Getting The Parameters Right

 To make maps you have to create a map file first. Then you put spectra into the file. Map files are similar to data files except they always have read/write access. The command to start a map file is open-map-file.

Some words of advice first:

• You will need to provide the number of points in each spectrum (they should all be the same), which means you need to know this and have it set to a sensible number before you get going in a production mode. The number of points is given on a laser plot at the extreme lower left, and if you use the command print-spectrum-header (p-s-h will do) you can see the same information. Once you set the number of points in the map set-up, it will not be possible to stuff a spectrum containing fewer (if, say, you smoothed the data first) or more points into that map.
• The number of points selected is a crucial item as far as memory and disk space goes also. It is wise to limit the amount of the spectrum you want in your map to as few channels (200 or fewer may be enough sometimes) as is practical. Otherwise you may run out of disk space. With the large maps which can be made using the raster mode this is an important point. Limit the number of channels in the spectrum using the drop-channels command. See below.
• Only one map can be open at any one time. This means that it is very inefficient to transfer data from one map to another, using the commands get-spectrum-from-map and add-to-map in a command file. It is much better to hold the data in a SPECX data file and use that as an intermediary.

The key to all this is to experiment first with the data you plan to put in a map. Let's illustrate this with an example of a real life situation.

The first thing to do is open a map file. The following dialog ensues once we type the command open-map-file:

>> o-map
File name? (extension will be .MAP) [    ] s140_core

Inquiring about file: s140_core.MAP
Map file does not exist: open a new one? (Y/N) [N] y
File title? S140_core_data
File owner? Robert_Simon
Set map centre automatically from first scan added to map? (Y/N) [Y] y
x (R.A.) & y (Dec) cell sizes? (arcsec) [  0.0   0.0] 6 6
Position angle of map y-axis? (degrees) [   0.0]
Maximum number of cells on x & y axes? [  0   0] 15 13
Number of spectral channels in map? 247

Map centre (i.e. pos'n corresponding to centre pixel)
will be the map centre of first spectrum ADDed to the
map after it is created. Use ED-S-H on first spectrum
if you want to force some other map centre.

>>

Some explanation of the exchange is given below:

• On issuing the open-map command and a filename, one is prompted for a number of inputs if the map doesn't already exist (if it does, that's the end of the conversation, and the map is just opened with no further fuss).
• You give a map title and owner (note that these strings, like the ones associated with data files, should not contain spaces, etc).
• Next you specify whether the map centre is determined automatically from the first spectrum added to the map (this does not have to be the central point); if you answer negatively then you can provide a different central position. In the latter case note that the new map centre should be as closely as possible an integral number of pixels from the original observation centre in both coordinates.
• Next give the cell spacing and angle; these will probably be the same as those used during the observations. However, you could make the cell spacing half size, say, if you might want to fill in a sparse grid with more points later.
• Give the map size in numbers of the above map cells in both dimensions. Usually it is useful to choose a map size a little larger than the actual map. You might want to expand it later with more observations at the edges.
• Lastly, provide the number of channels in each spectrum. In this example I happen to know that I will want 247 points, having done some experiments beforehand. The baselines were a bit wiggly, so in this case I was more interested in fitting only part of the baseline, rather than saving disk space.