5.3.6 Mapping

Mapping is supported by all of the three regular observing modes. It can be done on a rectangular grid with a user-defined spacing and extent. It is also possible to supply a list of mapping positions to achieve a map with a custom shape optimized to the source geometry. For both map types, a spacing of half a red array or 30'' might be a good choice, providing half pixel steps to achieve super-resolution with a good overlap for the red array and full coverage (but no overlap) for the blue array. Similarly, a spacing of 15'' yields super-resolution with a good overlap for the blue array and a very strong overlap for the red array. These details need to be specified only in Phase II of the proposal process (see the FIFI-LS Cycle 4, Phase II web page). In Phase I, the effective map area needs to be entered in SPT and the proposal should explain the suggested mapping strategy. The on-source integration time to be entered in SPT has to be the on-source integration time per raster point multiplied by the number of raster map points N ton.

If the source geometry allows the off-beam to be positioned symmetrically on both sides of the source, then one should use the much more efficient Symmetric Chop Mode for mapping. If that is not possible the Asymmetric Chop Mode has to be used. An asymmetric chop is also used in the bright object mode. Figure 5-6 illustrates mapping with an asymmetric chop. The off-beam (positions B1 to B3) covers an area while chopping that is the same size as the map itself. If this is undesirable, the map needs to be broken up into sub-maps with varying chop parameters to be specified in Phase II. The availability of guide stars might be another reason to break up a large map into sub-maps. In this case the sub-maps will be identified between Phase II and the actual observation by the support scientist in close collaboration with the guest investigator and the telescope operator.

When estimating the on-source integration time (Sect. 5.2.2), take into account the differing overlap of the red and blue FOV at the desired raster map spacing. The SNR entered into the calculation of ton is the SNR for a single raster map point. The final SNR for a point in the map should reach &radic(n) x SNR with n being the number of raster points from which a point is covered by the respective FOV. For example in Fig. 5-6, the area of the pixel in the middle is covered by 3 FOVs while 16 pixels are covered by 2 FOVs and the outer parts of the map are covered by 1 FOV.

The geometry of chopping and nodding while mapping using the asymmetric chop mode

Figure 5-6: The geometry of chopping and nodding while mapping using the asymmetric chop mode.

Download the PDF Version

Share This Page