Traditionally, spectral line maps are observed in a raster mode, switching the telescope between reference and source positions for each individual raster point. This method is very inefficient because of the overhead introduced by initializations and telescope movement. At the SMT the overhead per subscan is on the order of 10 seconds. For a typical 5 * 5 map integrating 20 seconds ``On'' and 20 seconds ``Off'' (i.e. a total of 1000 seconds) the overhead amounts to 2 (On+Off) * 5 * 5 * 10 = 500 extra seconds. Thus, the efficiency is only about 66%.

In ``On-The-Fly'' (OTF) mode, the telescope continuously scans along a row or column with a given velocity. Data is written every couple of arc-seconds which corresponds to typical integration times of a few seconds or less (in the future down to 100 milliseconds at the SMT). OTF is more efficient because the telescope system is not initialized for each point along the scanning axis. For the above map it would save 40 subscans or 400 seconds and the efficiency would go up to about 91% ! Therefore, it is recommended to use OTF whenever mapping a region larger than a few beams in size. There should not be a limit concerning the line temperatures (although, of course, weak lines require several maps that have to be co-added). There are the usual limits in baseline quality for ``On-Off'' type observations like PSWITCH or PRASTER.

Please read our Spectral Line On-The-Fly Guide (Updated version (1.1): HTML or PostScript) to learn how to set up and reduce OTF observations at the SMTO.