Outline

  For extended fields with relatively bright line emission, the raster (`on-the-fly') mapping technique is a sensible choice. In this mode the telescope is scanned along a line of constant latitude (usually RA, but GA is possible) while the backend continously integrates the incoming signals, forming the average once every few seconds and saving the result. The method is analogous to that normally used for mapping extended objects in the continuum with a single-beam receiver (see Part 4 of this manual). Because double-buffering of the backend is required, this method can be used only with the DAS.

Raster mapping is a relatively fast, efficient way of mapping a large area when sky conditions are reasonably good, and the line strengths are reasonably high. Typically it is possible to map 0.5 K T emission when the system temperature is as high as about 1000K (as the image should be fairly heavily oversampled). The on-source efficiency is 50--77%, which compares with 12--30% with regular grid maps.

Maps of total time up to hour can be made easily without a break in observing, and a data rate of 400--1000 spectra per hour can be achieved using this technique. Naturally, raster mapping will produce large data files, and if disk space is short this can be a problem. Contact your support scientist beforehand if you expect to produce Gigabytes of data.

Raster mapping is a relatively new technique at the JCMT, and because it was only retrospectively integrated into the software architecture there are a number of special steps to be taken if correct data are to be obtained. Raster mapping runs only in the position-switched mode ( PSW), with an integration at a user-supplied reference position once every row (i.e. the data are calibrated like a series of regular single-calibration grids).