HST Data Handbook for WFPC2

C.1 Observation Log Files

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Observation log files associated with each HST dataset contain pointing and specialized engineering data taken during the observation. These data files are produced by the Observatory Monitoring System (OMS), an automated software system that interrogates the HST engineering telemetry and correlates the time-tagged engineering stream with HST's Science Mission Schedule (SMS), the seven-day command and event list that drives all spacecraft activities. This system reports the status of the instruments and observatory and flags discrepancies between planned and executed actions. OMS provides observers with information about guide star acquisition, pointing, and tracking that is not normally provided in the science headers.

The observation log files share the same rootname as the observation they are associated with, except for the final character, which for observation log files is always a "j" (see appendix B for more on the names of HST data files). When OMS was installed in October 1994, it initially generated files with the suffixes cmh, cmj, cmi, which contained header information, high time resolution pointing data, and three-second average pointing data, respectively (see table C.1). OMS observation logs changed to the jih/jid/jif image format after August 1995, at which time the cmi table was renamed jit to keep the naming convention consistent. In the OMS version of August 1995, cmj tables were replaced with a jitter image, which is a two-dimensional histogram of jitter excursions during the observation. The suffixes of the GEIS jitter image are jih for the header and jid for the image data. The jit table accompanies the jitter image. The header file of the image replaces the cmh file but includes the same information with the addition of some image-related keywords.

A detailed description of the observation log files can be found on-line: http://www.stsci.edu/ftp/instrument_news/Observatory/ obslog/OL_1.html

Table C.1: OMS Observation Log Files

Suffix	Contents
October 1994 to August 1995
cmh	OMS header
cmj	High time resolution (IRAF table)
cmi	Three-second averages (IRAF table)
_cmh.fits	Archived FITS file of cmh
_cmj.fits	Archived FITS file of cmj
_cmi.fits	Archived FITS file of cmi
August 1995 to February 1997
jih/jid	Two-dimensional histogram and header (GEIS)
jit	Three-second averages (IRAF table)1
_jif.fits	Archived FITS file which bundles the jih/jid files.
_jit.fits	Archived FITS file of jit.
February 1997 onward
_jif.fits	Two-dimensional histogram (FITS)
_jit.fits	Three-second averages table (FITS)

1 After May 11, 1995, the jit tables for exposures shorter than 6 seconds contain higher-resolution, one-second average pointing data.

 

Pointing and tracking information prior to October 1994 is not routinely available. Interested observers with data from this epoch, can send E-mail to help@stsci.edu.

C.1.1 Observation Log File Contents (October 1994 version)

Observation logs created between October 1994 and August 1995 contain:

• rootnamej.cmh: This ASCII header file contains the time interval, the rootname, averages of the pointing and spacecraft jitter, the guiding mode, guide star information, and alert or failure keywords. Figure C.1 shows a representative observation log header file.
• rootnamej.cmj: This table presents the data at the highest time resolution for the telemetry mode in use. It contains the reconstructed pointing, guide star coordinates, derived jitter at the instrument aperture, and guiding-related flags. The intent is: (1) to provide high-time resolution jitter data for deconvolution or for assessing small aperture pointing stability, and (2) to display the slew and tracking anomaly flags with the highest resolution. Table C.2 lists the table column heading, units and a brief definition.
• rootnamej.cmi: This table contains data that were averaged over three-second intervals. It includes the same information as the .cmj table and also includes orbital data (e.g., latitude, longitude, limb angle, magnetic field values, etc.) and instrument-specific items. It is best suited for a quick-look assessment of pointing stability and for studying trends in telescope or instrument performance with orbital environment. Table C.3 lists the table column heading, units and a brief definition.
• rootnamej_cmi/j/h.fits: The above three GEIS files are actually archived as FITS files. They may be worked with as such, or run through the STSDAS task strfits, to convert them.
• Observation Log File Contents (August 1995 version)
• The contents of observation log files created between August 1995 and February 1997 are as follows:
• rootnamej.jih: This GEIS header file, the analog to the cmh file, contains the time interval, the rootname, averages of the pointing and spacecraft jitter, the guiding mode, guide star information, and alert or failure keywords. Figure C.1 shows a representative observation log header file.
• rootnamej.jid: This GEIS image-a significant enhancement of the old cmj file-presents a two-dimensional histogram of the pointing fluctuations during the observation. You can display it to visualize the spacecraft stability during you observation, and is information for deconvolutions and PSF analyses.
• rootnamej.jit: This table, the analog to the cmi table, contains data that were averaged over three-second intervals. Its content is identical (see table C.3).
• rootnamej_jif.fits: FITS file that is actually the de-archived product. This file can be converted to the jih/jid GEIS file via the strftis routine.
• rootnamej_jit.fits: The de-archived FITS file corresponding to the jit IRAF table. It can be converted via strfits.

C.1.2 Observation Log File Contents (February 1997 version)

The contents of observation log files created since February 1997 are as follows:

• rootnamej_jif.fits: The de-archived FITS file. Unlike the previous OMS epoch, this FITS file does not bundle a GEIS file and cannot be converted with strfits. This was done to more closely correlate the observation log files with the STIS and NICMOS FITS files with extensions and associations. OMS will normally put all associated observation logs into a single file, to correspond to the associated science exposures. However, if even one science exposure is orphaned (not associated) then an individual observation log FITS file will be produced for every exposure in that association. For a description of STIS and NICMOS association files, see appendix B. All of the information contained in the old cmh/jih ASCII header is now available as keywords in the FITS files.
• rootnamej_jit.fits: The FITS file containing the table information. The comments for the _jif file apply here as well.

Table C.2: Contents of .cmj Table

Parameter	Units	Description
seconds	seconds	Time since window start
V2 dom	arcseconds	Dominant FGS V2 coordinate
V3 dom	arcseconds	Dominant FGS V3 coordinate
V2 roll	arcseconds	Roll FGS V2 coordinate
V3 roll	arcseconds	Roll FGS V3 coordinate
SI V2	arcseconds	Jitter at aperture reference
SI V3	arcseconds	Jitter at aperture reference
RA	degrees	Right ascension of aperture reference
DEC	degrees	Declination of aperture reference
Roll	degrees	Angle between North and +V3
DayNight	0,1 flag	Day (0) or night (1)
Recenter	0,1 flag	Recentering status
TakeData	0,1 flag	Vehicle guiding status
SlewFlag	0,1 flag	Vehicle slewing status

 
Figure C.1: A Representative .jih or .cmh Header
 
Figure C.2: Representative .jih or .cmh Header
 
Table C.3: Contents of .jit or.cmiTable, Three-Second Averaging

Parameter	Units	Description
seconds	seconds	Time since window start
V2 dom	arcseconds	Dominant FGS V2 coordinate
V3 dom	arcseconds	Dominant FGS V3 coordinate
V2 roll	arcseconds	Roll FGS V2 coordinate
V3 roll	arcseconds	Roll FGS V3 coordinate
SI V2 AVG	arcseconds	Mean jitter in 3 seconds
SI V2 RMS	arcseconds	rms jitter in 3 seconds
SI V2 P2P	arcseconds	Peak jitter in 3 seconds
SI V3 AVG	arcseconds	Mean jitter in 3 seconds
SI V3 RMS	arcseconds	rms jitter in 3 seconds
SI V3 P2P	arcseconds	Peak jitter in 3 seconds
RA	degrees	Right ascension of aperture reference
DEC	degrees	Declination of aperture reference
Roll	degrees	Angle between North and +V3
LimbAng	degrees	Angle between earth limb and target
TermAng	degrees	Angle between terminator and target
LOS_Zenith	degrees	Angle between HST zenith and target
Latitude	degrees	HST subpoint latitude
Longitude	degrees	HST subpoint longitude
Mag V1,V2,V3	degrees	Magnetic field along V1, V2, V3
EarthMod	V Mag/arcsec2	Model earth background light
SI_Specific	-	Special science instrument data
DayNight	0,1 flag	Day (0) or night (1)
Recenter	0,1 flag	Recentering status
TakeData	0,1 flag	Vehicle guiding status
SlewFlag	0,1 flag	Vehicle slewing status