CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
55 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
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Because the AO correction degrades with airmass, we strongly recommend limiting
AO observations to airmass values of 1.4. This is especially important for off-slit
NGS, as the AO correction further degrades with increasing distance between NGS
and the slit.
Seeing and IQ
tab:
•
For NoAO observations, the desired seeing condition is defined either in terms of
requested Turbulence Category (TC) or Image Quality (IQ) at a given reference
wavelength. The ETC provides automatic conversion between the TC (Phase 1
constraint) and IQ (Phase 2 constraint).
•
The seeing is given exclusively in term of TC when the AO mode is selected. Hence,
in this case, TC is the relevant constraint for both Phase 1 and 2. AO observations
are restricted to TC= 10%, 20%, 30%, 50%, 70% and 85%. S/N ratio calculations
for larger values in this mode are not supported by the ETC as AO in those
conditions would result in marginal correction.
•
When the AO mode is selected, the R mag, B-V colour, spectral type and target
separation of the NGS must be specified. Suitable NGS can be retrieved from online
catalogues available at Simbad/Vizier, or from Gaia-DR2 after having transformed
the Gaia magnitudes to the Johnson-Cousins system
5
.
Instrument
tab:
•
The instrument setup is defined in terms of setting (i.e., Y1029;; see Table 19, column
1), and grating orders. For observations below 1400 nm, mind the wavelength
regions of the optical ghosts on the detector (see Section 4.6.1.1), as they are not
displayed in the ETC
.
Exp.Time and S/N
tab:
•
Requested output can be either the exposure time to achieve a given signal-to-noise
ratio (S/N ratio) or the S/N ratio achieved for a given exposure time. In both cases
the DIT needs to be specified.
•
The S/N ratio is given per spectral pixel in the spectral dispersion (and not per
resolution element);; the S/N ratio is calculated by integration over a box being a)
one detector pixel wide in spectral dispersion and b) a user-defined number of pixels
long in cross-dispersion direction ("Extraction Aperture").
•
For polarimetry, all calculations are based on one of the two polarized beams only,
and all values are shown for one exposure (i.e., nothing combined).
•
Extraction Aperture
: this is the number of detector pixels along the slit to be used for
S/N ratio calculation. In the L and M-bands, users should carefully select the
Extraction Aperture to avoid contamination from the dominating thermal background
(this will happen if it is set to a large value).
•
ETC
outputs the on-source integration time only. Depending on observing technique
and accounting for overheads, the total execution time can be much longer (see
Section 5.6).
5
Gaia DR2 Photometric Transformations are available at
https://gea.esac.esa.int/archive/documentation/GDR2/Data_processing/chap_cu5pho/sec_cu5pho
_calibr/ssec_cu5pho_PhotTransf.html
