In previous blogs (e.g. Suppressing large-scale noise with zero-masks), Ed has described how masking the AST model - the astronomical signal - between iterations can reduce the amplitude of low frequency structures in the final map. Over the last month or two, I have extended the uses of masks to allow the COM and FLT models to be masked in a similar way to the AST model.
COM masking: The common-mode signal is the average of all available bolometer time streams. Once found, a least-squares linear fit is performed between the average and each individual bolometer time stream. This fit generates a scale factor and offset, together with a correlation coefficient. Bolometers that have a low correlation with the common-mode are rejected as bad. At any one time, the astronomical source is seen only by a subset of the bolometers, and so will be largely absent in the common-mode estimate. This means that bolometers that see the source will have a lower correlation to the common mode than those that do not see the source, which can result in bolometers being rejected unnecessarily. To avoid this, it is now possible to exclude samples that see the source from each time stream when finding the common-mode signal, and also when performing the least squares linear fit. This can produce a better estimate of the common-mode, unpolluted by bright sources samples, and also better estimates of the correlation coefficients, resulting in fewer bolometers being rejected.
The masking of the COM model is controlled by the new configuration parameters COM.ZERO_MASK, COM.ZERO_CIRCLE, COM.ZERO_LOWHITS and COM.ZERO_SNR. These are all analogous to the corresponding set of AST.xxx parameters used for masking the AST model. As an example, a configuration of:
^/star/share/smurf/dimmconfig_bright_compact.lis
com.zero_circle = 0.02
was used to produce an 850 um map of Mars from observation 40 on 20120202. This excludes samples from the COM model that are within a circle of radius 0.02 degrees, centred on the source. The resulting final map contained 6744407 samples (or 2877.31 bolos). Without the COM masking, the final map contained 4676093 samples (1994.92 bolos).
Sadly, not all observations show such an improvement using COM masking. Indeed, some show little or no improvement, and some actually show a decrease in the number of samples in the final map. For this reason, COM masking is not switched on by default in any of the dimmconfig files distributed with SMURF.
FLT masking: A similar set of configuration parameters - FLT.ZERO_MASK, FLT.ZERO_CIRCLE, FLT.ZERO_LOWHITS, etc., are now available to control masking of the FLT model. The FLT model holds the low frequency background for each bolometer time stream, and is estimated by smoothing the time stream using a filter defined by FLT.FILT_EDGE_LARGESCALE, etc. On the first iteration, any bright astronomical source in the data will result in the smoothed background being over-estimated in the vicinity of the source. This in turn, produces a negative bowl around the source when the background is removed. It can take many iterations for the map-maker to reduce this bowling to acceptable levels. Using the new FLT masking parameters allow the source to be omitted before estimating the background in each bolometer time stream (the source samples are replaced by a values interpolated from the neighbouring data, with added noise). Masking the FLT model in this way on the first two iterations can have a dramatic effect on the number of iterations need to reach convergence. For compact sources, it can typically reduce the number of iterations required to 50% of the original number, and sometimes fewer.
For this reason, FLT masking is switched on by default in the dimmconfig_bright_compact.lis file, using the settings:
flt.zero_circle = 0.0167
flt.zero_niter = 2
The effect of FLT masking on extended sources is marginal, and so is not switched on by default in dimmconfig_bright_extended.
