Casatools API

Over the past few ALMA cycles, the documentation for the casatools has improved tremendously, obviating the need for many of the original visread routines. Therefore, we have removed that duplicate functionality from the visread package. Instead, we highlight several of the casatools that we use on a regular basis. Following the tutorials, we recommend that you familiarize yourself with these tools and then use visread conversion routines as needed. Be aware that there are many casatools available beyond those listed here, too.

msmetadata

As its name implies, the msmetadata tool is a very useful tool for getting metadata-like quantities from a measument set. We have found it very useful for obtaining things like number and characteristics of spectral windows (datadescids), channel frequencies, times of observations, antenna baselines, and more.

For example, to get an array of the channel frequencies (in Hz) from a measurement set, follow

import casatools
msmd = casatools.msmetadata()
msmd.open("my.ms")
swp_id = 0
chanfreqs = msmd.chanfreqs(spw_id)
msmd.done()

ms

In our opinion, the ms tool is the workhorse tool for obtaining data from the measurement set. We commonly use something like the following workflow

import casatools
ms = casatools.ms()
ms.open("my.ms")
swp_id = 0
# narrow down the selection to just the spw under consideration
ms.selectinit(spw_id)
# further narrow the selection to the shortest baselines
ms.select({"uvdist":[0.0, 100.0]}) # [m]

# get channel info, baselines, visibility data, etc in a dictionary
d = ms.getdata(['axis_info', 'data', 'uvw'])
print(d['uvw'].shape)

ms.reset() # clear selections
ms.done()

Making the appropriate selection before querying your data can really reduce the memory requirements and runtime of any operation. This can really make a difference when working with large measurement sets.

If your spectral windows are different shapes, you will find the ms.selectinit operation to be necessary to read one spectral window at a time.

table

Compared to ms, the table tool is a more direct way to access the sub-tables within a measurement set. If you are working with a measurement set that only contains a single spectral window, you may find these routines useful. In practice, though, we tend to prefer a combination of the msmetadata and ms tools because of their ability to handle measurement sets with heterogeneous spectral windows.