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Intro to HST Data Handbooks 8.0 May 2011
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Introduction to the HST Data Handbooks > Chapter 6: Observation Logs > 6.3 Using Observation Logs

Here are some simple examples of what can be learned from the observation log files. Note that for FITS format observation logs, current versions of STSDAS tools will handle the files with extensions properly. Keywords can be viewed with tools such as imheader or hedit, and data viewed, plotted, or displayed using the same tasks one might have for the GEIS files. For more information on FITS file structures, see Chapter 2.
Unless requested, all observations will be scheduled with FINE LOCK guiding, which may be one or two guide stars (dominant and roll). The spacecraft may roll slightly during an observation if only one guide star is acquired. The amount of roll depends upon the gyro drift at the time of the observation, the location during an orbit, and the lever arm from the guide star to the center of the ­aperture.
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 will not 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 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.
The dominant and roll guide star keywords (GSD and GSR) in the OMS header can be checked to verify that two guide stars were used for guiding, or in the case of an acquisition failure, to identify the suspect guide star. The dominant and roll guide star keywords identify the stars that were scheduled to be used, and in the event of an acquisition failure, may not be the stars that were actually used. The following list of observation log keywords is an example of two star guiding. These keywords are found in the jif file or, for older data, the cmh file.
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
The guide star identifications, GSD_ID and GSR_ID, are different for the two Guide Star Catalogs: GSC2 IDs are 10-characters in length, like those of GSC1, but consist of both letters and numbers. GSC1 IDs consist entirely of numbers.
The GSC2 catalog is the default catalog since Cycle 15 (June 2006). The keyword REFFRAME in the primary science header indicates which catalog was in use for an observation. This keyword is included in all Cycle 15 and later observations, and is either “GSC1” for Guide Star Catalog 1, or “ICRS” for International Celestial Reference System, upon which GSC2 coordinates are based. The same information is added to the HST Archive catalog file “shp_refframe” of the “shp_data” database table since June 2006. For more information about the catalogs and their astrometric accuracy, see:
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.
Some GHRS 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.
The guide star acquisition at the start of the observation set could fail if the FGS fails to lock onto the guide star. The target may not be in the aperture, or maybe only a piece of an extended target is in the aperture. The jitter values will be increased because FINE LOCK was not used. The following list of observation log header keywords indicate that the guide star acquisition failed.
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!
The observation logs for all of the exposures 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.
GUIDECMD= ‘FINE LOCK ‘ / Commanded Guiding mode
GUIDEACT= ‘COARSE TRACK ‘ / Actual Guiding mode at end of GS acquisition
If the observation dataset spans multiple orbits, the guide star will be reacquired, 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.
A type 51 slew is used to track moving targets (planets, satellites, asteroids, and comets). Observations are scheduled with FINE LOCK acquisition, i.e., with two or one guide stars. Usually, a guide star pair will stay within the pickle during the entire observation set, but if two guide stars are not available, a single guide star may be used, assuming the drift is small or the proposer says that the roll is not important for that particular observing program. An option during scheduling is to drop from FGS control to GYRO control when the guide stars move out of the FGS. Also, guide star handoffs (which are not a simple dropping of the guide stars to GYRO control) will affect the guiding and may be noticeable when the jitter ball is plotted.
In two-gyro mode, all observations are scheduled with two guide stars. Proposers cannot request the use of only one guide star.
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-axes) can be quite large for moving targets. Also, a special anomaly keyword (SLEWING) will be appended to the OMS header indicating movement of the telescope occurred during the observation. This is expected when observing moving targets. The following list of OMS header keywords is an example of expected values while tracking a moving target.
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
The spacecraft may shake during an observation, even though the guiding mode is FINE LOCK. This movement may be due to a micro-meteorite hit, jitter at a day-night transition, or for some other unknown reasons. The FGS is quite stable and will track a guide star even during substantial spacecraft motion. The target may move about in an aperture, but the FGS will continue to track guide stars and reposition the target into the aperture. For most observations, the movement about the aperture during a spacecraft excursion will be quite small, but sometimes, especially for observations with the spectrographs, the aperture may move enough that the measured flux for the target will be less than a previous group. Check the OMS header keywords (V2_RMS, V3_RMS) for the root mean square of the jitter about the V2- and V3-axes. The following list of header keywords, found in the jif or older cmh files, is an example of typical guiding rms values.
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)
Recentering 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.
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 will start after the guide stars are acquired (or reacquired), and the observation may stop before the “Loss-of-Lock” or “Recentering” event that occurred at the end of an OMS ­window.
The sgraph command in the package can be used to plot time vs. jitter along the direction of HST’s V2-axis (see Figure 6.2):
Figure 6.2: Plotting Jitter Along V3-axis
To get an idea of pointing stability, you can create a jitter ball by plotting jitter along the V2-axis vs. jitter along the V3-axis (see Figure 6.3):
Figure 6.3: Plotting V2 vs. V3 Jitter
The tstatistics task can be used to find the mean value of the si_v3_avg column—the amount of jitter (in arc seconds) in the direction of the V3. This value can be used to model jitter in a PSF. In this example, the mean jitter is ~3 mas, which is typical for HST data:
Figure 6.4: Averaging a Column with tstatistics
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 ( if you have any questions about the files. In particular please follow the following links for:

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