Combining multiple POL2 fields – James Clerk Maxwell Telescope

If you wish to combine POL2 observations for multiple overlapping fields, the best way to proceed is probably as follows (note, for this to work correctly you will need a Starlink build from 9th June 2019 or later):

1. Run “step 1” independently for each field. In other words, use pol2map to create an auto-masked total intensity map for each field. The following assumes that pol2map is run within directories called “field1″, field2”, etc, to create the auto-masked maps and the I/Q/U time-stream data for each field.
2. Co-add the auto-masked total intensity maps for all fields. First create a text file holding the paths to the separate auto-masked total intensity maps, and then run pol2map as follows:
```
% more infiles
field1/iauto
field2/iauto
% pol2map in=^infiles iout=iauto_mosaic \
          qout=! uout=! multiobject=yes
```
3. Run “step 2” and “step 3” for each field, using the mosaic created above as the mask field for all fields. For instance, for the first field:
```
% cd field1
% pol2map in=qudata/\* iout=iext qout=! \
          uout=! mapdir=maps mapvar=yes \
          skyloop=yes mask=../iauto_mosaic 
% pol2map in=qudata/\* iout=! qout=qext \
          uout=uext mapdir=maps mapvar=yes \
          skyloop=yes mask=../iauto_mosaic \
          ipref=iext cat=mycat debias=yes
```
  Then do the same for field2, field3, etc. If preferred, steps 2 and 3 can be combined into a single invocation of pol2map:
```
% cd field1
% pol2map in=qudata/\* iout=iext qout=qext \
          uout=uext mapdir=maps mapvar=yes \
          skyloop=yes mask=../iauto_mosaic \
          ipref=iext cat=mycat debias=yes
```
4. Co-add the external-masked I/Q/U maps for all fields and create a vector catalogue from the co-added maps. First create a text file holding the paths to the external-masked I/Q/U maps for all fields, and then run pol2map as follows:
```
% more infiles
field1/iext
field1/qext
field1/uext
field2/iext
field2/qext
field2/uext
% pol2map in=^infiles iout=iext_mosaic \
          qout=qext_mosaic uout=uext_mosaic \
          multiobject=yes cat=mycat_mosaic \
          debias=yes ipcor=no
```