XSHOOTER is a multi-wavelength (300-2500 nm), medium resolution spectrograph mounted at the UT3 Cassegrain focus. It consists of three spectroscopic arms, each with optimized optics, dispersive elements, and detectors. The Acquisition and Guiding camera can be used to obtain complementary images:
UVB, wavelength range 300-559.5 nm
VIS, wavelength range 559.5-1024 nm
NIR, wavelength range 1024-2480 nm
Each of the three spectroscopic arms is an independent cross dispersed echelle spectrograph with its own shutter and slit mask. The incoming light is split into the three arms via two dichroics (cut off wavelengths are 559.5 nm and 1024 nm for the separation of the UVB-VIS and VIS-NIR, respectively). For each arm the spectral format is fixed. The orders in each detector are highly curved and the sky/arc lines within each order are tilted.
Three piezo controlled mirrors, located in front of each arm, guarantee that the optical path is maintained aligned against instrument flexure and corrected for differential atmospheric refraction between the telescope guiding wavelength and the arm central wavelengths.
Two pairs of ADC (Atmospheric Dispersion Corrector) prisms in the optical path of the UVB and VIS arm compensate for the atmospheric dispersion. No ADC is mounted in front of the NIR arm.
The Acquisition and Guiding system consists of a 1.5'x1.5' FoV technical CCD for target acquisition. A filter wheel equipped with Johnson and SDSS filters is available. There is no secondary guiding.
The five positions slide, located before the UVB, VIS, and NIR spectrograph, allows to insert an image slicer in the optical path to perform IFU spectroscopy with a 1.8''x4'' FoV. The IFU is not offered anymore since ESO Period 111.
An internal calibration unit, equipped with Pen-ray (Ar, He, Ne, and Xe), Th-Ar, Halogen, and D2 lamps is used for the calibration frames (flats, arcs, order definition, format check, 2dmaps).