CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
43 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
employing the short gas cell (SGC) as a simultaneous wavelength calibrator with the 0.2”
slit (i.e., R~80,000). However, this value has not been confirmed on sky so far;; the best RV
precision that has been attained with CRIRES on sky is 10 m s
-1
.
For this reason,
we advise users to not expect any RV precision better than 10 m s
-1
for exposure times
DIT x NDIT
≤ 120s. For longer exposure times, the RV precision might be
even worse due to grating drifts. We note that the measured error of the RV measurements
does also depend on factors like the number of stellar absorption lines observed, the
broadening due to the stellar rotation and the signal-to-noise ratio.
Note that the pipeline does not calculate any RV values
;; data analysis for any CRIRES
RV data is left to the user. For this reason, we will only accept proposals with a clear outline
of the data analysis strategy when employing the gas cell.
Without a simultaneous wavelength calibrator during observations, the attainable RV
precision will be much lower due to a slow drift of the Echelle grating. During commissioning
it was observed that the drift in dispersion was somewhat higher (~0.2 px over 30 min;;
corresponding to a RV drift of 200 m s
-1
) following metrology alignment than it was without
metrology (0.05 px over 30 min;; RV drift ~ 50 m s
-1
).
For this reason, we recommend making use of the telluric absorption lines of the Earth’s
atmosphere as a long-term stable, simultaneous wavelength reference instead of employing
attached wavelength calibrations. In most wavelength settings, a large number of these
lines come for free and will be directly imprinted onto the science data. Figueira et al. (2010;;
A&A, 515, 106) demonstrated that telluric lines are intrinsically stable down to 10 m s
-1
(rms). Again, we recommend keeping the exposure times short (DIT x NDIT
≤ 120s) to avoid
the effects of grating drifts.
4.7 Throughput
The overall throughput of CRIRES has been measured on the spectro-photometric standard
Pi2 Ori by using the AO modes and the 0.4” slit width.
The total throughput was derived by scaling the observed spectrum, expressed in e
-
/s/px
and corrected for slit loss, by the theoretical. Figure 35 shows the overall efficiency
measured on the observed spectro-photometric standard in four different settings (i.e.,
Y1029, J1228, H1559 and K2148). The throughput over the whole spectral range can be
downloaded from the ETC webpage. The observation of spectro-photometric standards is
part of the instrument monitoring plan, and as such the results shown in Figure 35 should
be regarded as preliminary.
Based on these preliminary results obtained during the commissioning run in January 2021,
the throughput is found to be about 10-15% higher than the oCRIRES
