CRIRES User Manual
Doc. Number: ESO-254264
Doc. Version: P109.4
Issued on:
2021-12-01
Page:
67 of 99
Document Classification: ESO Internal Use [Confidential for Non-ESO Staff]
Polarimetry
CRIRES_pol_obs_AutoNodOnSlit
Linear and circular polarimetry taken
along the slit. Only on-slit guiding / AO
NGS possible.
The science observing templates must match the instrument mode (
‘Spectroscopy’ or
‘
Polarimetry’
) defined in the acquisition template. For example, the acquistion template
CRIRES_spec_acq_NGS
(spectroscopy mode) will not work with the observing template
CRIRES_pol_obs_AutoNodOnSlit
(polarimetry mode).
Table
11
provides a summary of the
available observing templates as a function of the instrument modes.
In the following, we describe the parameters present in the
observing templates
that have
not been introduced already in §6.3 and that the user is requested to define.
6.4.1 Exposure Time details
·
DIT
(TSF:
DET1.DIT
) defines the exposure time of an individual detector integration time,
in seconds. Only the following DITs are allowed in service mode: 1.4, 2, 3, 5, 7, 10, 15, 20,
30, 45, 60, 75, 90, 120, 150, 180, 240, 300, 450, 600 or 900s.
Any DIT<1.427 will be automatically converted to the minimum DIT of 1.427s;; DITs ≥600s
require a waiver because of the high risk of not meeting atmospheric quality requirements.
We recommend to set the DIT to such a value so that the detector counts remain
well below
the saturation limit
, but at the same time the number of detector read-outs (NDIT x
exposures) is kept as low as possible. Otherwise, the signal-to-noise ratio of the
observations may be strongly affected by the read-out-noise. The optimum setting shall be
evaluated by using the Exposure Time Calculator (ETC).
Owing to the bright thermal background in the L- and M-bands, any DITs longer than the
ones specified in
Table
12
will result in saturation of the detectors and will require a waiver.
Table 12: longest possible DITs for selected wavelength settings without saturation.
wavelength setting
DIT
0.2” slit
0.4” slit
3244.424
3262.140
90s
45s
all M-band settings
20s
10s
·
NDIT
(TSF:
DET1.NDIT
) determines the number of individual integrations averaged into one
exposure;; therefore,
DET1.NDIT x DET1.DIT
sets the exposure time of one exposure, which is
then written into a FITS file. The individual integrations (NDITs) are not stored in the FITS
file. Note that for each NDIT, the detector read-out noise listed in applies, and a large
number of detector read-outs can affect the overall signal-to-noise ratio of the observations.
NDIT >1 should be used for DITs
£
30 sec to keep the overheads for the detector read-out
short. For longer DITs, however, we recommend to set NDIT=1, and to increase the number
of exposures (
SEQ.NEXPO
) and nodding cycles (
SEQ.NABCYCLES
) instead. We have seen that
such a setting slightly improves the signal-to-noise ratio in the reduced data.
