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: http://www.stsci.edu/documents/dhb/web/cappc_oms.fm3.html
Дата изменения: Tue Nov 18 00:20:37 1997 Дата индексирования: Sat Dec 22 12:47:20 2007 Кодировка: |
There are three commanded guiding modes: FINE LOCK, FINE LOCK/GYRO, and GYRO. OMS header keywords GUIDECMD (commanded -guiding mode) and GUIDEACT (actual guiding mode) will usually agree. If there was a problem, they won't agree and the GUIDEACT value will be the guiding method actually used during the exposure. If the acquisition of the second guide star fails, the spacecraft guidance, GUIDEACT, may drop from FINE LOCK to FINE LOCK/GYRO, or even to GYRO, which may result in a target rolling out of an aperture. Check the OMS header keywords to verify that there was no change in the requested guiding mode during the observation.
Until new flight software (version FSW 9.6) came online in September 1995, if the guide star acquisition failed, the guiding dropped to COARSE track. After September 1995, if the guide star acquisition failed, the tracking did not drop to COARSE track. Archival researchers may find older datasets that were obtained with COARSE track guiding.
If you suspect that a target has rolled out of the aperture during an exposure, you can quickly check the counts in each group of the raw science data. As an example, the following IRAF commands can be used to determine the counts in each group.
GSD_ID = `0853601369 ` / Dominant Guide Star ID
GSD_RA = 102.42595 / Dominant Guide Star RA (deg)
GSD_DEC = -53.41362 / Dominant Guide Star DEC (deg)
GSD_MAG = 11.251 / Dominant Guide Star Magnitude
GSR_ID = `0853602072 ` / Roll Guide Star ID
GSR_RA = 102.10903 / Roll Guide Star RA (deg)
GSR_DEC = -53.77683 / Roll Guide Star DEC (deg)
GSR_MAG = 12.426 / Roll Guide Star Magnitude
cl> grlist z2o4040dt.d0h 1-24 > groups.lisSome observations can span several orbits. If during a multiple orbit observation the guide star reacquisition fails, the observation may be terminated with possible loss of observing time, or switch to other less desirable guiding modes. The GSACQ keyword in the cmh header will state the time of the last successful guide star acquisition.
cl> imstat @groups.lis
GSACQ = `136:14:10:37.43 ` / Actual time of GS Acquisition Completion
The observation logs for all of the following observations in the observation set will have the "DEGRADED" guide star message. This is not a Loss of Lock situation but an actual failure to acquire the guide star in the desired guiding mode. For the example above, the guiding mode dropped from FINE LOCK to COARSE TRACK.
V3_RMS = 19.3 / V3 Axis RMS (milli-arcsec)
V3_P2P = 135.7 / V3 Axis peak to peak (milli-arcsec)
GSFAIL = ` DEGRADED' / Guide star acquisition failure!
If the observational dataset spans multiple orbits, the guide star will be re-acquired, but the guiding mode will not change from COARSE TRACK. In September 1995, the flight software was changed so that COARSE TRACK is no longer an option. The guiding mode drops from two guide star FINE LOCK to one guide star FINE LOCK , or to GYRO control.
GUIDECMD= `FINE LOCK ` / Commanded Guiding mode
GUIDEACT= `COARSE TRACK ` / Actual Guiding mode at end of GS acquisition
The jitter statistics are accumulated at the start of the observation window. Moving targets and spatial scan motion will be seen in the jitter data and image. Therefore, the OMS header keywords V2_RMS and V3_RMS values (the root mean square of the jitter about the V2 and V3 axis) can be quite large for moving targets. Also, a special anomaly keyword (SLEWING) will be appended to the OMS header stating movement of the telescope during the observation. This is expected for observing moving targets. The following list of .cmh header keywords is an example of expected values while tracking a moving target.
/ LINE OF SIGHT JITTER SUMMARY
V2_RMS = 3.2 / V2 Axis RMS (milli-arcsec)
V2_P2P = 17.3 / V2 Axis peak to peak (milli-arcsec)
V3_RMS = 14.3 / V3 Axis RMS (milli-arcsec)
V3_P2P = 53.6 / V3 Axis peak to peak (milli-arcsec)
RA_AVG = 244.01757 / Average RA (deg)
DEC_AVG = -20.63654 / Average DEC (deg)
ROLL_AVG= 280.52591 / Average Roll (deg)
SLEWING = ` T' / Slewing occurred during this observation
/ LINE OF SIGHT JITTER SUMMARYRecentering events occur when the spacecraft software decides that shaking is too severe to maintain lock. The FGS will release guide star control and within a few seconds reacquire the guide stars. It is assumed the guide stars are still within the FGS field of view. During the recentering time, INDEF will be written to the OMS table. Recentering events are tracked in the OMS header file.
V2_RMS = 2.6 / V2 Axis RMS (milli-arcsec)
V2_P2P = 23.8 / V2 Axis peak to peak (milli-arcsec)
V3_RMS = 2.5 / V3 Axis RMS (milli-arcsec)
V3_P2P = 32.3 / V3 Axis peak to peak (milli-arcsec)
Be careful when interpreting "Loss of Lock" and "Recentering" events that occur at the very beginning or at the end of the OMS window. The OMS window is larger than the observation window. These events might not affect the observation since the observation start time will occur after the guide stars are acquired (or re-acquired), and the observation stop time may occur before the "Loss of Lock" or "Recentering" event that occurred at the end of an OMS -window.
The sgraph commend in the stsdas.graphics.stplot package will plot time vs. jitter along the direction of HST's V2 axis (see Figure C.4):
cl> sgraph "x3y60102j_jit.fits seconds si_v2_avg"
st> sgraph "x3660102j_jit.fits si_v2_avg si_v3_avg"
tt> tstat u26m0801j.cmi si_v3_avg
# u26m0801j.cmi si_v3_avg
# nrows mean stddev median min max
11 -0.003006443888 0.00362533 -7.17163E-4 -0.00929515 0.00470988
Understanding and interpreting the meaning of the table columns and header keywords is critical to understanding the observation logs. Please read the available documentation and contact the STScI Help Desk (help@stsci.edu) if you have any questions about the files. Documentation is available via the WWW at: http://www.stsci.edu/ftp/instrument_news/