Supplying an externally-generated zero mask

In an earlier post I described zero-masking , in which the map is forced to a value of zero in certain locations to help with convergence (i.e. reduce saddles, negative bowls around sources etc.). The two methods that have been available until now are: defining circular regions beyond which the map is set to zero (good for known compact sources); and using a threshold on the local SNR (an automatic way to identify source and blank sky).

Since SMURF still handles non-continuous pieces of data independently (i.e. multiple maps of the same field), the SNR of an individual map may not be sufficiently large to produce a good mask of source / blank sky, whereas the combination of all data sets do. For those cases, and other examples where you may know, a priori, where to expect emission, a new facility has been added so that the user can define a zero mask externally and then provide it to SMURF.

In this illustrative example, I use one of the OMC-1 maps from 2010 with the s4a array: obs 18,22,23 on 2010216 and obs 27,29,33,34 on 20100218. First, we make a map using the bright_extended configuration (which uses SNR-based zero masking; I also use a lot of down-sampling and big map pixels to speed things up for this example):

makemap ^names_450.txt map_450_be pixsize=8 \
method=iterate \
config='"^/stardev/share/smurf/dimmconfig_bright_extended.lis"'

The resulting image looks reasonably good, but there are obvious negative bowls around the bright extended sources. However, the SNR of the final image is quite good, so I create a new mask based by thresholding the SNR map (4-sigma), and then smoothing it with a 5-arcsec FWHM Gaussian to spread out from where the sources are slightly:

makesnr map_450_be map_450_be_snr

thresh map_450_be_snr mask_450 thrlo=4 newlo=0 \
thrhi=4 newhi=1

gausmooth mask_450 mask_450_sm fwhm=5

thresh mask_450_sm zero_mask_450 thrlo=0.05 newlo=bad \
thrhi=0.05 newhi=1

I can then feed this new "zero_mask" back into makemap and re-run the data:

makemap ^names_450.txt map_450_zm pixsize=8 \
method=iterate \
config='"^/stardev/share/smurf/dimmconfig_bright_extended.lis,itermap=1,ast.zero_mask=1,ast.zero_snr=0"' \
ref=zero_mask_450


The key things to note are: (i) I turned off the SNR thresholding, ast.zero_snr, that is the default for the bright_extended configuration, and (ii) the new zero mask is provided as the REF image (also ensuring that the map and mask are on the same pixel grid), and to use it we set ast.zero_mask=1.

In the following images I show (from top-left): (i) an arm sticking out towards the north of OMC-1 using the default bright_extended reduction; (ii) the same region after using the updated zero-mask; (iii) the first map subtracted from the second map to illustrate the improved response to large-scale structure; (iv) the original combined zero mask based on the SNR of map pixels in individual scans; (v) the new zero mask based on thresholding/smoothing the combined SNR map: