Allows subtraction of the signal from sky bolometers. Tests during the commissioning period showed that, for faint sources, the signal is often dominated by atmospheric variations or sky noise. Furthermore, such variations were found to be correlated across the arrays and can thus be corrected for. At present the sky removal algorithm simply subtracts from the signal bolometer a mean or median signal level from a user specified list of sky bolometers. The mean method allows bolometers that are a specified number of standard deviations from the mean to be dropped. Sky subtraction is done on a jiggle-by-jiggle basis and so the sky point is measured 9 seconds after the source point for the default 9-point jiggle pattern.
Sky removal should be used with caution. Possible pitfalls include subtracting the signal level from a bolometer at the chop position (for chop throws of less than about 90''), using the inner ring of bolometers for a source that may be extended, or selecting bolometers that are microphonic or dominated by 1/f noise. For point sources and the mean sky subtraction method, we recommend using the inner ring of the long-wave array (h6,h8,h13,h14,g15,g16) and the none-noisy bolometers from the second ring out on the short-wave array (d10,e2,d7,c12,c2,b5,b10,c5,c16). For the median method a longer list can be given.
The SCUBA photometry cookbook