Change to FLT Ringing Filter

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).

Interpreting SCUBA-2 map Quality Arrays

SCUBA-2 maps created using "AST-masking", such as is done by  dimmconfig_bright_extended, will contain a Quality array indicating the background pixels that were masked (i.e. forced to zero). In itself this is fairly simple - background pixels have a non-zero Quality value and source pixels have a Quality value of zero. However, it is also possible to have independent masks for the FLT and COM models, in addition to the AST mask. For instance, if the "auto-FLT masking" scheme is used the Quality array contains both a FLT and an AST mask and so some care needs to be used when interpreting the Quality array.

Each value in the Quality array is restricted to taking integer values between 0 and 255, and so can be thought of as 8 separate bits. Each "bit plane" within the Quality array holds a single mask - AST, FLT or COM.   The showqual command can be used to find out which mask is held by which bit plane:

% kappa
% showqual fred.sdf
   AST (bit 1) - "Set iff AST model is zeroed at the output pixel"
   FLT (bit 2) - "Set iff FLT model is blanked at the output pixel"

This means that the AST mask is stored in bit 1 (the least significant bit), the FLT mask is stored in bit 2, and there is no COM mask. Note, if a map was produced using FLT masking but no AST masking, then the FLT mask would be stored in bit 1.

The decimal integer value of any element of the Quality array is equal to the binary value formed from the bits listed by showqual. So in the above case the maximum Quality value is 3 (the decimal equivalent of binary "11" - both bits set). Remembering that a bit is set (i.e. is 1) for background pixels and cleared (i.e. is 0) for source pixels, it follows that the four possible decimal Quality values in the above case (0-3) are:

0 - neither bit set, so the pixel is inside both the AST and the FLT mask (a source pixel).

1 - bit 1 set but not bit 2, so the pixel is outside the AST mask but inside the FLT mask (a border-line pixel).

2 - bit 2 set but not bit 1, so the pixel is inside the AST mask but outside the FLT mask (a border-line pixel).

3 - both bits set, so the pixel is inside neither mask (a background pixel).

In the following screen shot of a Quality array displayed using GAIA, the black areas have value zero and are thus inside both masks, the dark brown areas have value 2 and are thus inside the AST mask but outside the FLT mask. The light brown areas have value 3 and are inside neither mask. In this particular case, there are no  areas with a quality value of 1, so the FLT mask is contained entirely within the AST mask.

There are several commands within kappa that manipulate Quality arrays in various ways. For instance, the setbb command allows pixel data values to be set bad if the associated Quality value has a specified collection of set bits. Thus:

% setbb fred 1

will set all pixels bad except for those inside the AST mask. Likewise, 

% setbb fred 2

will set all pixels bad except for those inside the FLT mask.  Note, the change made by setbb is temporary - it can be undone by doing:

% setbb fred 0

To display the fred.sdf map and then overlay the AST mask in blue and the FLT mask in red, do:

% display fred mode=perc percentiles=\[2,98\]
% setbb fred 1
% contour fred clear=no mode=good labpos=! style='colour=blue'
% setbb fred 2
% contour fred clear=no mode=good labpos=! style='colour=red'
% setbb fred 0