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              THE VLA SPECTRAL LINE DATA SET: NGC 5921
                         (Michael Rupen)

    This is an HI data set taken with the VLA in its D (most compact)
configuration on April 13, 1995.  The galaxy in question is a small
barred spiral with a reasonably large bulge, a LINER nucleus, and intense
star formation along the bar and inner spiral arms,

    The data reduction for this project can be divided into five
stages - (1) calibrating the raw multi-source pseudo-continuum (channel 0)
data; (2) calibrating the bandpass; (3) subtracting the continuum; 
(4) making images of the fully calibrated line-only data; and (5) trying
out some simple displays and analyses of the resulting cube.

Stage 1.  Calibrating the Channel 0 Data: 
  There are two raw, multi-source data files: 13/04/95.CH 0 and .LINE.
  The former contains the so-called "channel 0" data, consisting of the
  vector average of the inner 3/4 of the line data.  These files contain
  observations of the absolute flux density and bandpass calibrator
  (1331+305=3C286), the phase calibrator (1445+099), and NGC 5921.
  The observing mode was 2AC, indicating the use of a single VLA IF pair
  to obtain Stokes right and left circular pol'n correlator products (RR
  and LL); there were originally 127 channels, but I've averaged down to 63
  to make mapping a bit faster.  The data have not been flagged or processed
  in any way.

  This first step consists of flagging the Channel 0 data and using them to
  derive the overall, time-variable antenna gains.  For those that want to
  skip this step, the requisite tables can be copied from 13/04/95.CH0SAV.
  In any case, remember to copy the FG and CL tables to the LINE data when
  this step is finished.

  At this stage it's a good idea to make a quick map of the channel 0 data
  to see that everything's OK.

Stage 2.  Calibrating the Bandpass and Subtracting the Continuum:
  Having calibrated the time-dependent gains, the next step is to calibrate
  out the frequency variations, i.e. to calculate the bandpass.  Those
  wishing to skip this step can copy the BP table from 13/04/95.LINSAV.

Stage 3.  Subtracting the Continuum
  Now that the data are fully calibrated, the continuum emission may
  be removed, leaving a line-only data set.  This will require
    (1) figuring out which are the line-free channels
    (2) subtracting an average or a fit to those channels from the
      data, either in the uv- or in the map-plane
  This may require some iteration!

Stage 4.  Imaging the Continuum and the Line-only Data
  With the continuum and the line now stored in separate data sets,
  the next step is to image and deconvolve them.  

Stage 5.  Examining and Analyzing the Cubes
  Finally, play around with the various display options, and try making
  moment maps, lv-slices, and spectra.  The adventurous may wish to try out
  Karma or Gipsy at this point.

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