CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
29 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
allowed under Turbulence Category of 100% (see 5.3.1 for more details on the user
constraints).
Because the AO correction also strongly degrades with airmass, we
strongly recommend observing at low airmass (≤1.4) only.
This is especially important
for off-slit NGS, as the AO correction of the target further degrades with increasing distance
between the NGS and the slit. In any case, airmass values > 2.0 must be avoided whenever
employing AO.
Figure 22 illustrates the increased throughput when the AO system is employed. The graph
shows the spatial profiles of the spectro-photometric star Pi.02 Ori (R=4.29) at a wavelength
of 1559.245 nm taken in atmospheric condition corresponding to Turbulence category =
50%, in open and closed loop. In both cases, the exposure times were the same;; however,
in closed loop a flux level being about 1.8 times higher was attained than in the open loop
observations.
Figure 22: Improved throughput thanks to the use of AO. H-band flux of the star Pi.02 Ori
measured along the 0.4” slit in open (solid red line) and closed loop (dashed blue line).
4.2.1.1 AO natural guide stars
CRIRES wavefront sensing is done in the R band (Johnson/Bessell). The performance of
the AO system depends on the distance to the science target and on the brightness of the
selected natural guide star (NGS) for AO. The loop may be closed on non-stellar objects
such as the surface of Mars or the nucleus of comets, although in these cases NoAO is
often used (for example, for a diffuse comet nucleus).
4.2.1.2 The distance of the AO natural guide star from the slit centre
Although nominally the field selector allows the selection of the AO NGS star up to 25” from
the nominal position of the science target, the AO NGS star should be as close as possible
to the scientific target, usually closer than 10”: ideally, it is the science target itself. With
