Special Trouble or Action Header Keywords

The FOS or HRS has failed to locate a target. All observations following the failure and preceding the next baseline guide star acquisition will contain this keyword. This keyword is triggered by a status buffer message from the vehicle and it usually means that the science data are not valid. Note that the absence of this keyword does not insure a scientifically successful target acquisition. Only possible value is T.

The telescope has failed to acquire the guide stars. Each observation until the next baseline guide star acquisition will have this keyword appended to the header file. Possible values are:

• VEHSAFE: The HST vehicle went into a SAFE mode. No observations are executed when in this mode;

• DEGRADED: The commanded guiding mode is "Fine Lock" and the actual guiding mode is "Coarse Track";

• SREXCPn: The search radius, the region in which an FGSs searches for a guide star, was exceeded. The "P" indicates the primary guide star (or first of the pair to be acquired) and the "n" identifies the guide star pair. One or two guide star pairs per observation are available to the FGSs to insure that at least one is correctly acquired.

• SREXCSn: The search radius, the region in which an FGSs searches for a guide star, was exceeded. The "S" indicates the secondary guide star (or second of the pair to be acquired) and the "n" identifies the guide star pair.

• NOLOCK: The guide star(s) were found, but lock was not achieved. There are several possible reasons for this condition.

• UNKNOWN: The guidestar acquisition could not be assessed. This is usually due to a dropout in engineering telemetry during the acquisition.

• IN PROG: Similar to unknown. May occur if the engineering telemetry is partially interrupted and the data that is received shows only that the acquisition was taking place.

TAPEDROP

This keyword warns of the potential loss of science data due to changing tracks on the onboard tape recorder. Only possible value is T.

• TLM_PROB, start_aedp_cgap,' Start of Gap in AEDP data'

• TLM_PROB, end_aedp_cgap,' End of Gap in AEDP data'

• TLM_PROB, tlm_pacq_ctime, ' Earliest Acquisition time'

SLEWING

• FOS, HRS, WFII, or FOC SAFING: If an instrument is safed, the observation which was active at the time will contain this keyword. For observations which follow the SAFING event, observation logs will not be produced.

• DCF_NUM UNCHANGED: This value signifies missing exposures. Under some conditions, an observation may not be taken at all. For example if the first HRS observation of a pair does not complete in time, the second observation may not be taken. No observation will be sent to the ground, and the HST pipeline will not see the science data. The jitter file, however, will exist primarily to indicate the anomaly.

• SHUT: FOS aperture door failed to open.

• FAILED: An FGS astrometry observation failed due to inability to acquire the science target.