GMOS-IFU
Information for non-specialists (relating to Gemini press release)
The images on this web page may be downloaded and used for publication provided they bear the acknowledgement: "University of Durham, Astronomical Instrumentation Group"
The GMOS Integral Field Unit (IFU), used as part of the Gemini Multiobject Spectrograph (GMOS) installed on the Gemini telescope in Hawaii, produces images of astronomical objects with an extra dimension. Instead of the usual two dimensions of a normal image, the IFU also records the spectrum of each point in the image. This spectrum - in which the light is divided into its constituent colours - allows astronomers to study the composition of the material along the line of sight, and even to measure its speed, thus providing a virtually three-dimensional image. The GMOS IFU has the ability to produce 1500 spectra simultaneously, one for each point (or pixel) in the image. This is something which cannot be done with conventional imaging instruments, which lack the ability to produce a spectrum, or by conventional spectrographs, which can only produce a spectrum for a single slice - or sometimes only a single point - through the object. This is the first such instrument to be installed on the new generation of giant telescopes, providing a uniquely powerful tool for studying the mysteries of the universe. The first mystery to be probed is the power source of the active galaxy NGC1068 which is believed to be a super-massive black hole.
The way the IFU works is illustrated in the following graphic which uses the example of the observation of NGC1068.
In the illustration above, the picture at the top left is an ordinary image of the NGC1068 galaxy taken with GMOS (although GMOS's job is primarily to produce spectra, it can take excellent images as well!) without using the IFU. The picture at top-right shows a closeup of the the central part of the galaxy (displayed in false colour). Superimposed on it is the sampling pattern of the IFU. This consists of 1000 elements, or pixels, in a hexagonal pattern. The other 500 elements simultaneously observe a nearby clean patch of sky to provide a reference signal. For each element, an optical fibres extracts the light and rearranges it so that GMOS can produce a clean spectrum (without rearrangement the spectra would all interfere with each other and be useless). The observer can then choose how to display the result: either as a set of 1000 spectra as indicated at bottom right (only a few are shown here) or as an image using light only at the chosen wavelength (colour) as shown at bottom left (again only a few examples are shown - there would actually be thousands of such images). Using this information, astronomers can work out what is happening in the object. In fact, the detailed shape of the features in the spectrum shows that gas is being ejected from the vicinity of the blackhole in oppositely-directed cones.
The results can also be presented as a movie which shows the image at wavelengths corresponding to a particular radial velocity (i.e the recession speed along the line of sight) for one particular emission line (here [OII]5007).
The GMOS IFU is the first example of this type of advanced instrument to be installed on the new generation of giant telescopes, of which the two Gemini telescopes (one in Hawaii and one in Chile) are the latest example. These superb telescopes were built by an international consortium of the USA, UK and Canada together with Australia, Chile, Brazil and Argentina. The GMOS instrument was built by a UK-Canadian consortium of the University of Durham, the Astronomical Technology Centre, Edinburgh and the Herzberg Institute of Astrophysics, Victoria, Canada. The IFU, which gives GMOS its extra power, was built by the Astronomical Instrumentation Group of the University of Durham. The project group responsible for the design and construction of the IFU consisted of Graham Murray, Robert Content and George Dodsworth, led by Jeremy Allington-Smith. Once installed at the telescope the astronomical testing was led by Bryan Miller and Inger Jorgensen of Gemini, Isobel Hook of the UK Gemini office and Jeremy Allington-Smith.
GMOS and its IFU was installed on the Gemini telescope in Hawaii in the autumn of 2001. The GMOS instrument was first tested without making use of the IFU. This produced stunning colour images and spectra of astronomical object as described in a previous press release. When the IFU was tested, it quickly became clear that this gave GMOS a very powerful capability. NGC1068 was chosen to show off the capabilities of the instrument. This is one of the most spectacular galaxies in the sky, with an ultra-bright nucleus in which material falling towards a super-massive black hole produces jets of charged particles which shoot off in opposite directions. Data from the GMOS IFU allowed the motions of gas in the region surrounding the black hole to be mapped. Together with data from other instruments including the Hubble Space Telescope, this is providing new insights into the workings of this extraordinary galaxy and will help to prove that a black hole is the power source. Even the Hubble Space Telescope with its spectacular imaging ability due to its position above the distorting effect of the atmosphere has no IFU, so the GMOS IFU is providing a major boost to this kind of study through its three-dimensional ability..
Physically the IFU is an oddly-shaped box, about the size of a small suitcase. It fits inside the 2-ton GMOS instrument to convert a two-dimensional view of the object under study into a shape that can be passed to GMOS so that each pixel can be turned into a spectrum. This conversion is done using flexible optical fibres, each of which is finer than a human hair. For maximum efficiency the fibres are attached to tiny lenses at each end, each smaller than a full stop. Each fibre must be positioned in its proper place and mated to its two micro-lenses with hair-splitting accuracy. The assembly was so successful that almost every fibre was correctly positioned. The result is that the IFU provides an almost perfect picture and transmits light with near-perfect efficiency. One installed in GMOS the observer can move the IFU into position so that it can do its job with a click of a mouse button.
Information for specialisists
The GMOS IFU is a system for two-dimensional ( integral field ) spectroscopy in the optical (0.4-1 micrometre). It is one of the main modes of GMOS.
As well as the main Durham integral field spectroscopy page, some further information is included here.
The basic parameters of the system are 0.2 arcsec sampling over a total field area in excess of 50 square arcsec which includes a separate field for background subtraction displaced from the object field by 1 arcmin. The 2-D field is reformatted into two slits which allow a spectrum from each element of up to half the full length of the detector (6144 pixels) to be formed. Optionally, a mask may be added to block of one slit so that half the field (a contiguous region) can be used with full-length spectra.
The IFU is loaded into the GMOS input focal plane (after the ADC) by the mask changer, so that GMOS can switch easily between aperture and integral field spectroscopic modes.
Completion was in 2001, synchronised with the completion of the main part of GMOS.
Publications
Summary
Principle of Operation
Field To Slit Mapping
Mechanical Layout
