XMM-Newton Science Analysis System
omqualitymap (omqualitymap-1.5) [xmmsas_20211130_0941-20.0.0]
Any object with either a raw count-rate 70 counts/sec or flagged as having a mod-8 pattern is checked to see
if it might have diffraction spikes.
Setting of bit 3 (Diffraction-spikes)
The algorithm works as follows:
- 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.
- Compute the search area from:
SearchArea = (maxRadius - minradius) 4 maxDistance
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.
- 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 counts/sec (n1).
- Compute the number of stars expected within the search area, again ignoring objects
with a raw count-rate greater than 5 counts/sec (n2).
- 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. All quality-image pixels within rectangular
regions rotated from the vertical by
135 degrees have bit 2 set. The half-width of a rectangle is 10 unbinned pixels and they extend radially, 50 to 250 unbinned pixels
from the centre of the star.
XMM-Newton SOC -- 2021-11-30