Doing the observing

Once the receiver is tuned to the correct frequency, sideband, and velocity, and you are assured the pointing and focus has been established, the following steps should be followed for raster mapping.

• The DAS should be configured for the bandwidth required, and loaded in NON-continuous calibration mode. This is very important.
• Set up the cell definition required. Normally the cell size should be less than half the HPBW to avoid smearing the map in the scan direction. One-third HPBW sampling is a good default choice. If you want to cover a larger area you can set the cell size (corresponding to the distance the telescope moves between spectra) to a larger number (say, one beamwidth), as long as some beam smearing is acceptable. A rotated (wrt the normal coordinate frame) cell may be used if you want to map along a particular direction.
• Send the telescope to the center of the required map (including any offsets required in cell units).
• Specify the position switch (in arcseconds, as usual, remembering that the coordinate frame will be that defined by the cell setup).
• A calibration should be done at the reference position (don't forget to offset in cell units).
• Type raster. This will prompt for a number of entries:
  • The integration time per point (T), which should be 2--6 seconds per point: 2 seconds is the minimum time to ensure no overheads or synchronisation problems due to data transfer; if you want more than 6 seconds/point, then repeat the map at the same position and average them together (this will give better baselines and calibration than a longer integration time per point).
  • The number of points (N) along a row (in the scan direction). This must be an odd number. The maximum number of points/row is currently limited in software to 59. A longer row results in a higher on-source observing efficiency: measured in Jan 1995, 11 points/row = 50%, 31 points = 67%, 59 points = 77% (percentage is the time actually spent on source compared to elapsed time). However, as expected, longer rows will require better weather to give the flattest baselines (although for narrow lines, this is less important).

    The time on the reference position is automatically set to . This gives the optimum signal-to-noise ratio over the map in a given time.

  • Input the number of rows; this can be any odd number. The limit is the size of the storage file. The largest raster made as of March 1995, contained 1000 spectra (with 2 seconds per point). Larger maps can be constructed by mosaicing sub-maps together. The map should not take too long, otherwise line calibration may become unreliable when the atmosphere is unstable. CAL should be done at regular intervals, say once per 30, 60 or 90 minutes, depending on the sky conditions.
  • Finally, take defaults for the rest of the prompts.
  A map may be re-started at an intermediate row number, but not at a point within a row.

Now sit back and wait...

At the end of each row, the spectrum at the row centre is displayed, and that row may be accessed and reduced in SPECX.

Some rasters may occasionally show the message

Out of sync by 1 second

at the end of one of the rows. Unless the number of such messages is very large, this will not significantly affect the data.