Sensitivity for mosaicking
Note: No provision is made to estimate the sensitivity of a mosaic in
frequency cycling mode. This is to avoid mixing the complexity of both
modes...
Mosaicking is a particular case of wide-field imaging: The user wishes to
observe a given field of view larger than the primary beam size with a
sensitivity as uniform as possible. The targeted field (which area is
, define by the user) can be divided in a number of independent
resolution elements or independent (primary) beams
. We have:
 |
(33) |
where
is the area of the primary beam. It is linked to the telescope
full width at half maximum (
) by
 |
(34) |
The 0.8 factor represents the truncation of the beam at 20% of its
maximum, which is performed during the imaging process.
For the sensitivity estimation we assume a standard sampling of targeted
field and the on-source time is equally divided between the independent
primary beams
in the targeted field of view. We
thus yield at the center of a Nyquist sampled mosaic
 |
(35) |
with |
(36) |
where where
is the integration time per pointing
during each single mosaic cycle, and
is the time to slew between
two consecutive pointings. There are several subtleties in these
computations.
In summary, correctly setting up the sensitivity estimate for a mosaic requires
- To compute the number of independent primary beams in the targeted
field of view
 |
(50) |
- To compute the number of pointings in the mosaic and the number of
pointings per track
 |
(51) |
- To compute the optimal integration time per scan with
 |
(52) |
The minimum integration time per scan of 10 seconds implies the maximum
dynamic of scales that can be observed in a single track.
- To compute the number of points per track that define the limit
between small and large mosaics with
 |
(53) |
- To compute the maximum number of repeats per pointing and per cycle,
depending on the mosaic size.
- To check that the maximum number of pointings per track is not yet
reached with
 |
(55) |
If the PI wishes to observe an area that will require more that 150
pointings per independent track, the estimator will ask to either
increase the requested elapsed telescope time or to decrease the
requested field-of-view area.
- To adapt the optimal integration time per scan to ensure that all
pointings per track will be cycled in
only for a
large mosaic with
 |
(56) |
- To compute the actual mosaic efficiency and time to complete a full
cycle with
 |
(57) |
and
 |
(58) |
These two quantities should be given as feedback to the user.
- To finally compute the sensitivity with
 |
(59) |