Doc:
Issue
Date
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VLT-MAN-ESO-14650-4942
P96
24.06.2015
25 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
2.2.1.7 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.
Unfortunately due to some problems affecting the ADCs, they have been disabled since
August 1
st
, 2012. See the following section for more information about the observations
without ADCs.
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.