Whether it is knots of blue stars within a galaxy or optically overlapping objects, the SDSS takes great care to separate targets. Accurate analysis depends upon defining a sort of photon ownership within an image. The process of systematically assigning counts to individual objects within the SDSS is called deblending. The result of this process is consistent photometric measurements. A number of flags exist to inform researchers about the the performance and products of the deblending algorithm. The imaging products that result from deblending are referred to as children.
Blend and Deblend
Closely positioned or overlapping objects are flagged BLENDED. A computer algorithm is then applied to the photometric data that gives credit to each object for its photon contribution. Sometimes isolated galaxies with very diffuse edges are deblended from the sky, itself. If it is successful, the flag DEBLENDED appears in the Flags box.
NOTE: You rarely see this flag in Explore because this tool displays the final processed image by default. The mark PRIMARY in the mode box indicates that this image is marked as the best available, displaying the results obtained after running all appropriate analysis algorithms. Typically this flag is associated with the parent image. Parent images are discussed in the next section.
A number of flags are associated with how the deblending algorithm performed on a particular object. A complete list of flag definitions is located in the PhotoFlags table. Here are a few:
- NODEBLEND – there was a blended (BLENDED) object in the image but the deblending algorithm was not performed. Look for other flags for more information. This is a warning that all the photometry done on the object includes all the photons contributed by nearby or overlapping objects.
- NOTCHECK or NOTCHECK_CENTER – warns that there are pixels that have not been analyzed for peaks in the photon counts. This is a further warning to question the photometry on this object.
- DEBLEND_NOPEAK – This flag is a warning that for at least one of the filters no peak was found in brightness across pixels.
- TOO_LARGE – The object was either too large for the deblending process to work well or it was too fuzzy with no definitive edge.
- DEBLENDED_AT_EDGE – This object was recorded near the edge of the frame. Because it was at the edge of the recording surface, results might have been affected.
Children and Parents
Each object that is separated out as a result of deblending is referred to as a child and flagged CHILD. If a child appears to have more than one photon peak within its outline, it is again flagged BLENDED. Deblending is performed again on the image.
The original image is the parent and is referenced by the parentid number on the Explore page. If you pull up the Explore page for the parent (using the Search by objid command in the left hand frame) you can view the photometry associated with the original or parent image.
Galaxy deblends can get very complicated. SDSS provides a count of the number of children associated with a parent image under the nChild heading in Explore. In primary images, the nChild should always be zero. When viewing the Explore page for parent images, the nChild will be greater than zero.
Notice differences in photometry between parents and children.
To search for the image that was processed to produce the photometric results displayed in Explore, use the parentid in the Search by – obj id box.