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The goal of a time estimator is to find the elapsed telescope time
(
) needed to obtain a given rms noise, while a sensitivity estimator
aims at finding the rms noise obtained when observing during
. The
total integration time spent on-source
is shorter than the elapsed
telescope time due to several factors. As of Gildas Jul17 release, the
input time of the sensitivity estimator is telescope time. The actual on
source time is then computed taking into account the following two points:
- Instrumental setup time: At the beginning of an observing track
a significant time (
minutes according to history of
observations) is spent in receiver tuning and calibration observations
before observing the actual astronomical target. This means that even for
a very short ON source time, a project cannot be shorter than
. Also, for long projects observed in several (
) tracks
the time spent for tuning and calibration is
. We
thus define the time spent for observations (i.e. without instrumental
setup)
as:
 |
(19) |
The number of tracks is computed as
where
is the typical
duration of a track, which depends on the source declination:
- Sources above
are observed 8 hours at most,
- 8.2 hours for a declination of
(truncated to 8 hours),
- 6.5 hours for a declination of
,
- 3.9 hours for a declination of
,
- 0.0 hours for a declination of
.
- Sources below
can not be observed.
A linear interpolation with the declination is performed in the
appropriate range between
and
.
For short projects (
), the number of tracks
is set to 1. Otherwise, the floating value of
is
used in the computation of
. Since
is constant whatever
the length of a track the use of a floating value for
is
somehow unnatural but it ensures that the conversion from
to
is a monotonic function.
- Observing efficiency: After the initial phase of instrumental
setup, the observing mode does not dedicate 100% of the time to the
astronomical target. Part of the time is spent for calibration (pointing,
focus, atmospheric calibration,...) and to slew the telescopes between
useful integrations. The time actually spent on source
is defined
as
 |
(20) |
where
is the observing efficiency.
The exact computation depends on the observing mode. There are three main
observation kinds.
- Single-source, single-field observations
- where the telescope tracks
a single source during the full integration time. This mode is used when
the signal-to-noise ratio is the limiting factor.
- Track-sharing, single-field observations
- where the telescope
regularly cycles between a few close-by sources. This mode is used when
the sources are so bright that the limiting factor is the Earth
synthesis, not the signal-to-noise ratio.
- Single-source mosaicking
- where the telescope regularly cycles
between close-by pointings that usually follows a hexagonal compact
pattern whose side is
, where
is the diameter
of the interferometer antennas. This modes is used to image sources wider
than the primary beam field of view.
In the following, we will work out the equations needed by the sensitivity
estimator for each of these observing modes.
Subsections
Next: Single-source, single-field observations
Up: IRAM Memo 2015-2 NOEMA
Previous: Actual computations
Contents
Gildas manager
2023-06-01