## In IMAGER

The script @ fits_to_uvt handles the conversion from UVFITS to uv table format. The steps in GILDAS involve

1. Converting UVFITS to UVT
2. Collapsing the polarization information
3. Adjusting the weights to properly estimate the noise
4. Identify and flag bad data
5. 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 STOKES 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.