2.2.4.2 Effects on SASS products

When affected data are corrected, there is a marked improvement in the image. In figures 11 and 12, we show the central part of the image of the Cen A data. Each image has been smoothed with a 3 arcsec Gaussian. We do not expect that the background calculation will be significantly affected for normal observations.

  

Figure 11: The archival version of the Cen A observation. The data have been smoothed with a Gaussian of FWHM = 3''. The doubled contour level is roughly 50% of the peak of the core.

LINK TO POSTSCRIPT FILE for Figure 11

  

Figure 12: The corrected SASS version of Cen A. The data have been smoothed with a Gaussian of FWHM = 3''. The absolute values of the contour levels are slightly higher than that for the archival version because the live time is 3% greater. Note the significant changes to the morphology of the two features in the jet.

LINK TO POSTSCRIPT FILE for Figure 12

We expect a second order effect on the number of sources detected (and possibly on the association between detections with different cell sizes). Whereas the positions will change only slightly or not at all, we expect additional source detections near threshold. For CenA we find 63 detections with a 12'' cell for the archival data but only 60 for the reprocessed run. Although this is opposite to our expectations, it might be caused by a 3% increase in livetime at HIbackground level 8 (most of the data had extremely low background levels). With the 24'' cell, the archival run reported 46 sources, but the reprocessed data overflowed with more than 200 detections.

In addition to having more sources, we expect the following:

positions	minimal effects except for two close sources*
intensity	significant increase for point sources
s/n	significant increase for point sources
variability	second order effects
sizcor	minimal effects

* Further testing in 1999 March has convinced us that for some sequences, there are systematic offsets in addition to the general (random) smearing. Thus we would no longer be surprised to find a positional shift of one or two arcseconds in some instances.