[daip] Blanking of cubes

[Lincoln Greenhill]

Dear AIPSgroup,

In projects BG98 and BG118 we are mapping the distribution of
SiO maser emission on Orion BN/KL.  The distribution of emission
is extremely complex, even in individual spectral channels, with
dozens of detectable maser spots with a range of brightness of perhaps
1000.  Some channels do not clean well, because of extended flux that
is not well sampled by the VLBA/Y1 array.

When we apply XMOM to create moment maps that are intended to show the
complex structure including weak maser emission, the poorly cleaned
channels leave behind artifacts that reflect the beam response.  However,
because the source is near 0 declination, the problem is localized to a
roughly N-S band on the sky centered around bright maser components.

We would like to be able to search the cubes, and blank emission based on
a more complex rule set than is possible with XMOM.  I can think of
two routes -

1) In XMOM, apply a cutoff based on a local calculation of signal-free RMS
in each image plane - rather than only an absolute flux cutoff.  The size
of the region (e.g. a square) over which the RMS is computed is provided
by the user.


2) in BLANK or XMOM, apply an absolute flux cutoff (as is done now) and
further clipping based on an input maximum acceptable dynamic range for
a generic 2-D region around each strong emission features in each plane.
The generic region shape is defined by the user; the program overlays
it on each strong feature and blanks regions where emission is too weak.


Has this problem been introduced by other AIPS users?  Can you think of
other solutions to it?  Does the AIPS group see any general value in
implementing a more sophisticated blanking algorithm than is available
in XMOM/BLANK?

Regards,

Lincoln




Lincoln J. Greenhill      Harvard-Smithsonian Center for Astrophysics
Radio & Geoastronomy Division, 60 Garden St, Cambridge, MA 02138, USA
Telephone:  1 617-495-7194             FAX:  1 617-495-7345
Internet:  greenhill at cfa.harvard.edu   http://cfa-www.harvard.edu/~lincoln