CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
35 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
Figure 27 shows the FoV of the SV for the derotator in SKY mode, with a position angle of
0 degrees. In this setting, the position of the slit with respect to the centre of the FoV is
offset by 3.6” to the West and 0.2” to the North, thereby increasing the allowed maximum
separation between the target and the SVGS. Note that when the 0.2” slit is used, the
footprint of the 0.4” slit covers an area of 0.4”x10” about 8” to the East with respect to the
0.2” slit (see dark vertical strip in Figure 27).
For obvious reasons,
it is crucial to make sure that the SVGS is within the FoV of the
SV at all times
during the observations. This is especially important when applying nodding,
jittering or generic offsets. The maximum possible distance between SVGS and target is 20
arcsec, and this only for the case that no nodding and jitter are applied.
When the target is used as SVGS, guiding will be performed using the light reflected by the
jaws of the slit. Note that for polarimetry and spectro-astrometry, the SVGS needs to be the
target (i.e., no off-slit guiding is possible).
4.4.2 Slit viewer guide star - limiting magnitudes
The SV is equipped with 6 NIR filters: J, H, K, two neutral density H filters and one neutral
density filter in K.
The SV is sufficiently sensitive that any emitting point source for which one aims to obtain
a spectrum should be seen on the SV image. In AO mode, stars of H
~
15 are easily detected
in a 10 s exposure when located well away from the slit.
In NoAO mode, H ~15.5 stars are
barely detected (3
s
) in a similar 10s integration under 0.9” seeing when located well away
from the slit.
For guiding on target (TRG=SVGS), the limiting magnitude is much brighter as only a small
fraction of the light is reflected by the slit jaws to the slit viewer detector. During the on-sky
commissioning run, under good conditions (0.8” seeing) and a slit of 0.2”, reasonable
guiding was possible with stars of H=14.5 and 14.0 in NoAO and NGS mode, respectively.
Table
3
lists the SV guide star magnitude limits for different observing modes.
For off-slit guiding, the SVGS must be sufficiently separated from the target
, as
otherwise the quality of the observations could be severely affected;; we recommend a
minimum angular distance of 2 arcsec. Guiding with an angular distance smaller than 2"
works fine for targets that are at least 3 magnitudes (H-band) fainter than the SVGS, i.e.,
with target that are (almost) invisible on the SV detector.
Table 3: Slit viewer guide star magnitude limits
on-slit (SVGS=TRG)
off-slit (SVGS≠TRG)*
NGS mode with 0.2" slit
0 ≤ H ≤ 15
0 ≤ H ≤ 16
NGS mode with 0.4" slit
0 ≤ H ≤
14.5
0 ≤ H ≤ 16
NoAO mode
-2 ≤ H ≤
15
-2 ≤ H ≤
16
*For off-slit SVGS, we recommend a minimum angular distance of 2" between SVGS and
the target unless the H-band magnitude difference target - SVGS is larger than 3.
