### Typical use

The processing of mosaics for NOEMA is essentially similar to that of single fields. There are only two small changes

Creation of uv table
  A mosaic UV table should be created using the /MOSAIC option of command TABLE in CLIC.
Imaging
  is done through UV_MAP as for a single field. However, the process is different. The command takes into account the various fields, the primary beams, and select an optimum projection center (phase center). The later may need to be specified by the user using the MAP_CENTER string, or as argument to UV_MAP
Deconvolution
  is also similar, but not all algorithms are available: MX, SDI and MRC do not work for mosaics. The change of behavior of the CLEAN algorithms is visualized through the change of prompt from IMAGER> to MOSAIC>. Two additional variables are used for mosaic deconvolution
  CLEAN_SEARCH
    The minimum fraction of a primary beam below which no Clean component is searched for.
  CLEAN_TRUNCATE
    The minimum fraction of a primary beam cumulated response below which no image restoration is performed. Below this threshold, the Clean image is blanked.
  Finally, note that the mosaic deconvolution produces sky brightness images ( SKY variable) while single-field deconvolution produces images attenuated by the primary beam.

IMAGER makes no specific assumption about the uv coverage of individual fields. However, it uses a single Clean beam in the deconvolution, so Mosaics where fields have widely different uv coverage will not conserve flux properly. Mosaicing deconvolution will work better if all fields are equivalent in uv coverage and noise level.

A mosaicing session would thus just be like a single-field imaging:

           1 read uv gag_demo:demo-mosaic
           2 uv_map
           3 hogbom /flux 0 10
           4 fit /jvm
           5 uv_restore
           6 show sky
           7 write * demo

Comments:

Step 1
  Read the UV table
Step 2
  Image the mosaic
Step 3
  Deconvolve
  Step 4-5 Optimize residuals (see Sec.)
Step 6
  Look at the result. The result is in SKY.
Steps 7
  Save the result SKY, and the intermediate files ( BEAM, DIRTY, PRIMARY).

However, mosaics can be huge and require more memory than available (see Section ) To overcome this issue, IMAGER provides the script @ image-mosaic that splits the UV_MAP step into slices that fit into memory. The user can then read the resulting images and deconvolve them, eventually by blocks of channels if needed (in particular when a uv_restore step is desired) using a similar logic.