Frequency specifications and uvfits outputs

WODEN writes visibilities into uvfits files, which are output in linear polarisations of XX, YY, XY, YX, where X refers to a North-South aligned receiver and Y a East-West aligned receiver. These outputs are split up across frequency, into ‘coarse bands’. Each coarse band is split into ‘fine channels’ like so:

When running MWA simulations using a metafits file, these frequency options are filled in automatically for the user, and can be overridden / alternatively supplied by keywords to run_woden.py. The arguments that match the diagram above are listed here.

run_woden.py argument	Diagram label
--coarse_band_width	Coarse band width
--freq_res	Fine frequency channel width
--lowest_channel_freq	Lowest frequency channel
--band_nums=1,2,3	Band 01, Band 02, Band 03

As many coarse bands as needed can be run, allowing for straight-forward splitting of the simulation across multiple GPUs. We’ll look at an example of that later. If you run with following arguments:

run_woden.py \
--freq_res=10e+3 --coarse_band_width=1e+6 \
--time_res=1.0 --num_time_steps=10 \
--lowest_channel_freq=100e+6 \
--band_nums=1,2,3 \
--output_uvfits_prepend=epic_output

this will produce three uvfits files named with the following properties:

Output name	Lowest frequency channel (Hz)	Number frequency channels
epic_output_band01.uvfits	100e+6	100
epic_output_band02.uvfits	101e+6	100
epic_output_band03.uvfits	102e+6	100

Note

The command above won’t work as many arguments are missing; I’ve left them out here to concentrate on the arguments that define the uvfits outputs.

You can run with whatever band numbers you want:

run_woden.py \
--freq_res=10e+3 --coarse_band_width=1e+6 \
--time_res=1.0 --num_time_steps=10 \
--lowest_channel_freq=100e+6 \
--band_nums=4,7,24 \
--output_uvfits_prepend=epic_output

which will create uvfits files like:

Output name	Lowest frequency channel (Hz)	Number frequency channels
epic_output_band04.uvfits	104e+6	100
epic_output_band07.uvfits	107e+6	100
epic_output_band24.uvfits	124e+6	100