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Frequency cycling observations
In frequency cycling, the tuning frequency is regularly cycled between
predefined values inside the same receiver RF band. This
first imply that the on-source observing time must be split between the
different tuning of the frequency cycling. To do this, the user will have
to give the percentage of the time required per tuning. The sum of the
percentage will have to be equal to 100%. By default, PMS will divide
equally the on-source time between the tunings, and the user will have
the possibility to modify this time repartition.
Frequency cycling also has two consequences on the observational
efficiency.
-
The time to setup the tunings is increased with respect
to standard observations by
, where
is typically XXX minutes.
-
After the setup phase, each cycle observed at a given
frequency must be surrounded by gain calibration observations at
the same frequency. This means that the observing efficiency is
decreased: in practice this is like doubling the number of
calibrators, since each calibrator will have to be observed at
the 2 frequencies (the frequencies of the previous and of the
next cycle, whatever the number of cycles).
To take this into account, we first define the overhead factor as
 |
(38) |
The overheads are now split into generic overheads, independent of the
number of gain calibrators, and the calibration overheads that is
directly proportional to the number of observed gain calibrators. This
gives
 |
(39) |
We will use
and
,
or 2 for detection or imaging,
respectively, and
in frequency cycling mode, 1
otherwise.
-
The overall overheads are evaluated by computing the
total on-source time over the total telescope time:
 |
(40) |
In the context of frequency cycling, the overheads for a single
frequency are computed by weighting the times with the fraction
spent on this cycle (e.g. 50% for 1 among 2 frequencies):
 |
(41) |
which is identical to the previous formula. If the overheads reach
75%, a warning is raised.
When frequency cycling is combined with dual band observations, it
is emphasized that both receiver bands are affected by the
efficiency loss of the frequency cycling even though one of the two
bands could not require frequency cycling at all.
Subsections
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Gildas manager
2024-12-01