If, as in this case, you have multiple observations contributing to your map, each observation can be reduced separately, and then combined to make the final map. Currently there is no advantage in terms of data quality to reducing all observations simultaneously or separately. However, the latter does allow the option of assessing the individual maps before coadding and is the method followed in this example.
When running the DIMM we select the default configuration file dimmconfig.lis; the individual parameters of which are described in Section .
% makemap '$STARLINK_DIR/share/smurf/s8d20100216_00022_000?.sdf' Orion22 \ method=iterate config=^$STARLINK_DIR/share/smurf/dimmconfig.lis % makemap '$STARLINK_DIR/share/smurf/s8d20100216_00023_000?.sdf' Orion23 \ method=iterate config=^$STARLINK_DIR/share/smurf/dimmconfig.lis
The map from each observation is shown on the top row of Fig. . Note that these maps were observed as a series of rotating pong patterns to avoid repetition of any scan direction, hence their distinctive shape.
The default configuration file is designed to preserve maximum flux, however the difficulty of distinguishing between low frequency noise and real extended source emission inevitably means that some low frequency noise ends up in the final map. The maps show that although extended emission has been recovered the background is far from flat, displaying large scale patchiness as well as deep negative bowling surrounding the strongest sources.
Both of these effects can be mitigated by post-processing using the PICARD recipe REMOVE_BACKGROUND. A number of different techniques are available in this recipe to control how the background is removed. In this example we have selected CUPID findback which uses spatial filtering to remove structure on a size scale less than that specififed by the parameter FINDBACK_BOX. The modified parameter file (params.ini) is shown below where the method is set to findback and the findback box size to 30 pixels or 120arcsec when using 4arcsec pixels:
[REMOVE_BACKGROUND] BACKGROUND_FITMETHOD = findback FINDBACK_BOX = 30
Caution is advised when selecting the box size, with a smaller box giving a flatter background but at the expense of source flux. This is of particular importance to extended sources where the recovery of faint emission is paramount.
% picard -recpars params.ini REMOVE_BACKGROUND Orion22.sdf % picard -recpars params.ini REMOVE_BACKGROUND Orion23.sdf
The background subtracted maps are shown on the bottom row of Fig. where both the bowling and uneven background have been significantly improved. Before we combine the maps we will first crop them to their originally requested size using the PICARD recipe CROP_JCMT_IMAGES.
% picard CROP_JCMT_IMAGES Orion22_back.sdf % picard CROP_JCMT_IMAGES Orion23_back.sdf
These cropped maps are then coadded using MOSAIC_JCMT_IMAGES. This example utilises the default parameters where wcsmosaic with variance weighting is used for the mosaicking method although it can be configured to use makemos or to use a different wcsmosaic method.
% picard MOSAIC_JCMT_IMAGES Orion2*_back_crop.sdf
The key advantage to using the PICARD recipe over standalone KAPPA commands is that the exposure time image is also propagated correctly to the output mosaic (it is stored in the .MORE.SMURF.EXP_TIME extension).
The final, coadded map is shown in Fig. .
Note the output filename convention for each PICARD recipe: REMOVE_BACKGROUND creates output files with the suffix _back, CROP_JCMT_IMAGES creates files with the suffix _crop, while MOSAIC_JCMT_IMAGES creates files with the suffix _mos appended to the last input filename.
The SCUBA-2 SRO Data Reduction Cookbook