User Manual
Doc. Number: ESO-323064
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4.3.2 Correcting for detectors non-linearity
All common IR detectors suffer from non-linearity effects. In the case of CRIRES, deviation
from linearity is of the order of 5% of the detected flux at about 18k ADUs and increases
with flux. However, the CRIRES pipeline is able to correct for non-linearity effects at low-,
medium- and high- count levels (see Figure 24).
Figure 24: Correction for non-linearity effects implemented by the pipeline.
Left
: correction
factor from the polynomial fits (pixel-by-pixel), evaluated at certain ADU-levels and plotted
as medians and 1-sigma shaded regions over all pixels. Detectors 1,2,3 in red, yellow, and
blue respectively.
Right
: Normalized ADU/s as measured from the frames corrected for non-
linearity. For the three detectors, the median over the bins in ADU-level is given.
4.4 Characteristic of the spectrograph
4.4.1 Slit viewer camera field of view
Target centring and NGS acquisition are performed in the NIR via the Slit Viewer camera
(SV). Because CRIRES observations require the use of a guide star (SVGS) to ensure that
the target is properly kept centred along the slit during the science exposures, the SVGS is
also acquired through the SV camera.
With a pixel scale of 37.3 mas, the SV covers a maximum unvignetted sky projected field
of view (FoV) of
22.8” x 33.8”
. 33.8
’’ along the slit
and 22.8
’’ perpendicular to the slit,
thus
making the available FoV slightly smaller than that of the old CRIRES instrument. The
position of the slit within this window is off-centred by
3.6”
towards West, and
0.2”
Northwards, as illustrated in Figure 25, to increase the allowed maximum separation
between the target and the SVGS. When the target is also used as SVGS, guiding will be
performed using the light reflected off the slit viewer window around the slit. CRIRES also
utilises a de-rotator to control the alignment of the slit relative to the sky (namely to
compensate for field rotation in the Nasmyth focus) or to align the slit with the parallactic
angle so as to reduce slit losses. Finally,
it is worth noticing that when the 0.2” slit is used,
