMOS follow the link to Performance and Use on the left-hand menu. Further details on eg. filters, gratings, slits, IFU and other GMOS components can be found by following the link to GMOS Components on the left-hand menu.

Overview of the GMOSs' Capabilities

Science Modes:	Broad-band (SDSS filter system) and Narrow-Band imaging Long-slit Spectroscopy Multi Object Spectroscopy Integral Field Spectroscopy
Wavelength Coverage:	0.36 - 1.10 µm Note that the Gemini telescope primary and secondary optics are now silver coated, reducing the system throughput in the blue
Detectors:	Each GMOS has a detector package consisting of three 2048 x 4608 EEV chips The GMOS-N CCDs and GMOS-S CCDs have quite different spectral responses, with GMOS-N having more red sensitive CCDS and GMOS-S having more blue sensitive CCDs. The plate scale is 0.073 arcsec/pixel for each GMOS
Imaging:	Field of View is 5.5 arcmin x 5.5 arcmin, with two small gaps Images do not fill the entire CCD package
Spectroscopy:	Spectral Resolution (Longslit and MOS): 670 - 4400 (0.5 arcsec slitwidth) with a selection of gratings Spectral Resolution (IFU): 1080 - 7100 with Integral Field Unit (0.31 arcsec effective slitwidth) Longslit spectra cover the full extent of the CCD package (3 x 2048 pixels with two small gaps) Long-slit length is 5.5 arcmin with two small gaps at +/- 0.92 arcmin from the center Long-slit width is variable, with standard long-slits available Multi Object Spectroscopy uses custom designed, laser milled slitmasks Up to 600 MOS slitlets over 5.5 arcmin x 5.5 arcmin Field of View Integral Field Spectroscopy with 0.2 arcsec fibers over 35 square arcsec with half spectral coverage IFU can be configured to provide full spectral coverage and half the spatial Field of View IFU has separate sky IFU located 1 arcmin away for accurate sky subtraction
Nod and Shuffle:	Nod and Shuffle spectroscopy enables very accurate sky subtraction for the reduction of skyline residuals at the expense of slightly increased overheads. This is especially useful in the red and for faint targets or for very crowded fields. Nod and Shuffle is available with Long slit (1.83 arcmin slit length) and MOS for both GMOSs. Nod and Shuffle is also available with the GMOS-S IFU.
Wavefront Sensing:	Both GMOSs have an On-Instrument Wavefront Sensor that provides tip-tilt and focus corrected guiding. Guiding is essential with the Gemini Telescopes in order to achieve seeing limited image quality. The OIWFS should be used for all modes of GMOS, with the probable exception of non-sidereal tracking. The Peripheral Wavefront Sensors occult much of the GMOS Field of View and exhibit flexure with respect to GMOS
AO Correction:	The Gemini North AO unit Altair only transmits light longward of 0.835 µm to the science instruments. Given this limited wavelength range there are currently no plans to commission GMOS-N with AO. However, a planned future upgrade of the GMOS CCDs to more red sensitive devices may make this mode more attractive, particularly with the IFU.
Parallactic Angle:	Currently users can request to observe at the mean parallactic angle only. It is not possible to track the parallactic angle during an observation. There are no plans to commission the Atmospheric Dispersion Corrector at this time.
Polarimetry:	Not available. There are no plans to commission this mode.

Last update: October 20, 2006; Kathy Roth
Previous version: June 15, 2006; Bryan Miller