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.