CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
30 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
excellent atmospheric conditions, mild improvement in the amount of encircled energy can
still be obtained if an AO star with an R-band magnitude < 11 is used 20-25” away from the
scientific target. Targets further away than 20” from the main target will need a waiver;; this
is for the simple reason that the vignetting of the AO system more than 20 arcseconds from
the slit centre is not symmetric.
Note that any AO NGS being separated by more than 10 arcsec from the target must be at
brighter than R=12. This is for the reason that otherwise the AO system would become
unstable and the AO performance very poor. Likewise, for any AO NGS that is more than
10 arcsec away from the target, the high airmass limit is 1.4.
Note that for polarimetry as well as for spectro-astrometry, the AO NGS star needs
to be the target
(i.e., no off-slit AO star possible in these modes).
4.2.1.3 The brightness of the AO star
The flux on an APD is limited to 1 million counts in order not to damage the devices. The
optimal brightness of the AO star is R
~
11 mag. Brighter stars up to a bright magnitude
limit of R
~
0.2 mag can be dimmed using neutral density filters. Depending on the B-R
colour, some stars with slightly brighter R magnitude can be used. Good correction is still
obtained with stars as faint as R~14 mag under very good seeing conditions, while
moderate image quality improvement may be seen with stars as faint as R
~
15 mag under
excellent conditions (0.6”) and coherence time (5.2 ms). Stars fainter than R
~
15 mag will
not result in any improvement and NoAO must be used instead, as the AO loop would not
close on these targets.
Therefore, the allowed AO NGS magnitude range is: 0.2 < R < 15
(see 5.3.1 for details
on the user constraints).
4.2.1.4 The colour of the AO star
The B-R colour is important for precise atmospheric refraction compensation. The AO
system considers the di
f
ferential atmospheric refraction between the wavelength used for
the AO and the central wavelength of the spectrograph setup in the calculation of the tip-tilt
mirror orientation. A correct B-R is crucial for accurate centring of the target on the slit for
airmass > 1.2 when guiding with the slit viewer is not possible, as for example, if an off-slit
AO star is outside the field-of-view of the slit viewer detector.
4.3 Science detector characteristics
The focal plane of CRIRES is equipped with three 2048 x 2048 pixels Hawaii 2RG detector
arrays (6144 x 2048 in total) and a pixel size of 18μm (see Figure 6).
The detector read-out mode is always the “Sample-Up-The-Ramp” mode (
Figure
23
). The exposure time is set by the two following parameters: the detector
integration time (DIT) and the number of such integrations (NDIT) to be averaged into one
single exposure, whose total integration time is therefore NDITxDIT.
The minimum DIT is
1.427s;; it is not possible to integrate shorter than that.
