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Дата изменения: Wed Oct 22 17:31:27 2008
Дата индексирования: Tue Aug 18 14:54:21 2009
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Поисковые слова: вторая космическая скорость

XMM-Newton Science Analysis System


omdetect (omdetect-5.14.2) [???]

Meta Index / Home Page / Source Flagging Algorithms

Source on diffraction spike - bit 3

Any object with either a raw count-rate $>$ 70 or flagged as having a mod-8 pattern is checked to see if it might have diffraction spikes.

The routine for "detecting" diffraction spikes is a bit crude, and probably not very reliable, and probably needs overhauling. The algorithm works as follows:

1. Count the number of objects along position angles of 45 and 135 degrees from the centre of the star that lie within a radius of 10 and 120 pixels from the centre and a distance of no more than 5 pixels from the radius line.
2. Compute the search area from:

   SearchArea = (maxRadius - minradius) * 4.0 * maxDistance

   where minradius=10, maxradius=200, maxDistance=5 minradius is the minimum radius from the centre of the star maxradius is the maximum radius from the centre of the star maxDistance is the maximum distance from the centre of the star.

   These define a search area along the radius vector from the star (4 angles), 45, 135, 225, 315 degrees) for faint stars associated with a star spike.

3. Compute the number of objects within a box of width and height 200 pixels centred on the star centre, ignoring objects with a raw count-rate greater than 5 (n1).

4. Compute the number of stars expected within the search area, again ignoring objects with a raw count-rate greater than 5 (n2).

5. If the number of star along the spikes (n1) is > 3 * n2 then conclude that there are diffraction spikes and flag sources along the spikes. Stars between a radii of 10 and 120 and not more than 10 pixels away from the spike line are flagged.