Brief description of FEROS

FEROS is a bench-mounted fiber-linked echelle spectrograph built for the ESO 1.52m spectrographic telescope. With one exposure it covers the spectral range from 3600-9200 Å on a 2$\times$4k EEV CCD chip. The main dispersion axis runs along the longer side of the CCD. Read-out direction is such that the main-dispersion is along the y-axis. Due to the use of a prism cross-disperser the spectral orders are strongly curved on the CCD. The images of the fiber are sliced in order to increase the spectral resolution to about 48,000. The price to pay for the enhanced resolution is a complicated and broad cross-order profile.

FEROS uses two fibers one of which is used as object fiber. The second fiber can be used in two modes.

• In the first mode the second fiber is used for recording a sky spectrum simultaneously with the object exposure.
• In the second mode, the spectrum of a ThAr comparison lamp is recorded simultaneously with the object spectrum. This mode is used for increased accuracy in wavelength calibration and is typically used for planet search programs. The intensity of the ThAr lamp is attenuated (via a filter with variable attenuation) in order to allow long ThAr exposures. The sky spectrum cannot recorded simultaneously in this mode.

For the object-calibration mode it is especially important that there are no very strong lines in the calibration lamps, which could lead to blooming of the CCD and thus distort the stellar spectrum. Therefore, FEROS uses a filter to suppress the red part of the ThAr spectrum, where many strong lines of Ar are present. In order to have any useful calibration lines in the red part of the spectrum, a Ne lamp is used in addition to the ThAr lamp.

The spectrograph itself has no moveable parts, i.e. the spectral format is fixed. This allows relatively easy on-line reduction.