If you use the AST.SKIP and FLT.RING_BOX1 configuration parameters to prevent artificial blobs of emission appearing in your SCUBA-2 maps, then you may have seen cases where the centres of real bright sources are badly affected, as in the following example spotted by Harriet:
This issue may also cause complete holes to appear around bright sources. These problems are caused by the fact that the ringing filter is applied on the initial "pre-iterations" specified by AST.SKIP, as well as on all subsequent "real" iterations. If the ringing filter is restricted to just the "real" iterations, the following map is produced (left uses the same scale as above, right is a close up of the central region):
The stardev system at Hilo has been updated to include this fix.
For those interested in the details... The problem is caused by the fact that no FLT masking can be done on the first pre-iteration, since there is no map available at that point to use as a mask. Therefore bright sources cause heavy ringing on the first iteration. This ringing is then detected and the corresponding samples flagged by the ringing filter. This means that the bright sources are blanked out in the map created at the end of the first iteration. On the second pre-iteration, this map is used to locate the bright sources that should be masked out when estimating the FLT model - but the bright source is not present in this map and so is not masked out on the second iteration either, leading to heavy ringing again. And round it goes doing the same thing on each iteration. Restricting the ringing filter to "real" iterations solves the problem because bright sources have been masked out in the FLT model by the time the ringing filter is used for the first time, and so have not caused the heavy ringing that would otherwise have been detected and flagged by the ringing filter. In fact, in may have been sufficient just to inhibit the ringing filter on the first pre-iteration, rather than on all pre-iterations, but this would have been of little benefit since blobs only start to develop once the AST model is included in the iterative scheme (i.e. once all the pre-iterations have completed).
This issue may also cause complete holes to appear around bright sources. These problems are caused by the fact that the ringing filter is applied on the initial "pre-iterations" specified by AST.SKIP, as well as on all subsequent "real" iterations. If the ringing filter is restricted to just the "real" iterations, the following map is produced (left uses the same scale as above, right is a close up of the central region):
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For those interested in the details... The problem is caused by the fact that no FLT masking can be done on the first pre-iteration, since there is no map available at that point to use as a mask. Therefore bright sources cause heavy ringing on the first iteration. This ringing is then detected and the corresponding samples flagged by the ringing filter. This means that the bright sources are blanked out in the map created at the end of the first iteration. On the second pre-iteration, this map is used to locate the bright sources that should be masked out when estimating the FLT model - but the bright source is not present in this map and so is not masked out on the second iteration either, leading to heavy ringing again. And round it goes doing the same thing on each iteration. Restricting the ringing filter to "real" iterations solves the problem because bright sources have been masked out in the FLT model by the time the ringing filter is used for the first time, and so have not caused the heavy ringing that would otherwise have been detected and flagged by the ringing filter. In fact, in may have been sufficient just to inhibit the ringing filter on the first pre-iteration, rather than on all pre-iterations, but this would have been of little benefit since blobs only start to develop once the AST model is included in the iterative scheme (i.e. once all the pre-iterations have completed).
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