Command-line Applications

imcontsub

The image-plane application imcontsub is used to perform continuum subtraction on spectral line data cubes. It can handle large data cubes efficiently by processing them in chunks. The chunking is done along the RA-axis, i.e, the data cube is split into smaller sub-cubes that are processed in parallel.

A key advantage of image-plane continuum subtraction is that it allows for robust on-the-fly thresholding due to the high signal-to-noise ratio compared to the uv-plane. This is particularly useful on for data cubes unknown line emission, which, if not accounted for, can lead to significant errors in the continuum subtraction process. This on-the-fly thresholding can be enabled by setting the --sigma-clip parameter.

imcontsub --output-prefix output_prefix \
            --sigma-clip 5 \
            --order 3 \
            --segments 250 \
            --ra-chunks 64 \
            --nworkers 8 \
            input_fits_cube.fits

However, if the line emission is known, a binary mask can be provided to the --mask-image option. In this case, the --sigma-clip parameter will be ignored (if set.)

imcontsub also allows the user to specify multiple --sigma-clip, --order and --segments parameters. This allows the user to do the continuum subtraction in multiple iterations, each time using a different set of parameters. The advantage of this is that the user can start with a large --sigma-clip and wide --segments values to remove the most significant line emission, and then gradually decrease both to remove smaller line emission features. When using this mode, the --segments and --order must have the same length. Here’s an example

imcontsub --output-prefix output_prefix \
            --sigma-clip 5 5 3 \
            --order 3 2 2 \
            --segments 400 300 250 \
            --ra-chunks 64 \
            --nworkers 8 \
            input_fits_cube.fits