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| Joint Astronomy Centre |
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Raster Mapping available with the DASRaster (or 'on-the-fly') mapping has now been released using the DAS. This is a high-efficiency method of mapping reasonably bright spectral line sources. The telescope is scanned continuously while the DAS is integrating and, at the end of every row, a reference (sky) position is observed. This reduces overheads due to telescope moving, off-source integration, etc. The true on-source integration time using this method is typically 60-70%, compared with 25-30% with normal grid mapping.The minimum on-source integration time per point (Itime) is 5 seconds and the reference position is observed for sqrt(N)*Itime, where N is the number of points per row. This gives and optimum s:n over the map. The maximum number of points per row is 59. The distance between points should normally be 1/3 - 1/2 of the beamsize, to avoid smearing along the row. A data rate of ~450 spectra per hour can be obtained. See the DAS manual or contact WRFD for more information. Bill Dent / JAC |