Multiple Source Resolution

Multiple Source Resolution for 2MASS Final Product Generation




I. Overview

Objective - Select one apparition of multiply-detected sources for inclusion in the final Catalogs and Faint Source Augmentations.

Implementation - The selection algorithm is to be based on purely geometric criteria - select the apparition that is closest to center of its respective scan for inclusion in the Catalogs.

II. Nature of Multiple Sources

Figure 1 shows the distribution of separations of three-band sources, not in the same scan, taken from IDR2 Point Source Catalog. No three-band detections are closer than 2.02".

Figures 2 shows the distribution of separations of one and two-band sources from the same scan in the IDR2 Point Source Catalog. The peak at <2" separation are most likely in-scan duplicates resulting from close multiple sources inconsistently detected in different bands, which therefore did not bandmerge into a single source. This is illustrated in Figure 3 which shows the distribution of H-band only and JKs band sources in the same scan in the IDR2 PSC. In these cases, the H-band detection was offset far enough from the JKs-detections that bandmerge would not pull them together. Obviously, pairs with separations <1" are preferentially bandmerged.

Figure 1Figure 2Figure 3

III. Scan-to-Scan Multiple Source Resolution Algorithm

  1. Safety border around scans: consider only sources >10 arcsec (point sources) or >15 arcsec (extended sources) from scan edges to avoid band coverage problems (this is imposed during selection for Catalog Generation DB)


  2. Positionally correlate sources from adjoining scans in overlap region



  3. For all N>1 correlated source apparitions in or near overlap regions:




  4. Graphical Examples multiple source resolution algorithm at work .

  5. The dup_src and use_src flags


  6. The multiple source resolution results are encoded in the dup_src and use_src flags carried with each source record in the Catalog Generation databases. These flags also help to signify when confusion is encountered during the multiple source resolution process. These flags are used to drive the final catalog source selection.





IV. Scan Overlap Statistics Generated During Multiple Source Resolution

A side benefit of the Multiple Source Resolution processing is that statistics on photometric residuals between scans can be efficiently generated. For each overlapping pair of scans in the Catalog Generation Database, the mean and RMS of the J, H and Ks photometric offset of all sources and just sources with SNR>20 are calculated. Astrometric offsets can also be evaluated during the same step.


Last Updated: 11 December 2001
R. Cutri, S. Wheelock - IPAC, R. Stiening - UMASS