X-shooter
User Manual
Doc:
Issue
Date
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VLT-MAN-ESO-12000-0115
1
01.03.2009
15 of 60
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
The flexure compensation tip-tilt mirrors
Light reflected and/or transmitted by the two dichroics reaches, in each arm, a folding mirror
mounted on piezo tip-tilt mount. These mirrors are used to fold the beam and correct for
backbone flexure to keep the relative alignment of the three spectrograph slits within less
than 0.02” at any position of the instrument. They also compensate for shifts due to
atmospheric differential refraction between the telescope tracking wavelength (fixed at 470
nm for all X-shooter observations) and the undeviated wavelength of the two ADCs (for UVB
and VIS arms) and the middle of the atmospheric dispersion range for the NIR arm.
The Focal Reducer and Atmospheric Dispersion Correctors
Both UVB and VIS pre-slit arms contain a focal reducer and an ADC. These focal reducer-
ADCs consist of two doublets cemented onto two counter rotating double prisms. The focal
reducers bring the focal ratio from f/13.41 to ~f/6.5 and provide a measured plate scale at the
entrance slit of the spectrographs of 3.91”/mm in the UVB and 3.82”/mm in the VIS. The
ADCs compensate for atmospheric dispersion in order to minimize slit losses and allow
orienting the slit to any position angle on the sky up to a zenith distance of 60˚. The zero-
deviation wavelengths are 405 and 633 nm for the UVB and the VIS ADCs respectively. In
the AUTO mode, their position is updated every 60s based on information taken from the
telescope database.
The NIR arm is not equipped with an ADC. The NIR arm tip-tilt mirror compensates for
atmospheric refraction between the telescope tracking wavelength (470 nm) and 1310 nm
which corresponds to the middle of the atmospheric dispersion range for the NIR arm. This
means that this wavelength is kept at the center of the NIR slit. At a zenithal distance of 60°
the length of the spectrum dispersed by the atmosphere is 0.35”, so the extremes of the
spectrum can be displaced with respect to the center of the slit by up to 0.175”. If
measurement of absolute flux is an important issue, the slit should then be placed at
parallactic angle.
