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XMM-Newton Science Analysis System


omdetect (omdetect-5.14.2) [???]

Meta Index / Home Page / Algorithm

The detection algorithms

Omdetect uses two different algorithms for source-detection - one is optimised for detecting point-like sources, and the second is optimised for detecting extended sources.

To make the program execute as quickly as possible, all arrays are implemented as pointers, to avoid unnecessary copying.

Because it is not known how many sources may be detected, and to save memory, omdetect uses two linked-lists to store parameters of detected sources- one for probable point-sources and one for probable extended sources. Two separate lists are used mainly because the detection algorithms for point and extended sources are different, and also because the photometry of the two types is done differently.

The linked-lists store a structure for each source that contains, amongst other things, the following entries:

• The source x and y coordinates
• The source number
• imin - the left-most source-number pixel assigned to the source in the x direction.
• imax - the right-most source-number pixel assigned to the source in the x direction.
• imin - the lowest source-number pixel assigned to the source in the y direction.
• imax - the highest source-number pixel assigned to the source in the y direction.
• The semi-major axis
• The semi-minor axis
• The position angle of the semi-major axis (measured anti-clockwise from the +ve x-axis)
• The source extension flag (0 for point-like, 1 for extended)
• The quality flag value assigned to the source
• The confusion flag value assigned to the source
• The aperture radius that the photometry was computed with
• The number of pixels that an extended source contains.