User Manual
Doc. Number: ESO-323064
Doc. Version: 2
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6.3.4 OB Constraint set
In the
p2/Constraint Set
tab, users are requested to define the suitable observing conditions
for the OB execution. These are: i) Airmass; ii) Lunar Illumination; iii) Moon Angular
Distance; iv) Twilight; v) Sky Transparency; vi) Image Quality; vii) Turbulence; and vii)
Precipitable Water Vapour. As mention in §5.2, most of these constraint parameters (i.e., i,
ii, v, vii) have been already defined in Phase 1. We remind here that such requirements are
binding, and as such users are not allowed to defined OB constraint set more stringing than
that requested in the proposal. At Phase 2, observing constraints can be only relaxed.
I.
AIRMASS
(X): Most efficient AO corrections is obtained at
airmass up to ~1.4”
. At
higher values of airmass
(i.e., X>1.4”)
the performance decreases rapidly. In
addition, observations (NoAO and AO) with the derotator in SKY mode can be
strongly affected by differential refraction. User should adopt the same value
requested in the proposal.
II.
LUNAR ILUMINATION
(FLI): Most of the observations can be performed under
FLI=1. However, the Moon may affect AO observations as described in §5.2.3. User
should adopt the same value requested in the proposal.
III.
MOON ANGULAR DISTANCE
(MAD): In general, the Moon does not affect IR
observations. Please refer to §5.2.3 for more information
IV.
TWILIGHT
(TW): Bright objects may be observed during twilight. In that case, the
TW can be used to define the earliest time with respect to the end of the evening
astronomical twilight when the execution of the OB can be started. While the relation
between the time difference from the evening twilight end and sun elevation varies
during the year, for Paranal due to its low latitude this difference is small. Therefore,
the constraint is given in minutes as a difference in time with respect to the end of
astronomical twilight (i.e., the time when the solar elevation is -18 degrees). The
default TW value
–
15 (minutes). A negative number means that it is allowed to start
the observation before the end of the astronomical evening twilight, a positive
number means start the observation at least that many minutes after the end of the
astronomical twilight. The twilight constraint can take values between -30 and +15
minutes. In particular, K-, L- and M-band observations can already be taken well
before the end of the astronomical twilight.
V.
SKY TRANSPARENCY
: Thin clouds (THN) usually do not hamper CRIRES
observations for bright objects. Clear (CLR) conditions are justified for observations
requiring stable AO corrections.
VI.
IMAGE QUALITY
(IQ): Defined as the FWHM of a long exposure stellar images, is
a property of the images obtained in the focal plane of an instrument mounted on a
telescope observing through the atmosphere. It is therefore a quantity measured at
the requested airmass and wavelength of observing. The IQ is relevant only for
NoAO observations. User must use the IQ value calculated by the ETC for the
requested airmass and wavelength setting.
VII.
TURBULENCE
(TC): see §5.2.1 for the definition. At Phase 2, the TC constraint is
relevant only for OB using the AO mode. User should adopt the same value
requested in the proposal.
VIII.
PRECIPITABLE WATER VAPOUR
(PWV): Observations in the NIR can be
severely affected by the presence of PWV in the atmosphere: large amount of PWV
can significantly decrease the sky transmission, and, in particular, in the L and M
bands increase the sky emission background (see §5.2.2). For L and M-band
