Astron. Astrophys. 362, 585-594 (2000)

2. The CES instrument and the iodine cell

The Coudé Echelle Spectrometer (CES) at the ESO La Silla observatory is a conventional non-stabilized spectrograph offered to the whole astronomical community and for use in all kinds of high resolution spectroscopic observations. The setup of the instrument is changed frequently and reset before each observing run. Thus changes in the behavior of the spectrograph and most of all variations of the instrumental profile are clearly to be expected. All spectra included in this article were taken with the 1.4m CAT telescope, which fed the spectrograph with a direct beam from the telescope. The incoming light is reflected at the telescope from a third mirror into a tube which leads into the Coudé room, where the CES is located. After passing through the entrance slit and a pre-dispersing prism, the light is dispersed at the Echelle grating of the CES. The Camera optics image a small part of one spectral order onto the CCD detector. The CES is not stabilized in any way and the instrument setup (focus, alignment, CCD orientation) is redone frequently; this can lead to significant variations of the IP between individual nights. On smaller timescales the IP is also affected by seeing conditions and the guiding performance of the telescope. The CES I₂-cell (see also Kürster et al. 1994) is temperature-stabilized at  C and located directly in front of the entrance slit of the spectrograph. It is mounted on a short rail in order to move it easily in and out of the light path. All spectra of the CES planet search program prior to April 1998 (i.e. before the Long Camera at the CES was replaced by the new Very Long Camera) were taken at a central wavelength of 538.9 nm and with the Long Camera yielding a resolving power of and a spectral range of 4.85 nm. This spectral region is also free of telluric lines, which was confirmed by observing rapidly rotating B-stars. The great advantage of the I₂ self-calibration technique is the fact that all instrumental effects, like wavelength zero-point drifts, changes in the dispersion or resolving power and variations of the instrumental profile are recorded in the numerous I₂ lines. Therefore an appropriate modeling of the iodine spectrum should be able to exploit this information contained in the shape and position of the I₂ lines. One has to bear in mind that the goal is to measure radial velocities with a precision corresponding to a very small fraction of a CCD pixel. For the CES data one pixel represents a velocity span of while the typical FWHM of the IP is about wide. It is clear that any asymmetry of the IP has an impact on the velocity information one can retrieve from the data. For a long-term project like the CES planet search it is mandatory to deal with these instrumental effects in order to attain a very high long-term stability for measuring stellar radial velocities. The use of the I₂-cell for self-calibration and the application of our data modeling technique make it possible to reach this objective. We will now turn to the description of the data modeling.

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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