Chapter 2: Header Keyword Definitions
Special Trouble or Action Header Keywords
At the end of the header file, certain keywords and phrases will appear as a warning of an anomalous situation or to flag or emphasize a particular condition (e.g., slewing). These keywords will only appear if there is a condition to report. The special situations currently addressed are discussed below. Future enhancements will increase the special keyword reporting.
ACQ2FAIL
(Target acquisition failure)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.
GSFAIL
(Guide star acquisition failure)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
(Possible loss of science data.)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
The following three anomaly phrases indicate a problem with the engineering telemetry (called AEDP). These phrases warn the General Observer that some or all of the data in the observation log may be incorrect. The details specified in these three anomaly phrases are primarily for the operations staff, who will investigate the problem and, if so indicated, will reprocess the observation logs over those periods of time lost to gaps or bad data.
- 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
(Slewing occurred during this observation.)This flag specifies the telescope slew status. It serves as a warning that a slew occurred sometime during the time period, and because of this motion the observation log jitter statistics and apparent jitter on the sky will affected. Standard deviations will be very large because of the method of calculation. Slews occur during target acquisition observations, peak-ups, spatial scans, and moving target observations. See the definition of the SLEW flag for more details. Only possible value is T.
TAKEDATA
(Take Data Flag NOT on throughout observation.)This flag specifies the status of the TAKEDATA flag. The flag could be off for many reasons, some anomalous situations such as loss of lock, failed guide star acquisition, and others normal for the activity in progress, such as slewing during acquisition peak-ups or during spatial scans. The SIs, with the exception of FOS, will respond when the TAKEDATA flag is OFF. The response is an interruption of an exposure and/or shutter closures. The TAKEDATA keyword will be false only if the TAKEDATA flag was off for the entire observation. See the definition of the TAKEDATA flag for more details. Only possible value is F.
SI_PROBnn
The SI_PROBnn keyword can appear with nn = 01-99. Currently there are 7 possible values for this keyword:
- 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.