In a single-field observation, an interferometer tracks a particular
direction of the sky, named the phase center. The portion of the sky which
can be image around this direction is directly linked to the size of the
primary beam. The easiest way to image field-of-view larger than the
primary beam size is to track one direction of the sky after another until
the desired field-of-view is filled with small images made around many
different tracking directions. This observing mode is called mosaicing and
the tracked observations which constitute the mosaic are called fields.
There are many constraints to optimize mosaicing.
- Nyquist sampling of the mosaic field-of-view and mosaic pattern
- The
mosaic field-of-view must at least be Nyquist-sampled to obtain a
reliable image. Each observed field can produce a reliable image of the
same shape than the primary beam, i.e. a circular Gaussian (This assumes
that the short-spacing problem has been solved). Nyquist sampling thus
implies that the mosaic fields follow an hexagonal compact pattern as
this ensures a distance between all neighboring fields of half the
primary beam size. When the total observing time is fixed, Nyquist
sampling is the best compromise between sensitivity and total
field-of-view. Indeed, the distance between neighboring fields could be
less (in which case the mosaic would be oversampled) than half the
primary beam size. In this case, the sensitivity on each pixel of the
final image would increase with the share of the time spent to observe
this direction.
- Uniform imaging properties and quick loop around the fields
- Getting
uniform imaging properties is a desirable feature in the final result.
This implies that a uv coverage and a noise level as uniform as
possible among the different fields. Quickly looping around the different
fields is the easiest way to reach this goal. However, dead time to
travel from one field to another must also be minimized. At NOEMA,
the compromise is to pause at least 1 minute on each field and to try to
loop over all the fields between two calibrations every 20 minutes.
Hence, mosaic done in a single observing run is made of at most 20
fields. Larger mosaic must be observed by group of fields in different
observing runs.
