FLAMES — Fibre Large Array Multi Element Spectrograph
FLAMES (Fibre Large Array Multi Element Spectrograph) is almost a mythological monster: MEDUSA, one of three systems which feed GIRAFFE, the main spectrograph for FLAMES, has more than 130 fibres. Each of these fibres is positioned very precisely on a metal plate by the robotic arm of OzPoz. “The light from each star is collected by the corresponding fibre and sent to a spectrograph,” explains Dimitri Gadotti, Instrument Scientist for FLAMES. “In this way, FLAMES is able to take spectra of a large number of astronomical objects at the same time.”
Apart from MEDUSA, GIRAFFE can also be fed by two different integral field devices: IFU (Integral Field Unit) and ARGUS. An integral field unit can be visualised as a camera where each pixel is an individual spectrograph. So it can take images and, at the same time, the spectra of every part of the field of view and not only of selected targets. This is especially useful in very crowded fields.
A spectrograph is basically a kind of sophisticated prism: it is an instrument able to decompose the light into its component wavelengths. Instead of a simple rainbow, the output is a spectrum which is recorded on a CCD detector and finally saved in a computer file for further processing and analysis, allowing the astronomer to measure the intensity of the light at each wavelength or colour precisely.
But FLAMES, the multi-object intermediate/high resolution spectrograph installed on the second Unit Telescope (UT2) at the Very Large Telescope (VLT), is far more complex as it feeds two spectrographs: the main intermediate resolution GIRAFFE and the high resolution UVES (Ultraviolet and Visual Echelle Spectrograph, also installed on UT2).
With UVES, a much higher resolution can be achieved than with GIRAFFE, but it can only observe a maximum of eight objects at the same time and in the red part of the visible light region. The resolution of a spectrograph is the minimum difference in wavelength that can be detected for a specific wavelength of light. It is determined by the dispersion characteristics of the prism-like device.
“Spectra contain information about the motions, ages and chemical content of stars, as well as properties of the gaseous component,” explains Gadotti. “This makes FLAMES the perfect tool to study stellar populations and the structure and evolution of stellar systems; from our own galaxy, the Milky Way, and the nearby Magellanic Clouds, to more distant galaxies”.
The two plates each hold over 130 fibres. The
vertical plate is ready for observations at the
focus of the telescope, while the horizontal one
is being prepared by the OzPoz fibre positioner.
Maps showing the motion of gas and stars within
three very distant galaxies. The areas marked in
red move away from us, those in blue towards us.
The OzPoz robotic arm has positioned each of the 130 MEDUSA fibres on a different star in a cluster. The light from each fibre is dispersed by the GIRAFFE spectrograph, forming a spectrum that shows the intensity of the light for each constituent colour. On the image, each spectrum appears as an individual vertical line. Some are brighter as they correspond to brighter stars.
Each of the pixels of the ARGUS system is fitted with an optical fibre that brings the light into the GIRAFFE spectrograph, which disperses its individual colours to form a spectrum. Each spectrum appears as an individual vertical line. As the position of each fibre in the original image is known, data analysis software can re-assign each spectra to its original pixel, forming a kind of image where each pixel has the full details of the spectrum. In the case of this image, the telescope pointed ARGUS at a galaxy and the resulting spectra were used to build one of the images at the top of this page.