User Manual
Doc. Number: ESO-323064
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There is a priori no need to take flat fields at night-time, as the detector characteristics are
sufficiently stable in time, and the spectrograph wavelength setting sufficiently accurate.
However, flat fields may be important for very accurate radial velocity measurements.
Flat-fields part of the calibration plan (see Section 7.1) have a SNR per pixel larger than
200 at the peak efficiency of a given setting.
Users requiring flat fields with larger SNR
should indicate this in the ‘Special Calibration’
section of the proposal form or contact
when preparing the observations.
4.4.6 Spectrograph Field-of-View, slit width and seeing
The FoV of the spectrograph is
slit with
x 10”.
The appearance of a spectrum on the science
detectors mosaic is shown in Figure 27. The pixel scale of the science detector is 59 mas.
A slit width of
0.4” offers a close to maximal throughput in most AO observations
.
Figure 27: Illustration of the science detectors mosaic (H1582 setting).
4.4.7 Spectral resolving power
Spectral resolving power
for the 0.2” slit measured during commissioning was significantly
less than the expected
R
=100,000. There is some (expected) variation with echelle grating
angle and
R
>100,000 has been measured in some configurations. Nevertheless, for P108
users should not expect
R
>80,000. Full characterisation of the issue is ongoing, and several
options exist for recovering the expected spectral resolution.
4.4.8 Radial velocity precision
The short gas cell (SGC) provides a stable long-term wavelength reference in the H and K
bands. For a S/N of 150 per spectral pixel in the spectral continuum, an RV precision of 3
m s
-1
is expected to be attained by employing the short gas cell (SGC) as a simultaneous
wavelength calibrator in the K-band with
the 0.2” slit
(i.e., R~100,000). Note that the actual
error of the RV measurements will depend on factors like the number of stellar absorption
lines observed and broadening due to the stellar rotation.
If a lower RV precision is sufficient, users can make use of the telluric absorption lines of
Earth’s atmosphere
as a simultaneous wavelength reference. In most wavelength settings,
these lines will be imprinted on the science data. Figueira et al. (2010; A&A, 515, 106)
demonstrated that telluric lines are intrinsically stable down to 10 m/s (rms).
Without a simultaneous wavelength calibrator, the attainable RV precision will be much
lower due to a slow drift of the Echelle grating. During commissioning it was observed that
