SAAO - South African Astronomical Observatory

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The Spectrograph

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 ( tex2html_wrap_inline457 ). The conical angle is kept to the minimum possible, namely tex2html_wrap_inline459 . Auto-collimation of the échelle grating using a laser gave the true blaze angle to be tex2html_wrap_inline461 .

The central (ie. blaze) wavelength of an order is given as:

equation187

The resolution of GIRAFFE is:

equation194

where w is the `slit width', or in the case of GIRAFFE, the diameter of the optical fibre core (50 tex2html_wrap_inline463 m). With tex2html_wrap_inline465 = 447mm, this implies R = 39200.

The equation for the reciprocal dispersion is:

equation201

Finally, the free spectral range (FSR) defines the difference of two wavelengths, in successive orders, which are diffracted by the échelle grating at the sametex2html_wrap_inline467 . This difference, or the free spectral range, is given as:

equation217

For complete wavelength coverage, the CCD detector should have dimensions such that the FSR is tex2html_wrap_inline411 the wavelength coverage of the CCD. For GIRAFFE this is indeed the case for all wavelengths shorter than 8180Å (equivalent to orders tex2html_wrap_inline407 70). At the longest wavelengths (order 55), the FSR is tex2html_wrap_inline399 75% of the wavelength coverage of the CCD. In Table 2 the central wavelength, reciprocal dispersion and FSR are given for every tex2html_wrap_inline475 GIRAFFE order. Because of the conical angle ( tex2html_wrap_inline477 ), the central wavelengths of the orders are not precisely located in the centre of the CCD chip, except for the mid-range (n tex2html_wrap_inline399 100). For the bluer orders,

GIRAFFE: échelle orders

table228

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