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 $320 \times 512$ $30 \mu m^2$pixels. One pixel on the detector corresponds to an entrance aperture or $1.2 \times 0.7$ 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.

 

Table D.1: Resolution

Order	Resolution	Central Wavelength
	(Å/pixel)	(Å)
140	0.125$\pm$ 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 $\sim 900$ Å wide portion of the spectrum of objects with $V \leq 15$with a resolving power of $\sim 20,000$ and a signal-to-noise ratio $\geq 10$. This magnitude limit is set primarily by the readout noise of the chip (50 electrons rms) and by the maximum exposure time of $\sim 120$ 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 $\lambda$ is approximated by

$$R(\lambda) = {\sin^2 \pi \alpha X \over (\pi \alpha X)^2}$$ (20.1)

where $\alpha$ is a grating `constant' with value between 0.5 and 1, and $X=m(1-\lambda_c(m)/\lambda)$, in which m is the order number, and $\lambda_c(m)$ is the central wavelength of order m. Both parameters are related through the grating `constant' k by $k=m\lambda_c(m)$. In table D.2 we include approximate values for the parameters k and $\alpha$. 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.

 

Table D.2: Blaze parameters

Grating	k	$\alpha$
lines/mm
31.6	568746.	0.8
51.	344705.	0.8