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.
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 j
if 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: http://www-gsss.stsci.edu/Catalogs/Catalogs.htm
|
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 modeGUIDEACT= ‘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.
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.
/ 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
|
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.
/ LINE OF SIGHT JITTER SUMMARY
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
stsdas.graphics.stplot package can be used to plot time vs. jitter along the direction of
HST’s V2-axis (see
Figure 6.2):
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):
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:
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. In particular please follow the following links for:
