There seems to be a small amount of confusion regardng the presence
(or absence) of misplaced events in HRC-I images that result in what
are called ``ghost'' images. The HRC-I raw images contain about 1.5-2%
of the events from a central concentrated source that are
mislocated. Typically these events are displaced in the detector
negative u-axis direction by about 1/2 of a tap spacing.The root cause
of these events being misplaced is not competely understood, but it
appears that these are events where some form of saturation has
occurred that results in an unusual charge distribution being recorded
at the detector readout. Whether this is an electronics effect or a
physical effect at the MCP is not known. However, the event screening
algorithm that was developed to reduce charged particle background in
the HRC does an excellent job of surpressing these ``ghost'' image
events. In fact, based on tests with 3C273 the upper limit on
misplaced events in the core image is about 0.1% of the total event
flux when the data are screened. This is also evident in the images
from the Vela pulsar (PSR0833-45), particularly the second observation
(OBSID00364) where the jet and ``ghost'' are not co-aligned as shown
below:
Figure 1: The image on the left is not screened and shows both the jet
(diagonal) and the ``ghost'' (vertical). The image on the right is screened
and shows only the jet.
The point-like source in Vela has about 70,000 events in the core, the
jet (south-west) has about 800 events (about 1.1% of the core), while
the ``ghost'' (south) has about 1000 events. In the screened image,
the upper limit to the ``ghost'' is no more than 120 events as
expected based on previous screening tests.
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On 21 Mar 2000, 20:06.