AST.FILT_DIFF

Reduces spurious large scale structure in the final map - an experimental alternative or addition to AST masking

Description:

If ast.filt_diff is non-zero, the map created at the end of each iteration is modified by removing any large scale structures that have been introduced by the iteration. Such structures are a natural outcome of the iterative process and often take the form of ripples on the scale of the FLT filter size that get stronger with each iteration.

If " ast.filt_diff" is non-zero it gives the size of the largest features to be retained, in arc-sconds (a value equal to half of the FLT filter size seems to be a good starting point). At the end of each iteration the difference is taken between the new map and the previous map. This difference is filtered using a sharp-edged high-pass filter, of size specified by " ast.filt_diff" . The remaining high frequencies (i.e. small structures that have been introduced by the new iteration) are added back onto the map created by the previous iteration. This modified map is then used, instead of the original map, to define the AST model. [0.0]

Type:
real

SMURF Usage

MAKEMAP, CALCQU