This step creates a final catalog of source photometry and morphologies.
Sources are detected using image segmentation, which is a process of assigning a label to every pixel in an image such that pixels with the same label are part of the same source. The segmentation procedure used is from Photutils and is called the threshold method, where detected sources must have a minimum number of connected pixels that are each greater than a specified threshold value in an image. The threshold level is usually defined at some multiple of the background standard deviation (sigma) above the background. The image can also be filtered before thresholding to smooth the noise and maximize the detectability of objects with a shape similar to the filter kernel.
Note that overlapping sources are detected as single sources. Separating those sources requires a deblending procedure, such as a multi-thresholding technique used by SExtractor. Here we use the Photutils deblender, which is an experimental algorithm that deblends sources using a combination of multi-thresholding and watershed segmentation. In order to deblend sources, they must be separated enough such that there is a saddle between them.
Source Photometry and Properties¶
After detecting sources using image segmentation, we can measure their photometry, centroids, and morphological properties. Here we use the functions in Photutils. Please see the Photutils SourceProperties class for the list of the properties that are calculated for each source.