GIRAFFE is housed in the coudé room of the 1.9-m telescope. The various GIRAFFE components are all mounted on an optical table, which has a cover to protect it from dust and keep it as light-tight as possible. A schematic illustration of GIRAFFE can be seen in Fig. 1. See the Specifications for the various spectrograph components.
GIRAFFE is operated in the quasi-Littrow configuration ( ). The conical angle is kept to the minimum possible, namely . Auto-collimation of the échelle grating using a laser gave the true blaze angle to be .
The central (ie. blaze) wavelength of an order is given as:
The resolution of GIRAFFE is:
where w is the `slit width', or in the case of GIRAFFE, the diameter of the optical fibre core (50 m). With = 447mm, this implies R = 39200.
The equation for the reciprocal dispersion is:
Finally, the free spectral range (FSR) defines the difference of two wavelengths, in successive orders, which are diffracted by the échelle grating at the same . This difference, or the free spectral range, is given as:
For complete wavelength coverage, the CCD detector should have dimensions such that the FSR is the wavelength coverage of the CCD. For GIRAFFE this is indeed the case for all wavelengths shorter than 8180Å (equivalent to orders 70). At the longest wavelengths (order 55), the FSR is 75% of the wavelength coverage of the CCD. In Table 2 the central wavelength, reciprocal dispersion and FSR are given for every GIRAFFE order. Because of the conical angle ( ), the central wavelengths of the orders are not precisely located in the centre of the CCD chip, except for the mid-range (n 100). For the bluer orders,
GIRAFFE: échelle orders