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XMM-Newton Science Analysis System


omdetect (omdetect-5.14.2) [???]

Meta Index / Home Page / Algorithm / The detection algorithms

Point-source detection algorithm

This algorithm is designed to locate sources that have a shape roughly resembling the point-spread function. In the following binx refers to the image binning factor (1 for an unbinned, 2 for a binned image, respectively). Occasionally such a source may be extended.

1. Initialise a boolean mask image, having the size as the image, to false.
2. Initialise an integer 16 image, having the same size as the image, to zero (the source-number image).
3. Initialise the number of detected sources (npsources) to zero.

4. Where a quality array pixel is not zero, set the corresponding mask image pixel to true

5. Loop through all the pixels in the image, ignoring those where the mask image pixel is true and those where the source-number pixel is not zero.
   • If the current image pixel, i0, j0, is above a specified height above the background image pixel count how many neighbouring pixels are also above the specified height.

   • If the pixel i0, j0 has less than 4/binx - 1 neighbours set the mask image pixel i0, j0 to false and goto 1.

   • Add pixel i0, j0 to the list of pixels to be later checked as being part of a point-source.

   • If there are more pixels to check goto 8.

6. Sort the list of pixels into decreasing order based on pixel value.

7. Loop through the list of pixels

8. For each pixel call the specified point-source validation function to see if the pixel might be part of a valid point-source

9. If the validation function returns true
   • incremenent npsources by 1.
   • Call the pixel-numbering algorithm that assigns the pixels in the source-number image to the source- the assigned pixels are given the value npsources.
   • Call the centroiding routine that computes the x/y coordinates of the source and the semi-major/minor axes and position angle.
   • Call the routine that determines if the source is point-like or extended.
   • Add the source to the point-source linked list.
   • Move on to the next pixel in the list.

10. If there are still more pixels to check got
11. If any sources have been classified as extended, transfer them from the point-source linked list to the extended-source linked list. Also transfer their source-number pixels to the extended-source number image. Renumber the point-sources.