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7
7.1
Characteristics of the spectrograph
Wavelength settings
Wavelength settings are defined by a reference wavelength that corresponds to the wavelength
at the middle of detector 3. In a few cases (see §7), this reference wavelength is not sufficient,
and an additional number, the grating order is needed.
Set-ups are done by using the instrument model to position the spectrum with an absolute
accuracy to better than 30 pixels and a reproducibility ≈ 3 pixels. Reproducibility to a
fraction of a pixel is expected once the use of the metrology fibers is implemented sometime
in 2011. We note that at least initially these fibres will only be used in the standard and not
free settings.
Typically about 280 CRIRES settings are sufficient to cover the entire infrared spectrum in
the range of 955 ≤ λ ≤ 5248nm. Standard wavelength settings are detailed in Sect. 15.
Wavelengths can also be set using free wavelength settings: allowed ranges for the reference
wavelength are listed in Tab. 6a.
Tables 4a - 4k (Tables 5a - 5k) therefore now provide under the columns Unvignetted range the
wavelength (wavenumber) ranges for which reproducibility of the illumination is estimated to
be better than 1 % for a 0.400 slit. Wavelength outside of these ranges but still covered by a
detector are subject to contamination from adjacent orders. The wavelength range for which
the reproducibility of the illumination is better than 1% is smaller for a wider slit. For an
0.800 slit, the whole spectral range covered by the mosaic is likely affected by contamination by
adjacent orders. As a consequence, slit widths larger than 0.400 should be avoided in general.
Vacuum wavelengths are used throughout this manual.
7.2
7.2.1
Wavelength calibration
Standard means: ThAr lamp and N2 O and CO gas-cells
Wavelength calibration of CRIRES spectra is difficult. Depending on the wavelength and on
the required accuracy, different methods can be used. For an accuracy corresponding to 3
pixels, the start and end wavelengths and the derived dispersion assumed to be linear for each
detector is sufficient, in particular for the standard settings.
If present in raw data, either emission or absorption sky lines can be used in the data reduction
to improve the absolute wavelength calibration. The CRIRES team has started to obtain
data to quantify the absolute accuracy of such a method. However, the number of sky lines is
insufficient to achieve a high precision absolute wavelength calibration for many settings, in
particular those below 2500 nm.
The density of lines provided by arc lamps is usually small in the IR. However, the ThoriumArgon lamp can provide a large number of lines up to about 2500 nm. The ESO Calibration
and Instrument Modelling group in Garching together with the US National Institute of Standards and Technology have collaborated to increase the number of identified Thorium and
Argon lines in the NIR. This collaboration to support the wavelength calibration of CRIRES
lead to a paper available in the Tools section in the CRIRES web page. Users whose science
has benefitted from the ThAr calibration should cite Kerber, F., Nave, G. & Sansonetti, C.J,
ApJS, 2008, 178, 374.
The atlas and line catalogue is available through http://physics.nist.gov/PhysRefData/