SAAO - South African Astronomical Observatory




GIRAFFE: Introduction

The GIRAFFE spectrograph (Grating Instrument for Radiation Analysis with a Fibre Fed Échelle) (!) is a fibre-fed instrument used on the SAAO 1.9-m telescope, and is a copy of the MUSICOS spectrograph developed at Observatoire de Paris, Meudon Section (Baudrand & Böhm 1992, A&A, 259, 711). The instrument is capable of high dispersion spectroscopy (R tex2html_wrap_inline399 39000) over the wavelength region from 3820Å to 9240Å, or even out to 10400Å for bright stars. A 31.6 grooves mm-1 échelle grating, with a nominal blaze angle of tex2html_wrap_inline403 , disperses the light from an f/4 collimator. Two cross disperser prisms are optimised for the blue (3770-5560Å) and red (5200-10400Å) wavelength domains, which result in an inter-order spacing sufficient to allow simultaneous exposure of either the sky or an arc through a second fibre. The focal plane format is matched to the SAAO TEK6 CCD (1024 x 1024 x 24µm pixels) and the entire free spectral range (FSR) is covered for orders tex2html_wrap_inline407 70 ( tex2html_wrap_inline409 tex2html_wrap_inline411 8100Å). Intermediate regions can also be accommodated by suitable rotation of the prisms.

The spectrograph is permanently mounted in the coudé room of the 1.9-m telescope. A 16-m long pair of fibre optic cables relays the light from the Cassegrain focus of the telescope, down the polar axis, and into the coudé room. GIRAFFE employs a "double fibre'', which allows for either simultaneous star and arc exposures, or star and sky exposures. The former may be of some utility for precision radial velocity observations, where continuous monitoring of a suitably attenuated arc can be achieved. The latter "star/sky'' option will really only be appropriate for faint star observation, where background subtraction is an issue. At the time of writing, this option has not been attempted. Most users will probably use the single fibre mode of observing, taking objects, arcs and flat fields all through the same fibre.

The user controls GIRAFFE using the PC-based data acquisition program, GODOT, written by J. Menzies principally for CCD photometry, and adapted for use with GIRAFFE. The menu-driven program controls various CCD functions (e.g. exposures, readout, data storage) as well as spectrograph-specific operations in the `GIRAF' menu (e.g. moving the prisms or detector mount, turning arcs and flat field lamps on and off). Data can be stored either on the Sun Sparcstation 10 or on the Pentium II which runs Linux. As storage on the Sun is rather limited, by default data is stored on the Pentium II. Off-line processing of the data can be done while observing is in progress. On the Sun, this is achieved using a suite of GIRAFFE-specific C-shell scripts using the Figaro ECHOMOP échelle reduction software. These scripts were developed by M. Albrow and the Appendices to the Users Manual give instructions on how to proceed with ECHOMOP reductions. Alternatively users can make use of the échelle routines in the IRAF package. On the 's74' PC, the program xspec2 can be used to obtain fully reduced spectra while observing or at the end of the night, including radial velocities if desired.