Base line removal - scan_rlb

The SCUBA on-line software normalizes each scan in conventional (EKH) scan maps, which leads to baseline offsets, but even the ``Emerson 2'' maps have baseline uncertainties. Spillover, large spikes and sky noise add to these baseline uncertainties. It is therefore absolutely necessary to remove the baseline offset for each bolometer. If it is omitted one may end up with severe striping in the map. If your map is large enough, i.e. you have no source emission at the end of your scan, you can run scan_rlb, and fit a linear baseline for each scan (exposure and bolometer) by taking the end portions of the scan as a measure of the signal level. The default size of the region used for the baseline subtraction is the number of data points in one chop throw. This can be inadequate for the small chops, especially if the data are spiky or suffer from sky noise variations, and you may consider increasing the default to perhaps 100''. The output from scan_rlb is the basic map name, now appended by _rlb, which is the file you will use as an input for the next stage in the data reduction.

However, if your map is not large enough to start and end on a region free of emission scan_rlb will result in a gradient over the scan. Taking the default behavior of scan_rlb in this situation is probably the leading cause of poor results obtained from scan mapping. When you map galactic regions it is usually better to use the median rather than the default, which is linear. Another, sometimes more successful approach is to use SCUBA sections.

In scan map mode each `sweep' or `raster' is a section (exposure), and the complete map is an integration. If you believe that you have sections of the map that are free of emission, you may be in luck, and you can use these emission free sections to provide the baseline level for the rest of your map. To find out which section is which, display your rebinned map and then use scuover. To produce the image in Fig. we typed:

%display 20000721_0023_lon_reb
DEVICE - Name of display device /@xwindow/ > 
MODE - Method to define the scaling limits /'SCALE'/ > 
LOW - Low value for display /-0.86096328496933/ > 
HIGH - High value for display /12.873136520386/ >
% scuover exposure=1
DEVICE - Name of graphics device /@xwindow/ > 
Alignment has occurred within the AXIS Domain.

NDF - Image to display bolometers over /@20000721_0023_lon_reb/ > 
SURF: file contains data for 21 exposure(s) in 1 integration(s) in 1
measurement(s)

One can see that the scan map started at the top right of the map (a scan map of the moon's limb) and took 11 `sweeps' or exposures to complete the map. One can see that only exposures 1, 9, 10 and 11 started and ended off source. Therefore when one wishes to remove the baselines (remembering that one now has to go back 2 or 3 steps to do this) from this image the best method would be:

% scan_rlb
IN - Name of input file containing demodulated data 
/@20000721_0023_lon_ext/ > 
SURF: run 23 was a MAP observation of object MOON
SURF: file contains data for 21 exposure(s) in 1 integration(s) in 1
measurement(s)
OUT - Name of file to contain restored data /'20000721_0023_lon_rlb'/ 
> 
METHOD - Method to use for baseline removal /'LINEAR'/ > section
RLB - Remove fitted baseline from output data /YES/ > 
SECTION - Please enter a section (including {}) > [{e1}{e9}{e10}{e11}]
REMOVE_DC_OFFSET_SECTION: Processing integration 1

Its obvious with a source like the moon when you are on and off source and because the moon fills most of the image, the data were additionally masked. This is a rather special case, where it would have been very difficult to do the baseline subtraction any other way without ending up writing special purpose software. For molecular clouds and star forming regions it is often very difficult to find emission free regions. For really extended emission, like the Galactic center, it was therefore found that the best way to do baseline removal is to use the median level of all the scans for each bolometer (Pierce-Price et al. [15]), which corresponds to specifying SECTION as {}. Quite often you will find that you have to do an initial map reduction to see how extended the region is and where you can find emission free areas. Once you know this, it is much easier to go back to noisy sub-maps and redo the baselines.

Figure: An example of a scanmap of the moon's limb. The SCUBA array's position at the beginning of the observation is shown. In total it took 11 `sweeps' or exposures to map the area requested.