VLT-MAN-ESO-14650-4942
P96
24.06.2015
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ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
6.5 Attached calibrations
It is possible to include arc and flat calibration in an observing OB. For the selection of
offered night time attached calibrations, see Table 40 and following). However, we strongly
discourage taking night time attached arcs in the VIS arm (SLIT or IFU) because of remnants
caused by a few strong ThAr lines. These remnants persist in the following exposure for up
to one hour affecting the subsequent observations. Therefore, attached VIS arcs can be
granted only in visitor mode or in service mode, which will be executed only at the very end
of the night, if possible. The user should refer to Table 17 to select the exposure time of the
attached calibrations.
On the UVB side, an arc exposure of, 6s in the 1x1 binning, 3s in the 1x2 binning, 1s in the
2x2 binning with the normal readout speed does not produce remnants and should provide
enough lines for an accurate radial velocity calibration.
NOTE: the wavelength calibration in the pipeline is not performed with the ARC frames
but with the 2dmap frames (9 pinholes+ARC lamp). The latter provides better results.
Those calibrations (2dmap) are not attached calibrations and it will be possible to add
those templates in the science OBs in case you need higher accuracy of wavelength
calibrations.
To take attached calibrations, the attached calibration template MUST come after the
corresponding science template because it will use the setup of the instrument performed by
the science template.
Therefore if one needs to bracket the observations by attached flat fields, he/she needs to
create an OB like this:
Acquisition template-dummy exposures in a science template for instrument setup-attached
calibration here flat fields-normal observation with the science template-attached calibration.
If one does directly the attached calibration after the acquisition template, the system will use
the setup corresponding to the AFC.
6.6 Spectrophotometric calibration
6.6.1 Telluric absorption correction
The visual-red and a near-IR part of the spectrum are strongly affected by the absorption
lines of the Earth’s atmosphere. Many of these telluric lines do not scale linearly with
airmass, so it is necessary to observe a star with a well-known spectrum at the same
airmass and with the same instrument setup as that used for the science target.
Furthermore, the strength of the telluric lines varies with time, so it is also necessary to
observe the telluric standard soon after or just before the science observation. Two
templates are designed for this purpose: XSHOOTER_slt_cal_TelluricStd and
XSHOOTER_ifu_cal_TelluricStd.
In general, we use either main sequence hot stars (B0 to B4 whenever possible, or to B9
otherwise) or solar analogs as telluric standards selected from the Hipparcos Catalog.
Unfortunately, hot stars still contain some features, usually lines of hydrogen and helium,
which can be difficult to remove. If the regions around the hydrogen and helium lines are of
interest, then one can also observe a late type star, which should have weak hydrogen and
helium lines. This star is then used to correct for the helium and hydrogen absorption in the
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