The FEP0 Problem
February 2, 2000
The six Front End Processors (FEPs) are computers that process pixel
data from the ACIS CCDs. Each CCD is assigned to one of the six FEPs.
During the early phase of the mission, an intermittent problem was
discovered with one of the FEPs. Specifically, FEP0 would suddenly
experience a corruption in the bias map from rows 512 to 1024 at an
indeterminate point in the observation. This would lead to many
spurious events being reported from the top half of that CCD for the
remainder of the observation, flooding the telemetry stream. A patch
has been developed and installed which detects when such an error has
occurred, and prevents data from the top half of the CCD from being
telemetered from that point in the observation. Observers should be
aware that they might see the event data from the top half of one CCD
suddenly stop during an observation as a result of this effect. The
error is intermittent and appears to occur only during long
observations (longer than 40 ks). FEP0 should therefore be assigned
to the CCD which produces the least useful data for an observation.
By default, FEP0 will be assigned to the S0 CCD when CCDs S0 through
S5 are in use, to the S2 CCD when CCDs I0 through I3, S2 and S3 are in
use and the aim point is on the I array, and to the I2 CCD when CCDs
I2,I3,S1 through S4 are in use and the aim point is on S3. The GO may
request that FEP0 be assigned to a CCD other than the defaults but
there must be a compelling scientific reason to do so.
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