Standard 3x3

Standard event processing is based on 3x3 pixel neighborhood event detection and extraction. A local maximum is specifically defined as any pixel, p₀, that meets the following comparison rules.

p0 > p6	p0 > p7	p0 > p8
$p_0 \geq p_4$	$p_0 \geq$threshold	p0 > p5
$p_0 \geq p_1$	$p_0 \geq p_2$	$p_0 \geq p_3$

where the pixels are in READX and READY coordinates. Any event that meets the above comparison rules is written into the event list as a 9 pixel vector with the pixels arranged from p₀ to p₈ as illustrated above.

In the ACIS-2C processing, events are ignored if they occur at quadrant boundaries (i.e., the pixel which forms the local maximum signal value falls on the boundaries between regions read by a single output node). Therefore there will always be a 2 pixel gap in images where nodes meet (one pixel on each side of the node). Events are also not detected at the edges of windows or full-frames. Note that in the flight instrument events are recorded as recognized events, even if the local maximum pixel falls on a quadrant boundary (but no effort is made to adjust for differential gain between output nodes; the effect of gain differences should only be modest changes in the event grade distributions).