|Introduction to SQL|
|Form Query User Guide|
|SkyServer Traffic Page|
|Contact Help Desk|
Deblending Overlapping Objects
One of the jobs of the frames pipeline
Bright objects are measured at least twice: once with a global sky
Once objects are detected, they are deblended by identifying individual peaks within each object, merging the list of peaks across bands, and adaptively determining the profile of images associated with each peak, which sum to form the original image in each band. The originally detected object is
referred to as the "parent" object and has the flag
The list of peaks in the parent is trimmed to combine peaks (from different bands) that are too close to each other (if this happens, the flag PEAKS_TOO_CLOSE is set in the parent). If there are more than 25 peaks, only the most significant are kept, and the flag DEBLEND_TOO_MANY_PEAKS is set in the parent.
In a number of situations, the deblender decides not to process a BLENDED object; in this case
the object is flagged as NODEBLEND. Most objects with EDGE set are not deblended. The exceptions
are when the object is large enough (larger than roughly an arcminute) that it will most likely not be
completely included in the adjacent scan line either; in this case, DEBLENDED_AT_EDGE is set, and
the deblender gives it its best shot. When an object is larger than half a frame,the deblender also
gives up, and the object is flagged as TOO_LARGE. Other intricacies of the deblending results are
recorded in flags described on the Object Flags section of the Flags page
On average, about 15% - 20% of all detected objects are blended, and many of these are superpositions of galaxies that the deblender successfully treats by separating the images of the nearby objects. Thus, it is almost always the childless (nChild=0, or !BLENDED || (BLENDED && NODEBLEND)) objects that are of most interest for science applications. Occasionally, very large galaxies may be treated somewhat improperly, but this is quite rare.
The behavior of the deblender of overlapping images has been further improved since the DR1; these changes are most important for bright galaxies of large angular extent (> 1 arcmin). In the EDR, and to a lesser extent in the DR1, bright galaxies were occasionally "shredded" by the deblender, i.e., interpreted as two or more objects and taken apart. With improvements in the code that finds the center of large galaxies in the presence of superposed stars, and the deblending of stars superposed on galaxies, this shredding now rarely happens. Indeed, inspections of several hundred NGC galaxies shows that the deblend is correct in 95% of the cases; most of the exceptions are irregular galaxies of various sorts.