Spectroscopic Techniques: The techniques described below have been developed in order to obtain the maximum amount of information from the light captured by the telescope. The main goal of Multi Object or Area Spectroscopy (techniques 2 and 3 below) is to obtain many spectra from different parts of the telescope's field of view. With this type of data, many aspects of an object's physical characteristics can be studied such as velocity, temperature and chemical composition. These techniques are primarily done using a prism or a grating to disperse the light.
1) Long-Slit Spectroscopy: This traditional technique uses a long opened slit to produce a spectrum that covers a large linear portion of an object. Using this technique a broad swath of an object can be analyzed spectroscopically and reveal characteristics along the slit's long axis. Gemini can use this technique on GMOS.
2) Multi-Slit Spectroscopy: A mask is produced that has thin slits that are aligned to different objects in the field of view as seen by the telescope on the sky. Light from each galaxy or object being studied passes through individual slits and a number of spectra are obtained corresponding to each slit on the mask. At Gemini this technique is used by the Gemini Multi-Object Spectrograph (GMOS) where over 100 individual spectra can be obtained in one image.
3) Integral Field Spectroscopy: This new technique simultaneously produces hundreds of spectra over a small, 2-dimensional region of an object. To accomplish this, astronomers use several approaches. Designs like the GMOS IFU make use of optical fibers thinner than a human hair to collect light from the telescope and send it to the spectrograph. In GMOS, one spectrum is produced by every one of the 1500 fibers. Infrared IFUs usually use thin mirrors to "slice" an image into little slits. The mirrors are easier to cool and have better infrared properties than fibers. See illustrations and diagrams of this technique here: http://www.gemini.edu/media/IFUImages.html.
Dr. Jeremy Allington-Smith, the scientist from the University of Durham in the United Kingdom who oversaw the design and construction of the GMOS Integral Field Unit describes the advantages of IFUs as follows: "Images give spatial information but one can only measure the brightnesses of the objects. Traditional longslit spectroscopy spreads the light along only one dimension of each object into its component wavelengths, revealing the velocities, temperatures, and compositions of the gas and stars. Having only one spatial dimension makes it difficult and inefficient to study complicated regions where gas or stars may be moving in many directions. Integral field spectroscopy gives the best of both imaging and spectroscopy, simultaneously producing a spectrum at each position in a 2-dimensional region."
GMOS-IFU Credit:
The IFU was designed in Durham by Robert Content (optical) and George Dodsworth (mechanical), and constructed and tested by the project manager, Graham Murray. We are indebted to the other members of the GMOS team in Edinburgh and Victoria, as well as Gemini staff involved in GMOS and its IFU.
Allington-Smith, Dr. Jeremy et al. "The GMOS integral field unit: first integral field spectroscopy with an 8m telescope." Paper presented as part of Galaxies: The third Dimension of the ASP Conference Series, Cozumel Mexico, 3-7 December 2001.
Cecil, Dr. Gerald et al. "Spatial Resolution of High-Velocity Filaments in the Narrow-Line Region of NGC 1068: Associated Absorbers Caught in Emission." The Astrophysical Journal, 1 April 2002.
Miller, Dr. Bryan et al. "Integral Field Spectroscopy with the Gemini 8-m Telescopes." Paper presented as part of Galaxies: The third Dimension of the ASP Conference Series, Cozumel Mexico, 3-7 December 2001.