X-shooter
User Manual
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Issue
Date
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VLT-MAN-ESO-12000-0115
1
01.03.2009
28 of 60
ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
3.3
Spectroscopic observations
X-shooter science templates support different observing strategies: staring (commonly used
for UV and visible observations), nodding along the slit (classical near-IR observations, for
SLIT only), offsetting to a fixed sky position (for extended objects) or lets the user free to
choose any sequence of offsets (e.g. for mapping). Note that due to the small field of view of
the IFU, we recommend to offset to a pure sky position in case good sky subtraction is
needed.
3.3.1 Staring (SLIT and IFU)
With the
SHOOT_slt_obs_Stare
and
SHOOT_ifu_obs_Stare
templates, one or more
spectra are taken with each arm independently at a fixed position on sky. For each arm, the
user chooses the exposure time and the number of exposures. Exposures are completely
asynchronous i.e. in each arm, whenever an exposure is finished the next one starts
immediately, independently of what is happening with the other arms.
3.3.2 Staring synchronized (SLIT and IFU)
Whenever exposures in the three arms have to be parallel, the templates
SHOOT_slt_obs_StareSynchro
or
SHOOT_ifu_obs_StareSynchro
should be used. In
this case, the number of exposures is fixed to one per arm. Exposure times can still be
different in each arm but the exposures are synchronized to their mid-time. In case the
exposure times in all three arms are identical, exposures in the three arms will have the
same start time within approximately one second. In case of different exposure times, the
mid-exposure time of the three will coincide within about one second.
3.3.3 Nodding along the slit (SLIT only)
This corresponds to the standard way of observing in the near-IR primarily aimed at a double
pass sky subtraction. The template
SHOOT_slt_obs_AutoNodOnSlit
automatically nods
the telescope between two positions (A and B) along the slit. The user defines a
Nod Throw
and optionally a small
jitter
box (in the slit direction). The
Nod Throw
is defined as the
distance between the two nodding position i.e. the center of the two jitter boxes inside the slit
(see Fig. Figure 10). One cycle is a pair of AB or BA observations. Cycles are repeated in
ABBA sequences. For each arm, the user chooses the number of exposures at each position
and the exposure time (both identical for all A and B positions). Exposures are
asynchronous. Note that nodding is not offered in IFU mode because the field of view
(4”x1.8”) is too small to nod within the IFU.
