[Issues] AssociateList mixes up SIDs

Hugo Buddelmeijer astrowise at hugo.doemaarwat.nl
Tue Mar 11 11:35:27 CET 2008

Hi all,

I noticed some strange behavior in the AssociateList. Under some
circumstances, the AssociateList seems to store the wrong SIDs with an
association. It appears to calculate the associations correctly, but
mixes them up when storing the associations, or something like that.

The problem is described in the example below. The images referenced to 
can be found on http://www.astro.rug.nl/~buddel/awe/associatemixup/ .
The awe scripts used to recreate the problem and the images are attached.

* Setup *

I create a master association of three SourceLists, in a two step
process. The set_single_out_closest_pairs is set to False to preserve
all associations. The search_distance is sufficiently low to prevent
unwanted associations (1 arcsecond). The search_area is small enough to
only have a handfull of sources in each input SourceList. See
associateSmallAreaShort.py for the script.

The input SourceLists are from SDSS and UKIDSS. Their SLID and the
number of sources in the search_area are in order of association:
SLID   Name               number_of_sources
61804  PhotoObjAll (POA)  3
155041 Photoz (PZ)        3
244531 lasSource (LAS)    3

Figure allsourcesrealpos.png shows the 9 sources in their real position
on the sky. Figure allsources.png shows the sources shifted by about 1/3
arcsecond for enhanced visibility. There are clearly 3 groups. The
upper-right group contains a LAS singleton which will further be
ignored. The lower-right group contains a group with one source in each
SourceList. The upper-left group contains two sources in both POA and
PZ, and one in LAS. In reality there is only one source there, but it is
measured twice by the SDSS people, hence it is twice in the POA and PZ

* The associations *

The first association between POA and PZ goes as expected. The 2
lower-right sources are associated as 1 association and the 4 upper-left
sources are associated in 2x2=4 ways giving a total of 5 associations.
See figure assoc100651.png, the colored lines are drawn between the
associates, the cross represents the search_distance of 1 arcsecond.

The second association between the first and LAS goes wrong. 3 of the
previous 5 associate-groups are associated with the correct LAS source,
but 2 are associated with the incorrect one. The incorrectly associate
sources seem to be switched. (As expected there also is a LAS singleton,
resulting in 6 associations.) See figure assoc100661.png.

* Afterthoughts *

The resulting associations are way beyond the search_distance. The
search_distance can be sufficiently decreased so it is small enough to
not be able to make the correct associations between POA and LAS
anymore. When this is done, the incorrect associations disappear as
well. This lead me to believe that the associations are correctly
determined, but then incorrectly stored.

The file plotSmallAreaShort.py contains the plotting script. The
plotting algorithm can be somewhat obscure, mainly because it shifts the
sources and the association lines by some random amount for improved
legibility. At the bottom of the script is included a sample awe code to
plot the associations as simply as possible to show that the strange
associations are not an artifact of the plotting script but really in
the AssociateList.


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