Instrument Description: CASPEC

  In the present version we will be concerned with the standard configuration of CASPEC, the Cassegrain Echelle Spectrograph which is in operation at the f/8 Cassegrain focus of the ESO 3.6 m telescope. The instrument has been described in detail by D'Odorico and Tanné (1984) we will include here a summary of the main instrumental characteristics which are relevant for the analysis of the data.

In its standard configuration CASPEC uses a a 31.6 lines/mm echelle grating together with a 300 lines/mm grating cross disperser. A short focal length camera (f/1.46) focuses the beam into a thinned, back illuminated CCD consisting of pixels. One pixel on the detector corresponds to an entrance aperture or seconds of arc on the sky, the first dimension being in the dispersion direction. The table D.1 shows the change in resolution with the order number.

 

(Å/pixel) (Å)

140 0.125 0.01 4062.21

130 0.133 4374.95

120 0.143 4739.34

110 0.157 5170.49

100 0.174 5687.50

90 0.194 6319.20

80 0.217 7109.60

70 0.244 8120.00

In this configuration, the spectrograph records in a single CCD frame an Å wide portion of the spectrum of objects with with a resolving power of and a signal-to-noise ratio . This magnitude limit is set primarily by the readout noise of the chip (50 electrons rms) and by the maximum exposure time of min before contamination by cosmic rays becomes a problem. Fainter objects can be observe at lower resolution by binning the CCD data.

Scattered light.

Assuming a plane grating, used in near-Littrow mode, the blaze function R at wavelength is approximated by   where is a grating `constant' with value between 0.5 and 1, and , in which m is the order number, and is the central wavelength of order m. Both parameters are related through the grating `constant' k by .In table D.2 we include approximate values for the parameters k and . These are mean values, given that the actual values for a given observation are a function of the order number and depend also on the instrumental set up.

 

31.6 568746. 0.8

51. 344705. 0.8