VLT-MAN-ESO-14650-4942
P96
24.06.2015
32 of 161
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
At the Cassegrain focus there is no possibility yet to do a secondary guiding. Therefore the
evolution of the parallactic angle is not followed during the exposures but the slit is setup at
the parallactic angle at the moment of the acquisition. This angle is followed during the
exposure.
It implies that the atmospheric dispersion direction will change with the time with respect to
the slit angle.
The (approximate) relative evolution of the parallactic angle post-meridian crossing (for
different hour angles) is shown in the following figure for different declinations.
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