Importing Data from MIRIAD

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Note: A filler routine is currently being developed to convert native MIRIAD format data into a CASA measurement set, but it has not yet been completed. The following steps describe how to use FITS format files to transfer CARMA data into CASA, and should still be useful even when the filler is operational. At that time, a description of how to use the CARMA filler routine will be added to this page.

This page assumes you have MIRIAD already installed. All MIRIAD commands are given in their shell-script (csh) form, assuming that a number of variables have been defined at the top of your script.

For this example, the following items would be defined at the beginning of the csh shell script:

# in csh script

#!/bin/csh
# FILE = name of original, raw MIRIAD-format file
set FILE="../c0xxx.1E.miriad"
# PROJECT = base file name for processed data files
set PROJECT="c0xxx"
# ANTPOS = the location of the file which contains updates to the CARMA antenna positions
set ANTPOS="../antpos.YYMMDD"

Preparing MIRIAD data for export to FITS

To minimize the data you are transferring over, select only the astronomically useful data:

# in csh script
uvcat vis=$FILE  select='-source(NOISE),-auto' out=$PROJECT.vis

If you need to apply an update for the antenna positions, or linelength corrections, you should do these before exporting to FITS.

To update antenna positions:

# in csh script
uvedit vis=$PROJECT.vis out=$PROJECT.ant apfile=$ANTPOS

To apply linelength corrections:

# in csh script
linecal vis=$PROJECT.ant
uvcat vis=$PROJECT.ant out=$PROJECT.lc

Currently, MIRIAD writes out the identity of the array as identified by Antenna 1, which is a formerly-OVRO antenna. Thus, without correction, the dataset will identify the array as OVRO rather than CARMA, which means that CASA will make the same mistake. To fix this problem, simply do a small header edit:

# in csh script
puthd in=$PROJECT.lc/telescop value=CARMA

One last recommendation for preparing the data: it is still valuable to do some initial hanning smoothing to dampen ringing in the bandpass. (This was still true, at least, for data taken in summer 2009.)

To hanning smooth:

# in csh script
uvcal vis=$PROJECT.lc options=hanning out=$PROJECT.hann

Writing out the FITS files

Now you should be ready to write out your CARMA visibility data. You will need to write out each spectral window as a separate FITS file in MIRIAD, and then concatenate them back together within CASA. In your shell script, just use a "foreach" loop.

In this example, I am writing out the upper sideband windows only (windows 4,5, and 6). This loop produces one file for each spectral window, including data from all sources observed with that window.

# in csh script
foreach i (4 5 6)
    fits in=$PROJECT.lc op=uvout select="win($i)" out="./fits/$PROJECT.$i.fits"
end

Constructing the CASA Measurement Set (MS)

You now have everything you need to tell CASA how to create an MS from these data; the remaining commands on this page are formatted as if they were part of an executable python script (e.g., "execfile 'script.py' in CASA).

If you have more than a couple spectral windows to import, I'd recommend using a little python to assemble the list of files for you, rather than having to tediously type out the whole list by hand. The following lines of code will produce an array of filenames that you can give CASA as an input for the "importuvfits" command:

# in CASA
from glob import glob as filesearch
import os

fitsdir='fits/'
msdir='./'
project='c0xxx'
catfile=msdir+project+'.ms'

inlist = sorted(filesearch(fitsdir+'*.fits'))

In the same python script, you can now use the following "for" loop to read each of the individual FITS files into an MS of the same name (e.g., for a FITS file named "c0xxx.4.fits", produce an MS called "c0xxx.4.ms" .)

# in CASA
for item in inlist:
    infile=item.split('/')[-1]
    outfile=infile.replace('.fits','.ms')
    importuvfits(fitsfile=item, vis=msdir+outfile)

Lastly, concatenate the individual spectral windows MS directories into one big MS containing all spectral windows, for all sources:

# in CASA
files = sorted(filesearch('*.ms'))
concat(vis=files,concatvis=catfile,freqtol="",dirtol="1arcsec",async=False)

A final check

Of course, if you'd like to see whether or not concat produced something sensible, just run a "listobs" to make sure things look all right:

# in CASA
listobs(vis=catfile)

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--Michele Thornley 28 December 2009