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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