CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
65 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
6.3.5 Instrument details
·
Entrance slit width
(TSF:
INS.SLIT1.NAME
): w_0.2
or w_0.4, respectively for a minimum
spectral resolution of R = λ/Δλ = 80 000 or R = 40 000.
·
Reference wavelength
(TSF:
INS.WLEN.CWLEN
) allows the user to select from a pull-down
menu the wavelength value that uniquely identify the wavelength setting (i.e., cross-
disperser + order sorting filters + echelle grating). The settings and corresponding
wavelength ranges are provided in Table 19. The selection of a specific standard setting
automatically includes the positioning of the corresponding photometric band of interest for
the cross-disperser wheel (YJHKLM) and for the order sorting filter wheel + the setting of
the relative echelle grating angle. Specifying the wavelength in the acquisition template
allows for (a) differential atmospheric refraction between target and a separate guide star,
and (b) parallelisation of target acquisition and instrumental setup.
·
Derotator Mode
(TSF:
INS.DROT.MODE
): can be set to either ELEV
or SKY. In the ELEV
mode, the slit is always aligned with the parallactic angle (“pupil stabilized”), while in the
SKY
mode the field rotation of the telescope is compensated (“field stabilized”).
The
SKY
mode is relevant for extended sources or for placing multiple objects
simultaneously on the slit. The SKY mode must be used with the Spectro-Astrometry and
Generic Offsets templates. Likewise,
the SKY mode must be used whenever employing
an off-slit SV guide star and/or off-slit AO NGS.
The
ELEV
mode should be used for point sources, as it reduces slit loss introduced by
differential refraction. This is particularly important when SV guiding is done in one band
(e.g., in K) and the observations in another one (e.g., M). It can be only used for on-slit
guiding (i.e., SVGS = Target).
·
Position angle
(TSF:
INS.DROT.POSANG
) sets the position angle (PA) of the slit when the
SKY derotator mode is selected. The PA is counted from North (0 deg) via East (90 deg).
·
Gas cell
(TSF:
INS.OPTI1.NAME
) sets the name of the optical gas cell to be used during the
science. The default value (FREE) should be set for observations without any gas cell, while
GAS_SGC
or GAS_N2O should be set for observations at short (i.e., YJHK) and long (i.e.,
LM) wavelength, respectively. Gas cells provide for simultaneous wavelength calibration by
imprinting the absorption spectrum of the gas onto the spectrum of the target. Note: the
option of using the gas cell is only available for spectroscopic mode, but not for polarimetry.
·
Run Metrology?
(TSF:
SEQ.METROLOGY.ST
): To ensure the highest reproducibility of the
slit image position on the detector, the Metrology (see Section 4.6.2) should be activated
(
SEQ.METROLOGY.ST
="Yes") in the acquisition template. When activated there, no additional
overheads are produced as the Metrology runs in parallel to the target acquisition. Note that
the metrology is only available for wavelength < 4000 nm (YJHKL bands).
Note that
without employing metrology, the spectrum may be shifted by 5-10 pixel.
6.3.6 Differential Tracking parameters
·
Reference star tracking table file
(TSF:
SEQ.TRACK.REF
) and
Target tracking table file
(TSF:
SEQ.TRACK.TARG
):
These parameters expect the names of PAF files. PAF files are
ASCII files compatible with the VLT parameter file (PAF) format and contain the ephemeris
of the right ascension and declination used as a reference object ("AO star" and "slit viewer
