The script @ fits_to_uvt handles the conversion from
UVFITS to uv table format.
The steps in GILDAS involve
- Converting UVFITS to UVT
- Collapsing the polarization information
- Adjusting the weights to properly estimate the noise
- Identify and flag bad data
- Setting the frequency and velocity references
It also handles some nasty details, like the source coordinates
being hidden in different places depending on the CASA version.
- Step 1
fits 'name'.uvfits to 'name'.uvt
will create a GILDAS UV table from the UVFITS file. The signal
is assumed to be unpolarized at this stage. With the /STOKES
option, polarization information would be carried on properly at this stage.
- Step 2
This step is only needed with the /STOKES option. It could be
done manually by using command magentaSTOKES that handles Stokes parameter
conversion. The desired
polarization state should be set to NONE to optimize signal to noise
ratio for unpolarized sources, to I if polarization
is a concern.
- Step 3
This step is only carried on if the /WEIGHT option is present.
Depending on CASA version, the weights are not handled in the same way.
In Casa 3.4, they are approximately correct. In Casa 4.1, they are off
by a large factor (perhaps the number of channels...). Task
uv_noise utilizes the many channels available (3840 per
spectral baseband in FDM mode) to compute a statistic per visibility, and adjust
the weight through a median scaling factor. Task uv_noise
will also flag data with ``unusual'' weights (deviating from the median
by more than some factor (user specified, default 3)).
- Step 4
Task uv_trim will remove the flagged data from the UV
table, saving space.
- In practice, the optional Steps 2 to 4 are handled by a single
task called uv_casa, which saves 2 intermediate files
(and thus 2 read and 2 writes of large files).
- Step 5
At this stage, one could start usual imaging using the
standard imaging commands READ UV 'name'; UV_MAP.
Commands SPECIFY FREQUENCY Value and SPECIFY
VELOCITY Value will set the desired correspondence between
Velocity and Frequency axis.
Next: Transfering Image data
Up: IMAGER versus CASA
Previous: In CASA
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Gildas manager
2023-06-01