Here Come the Observation Log Files

or How to Avoid Space Telescope Engineering Xenophobia!

Olivia Lupie, Bruce Toth, and Stefi Baum

New Guest Observers will now have access to the "Observation Log Files" along with their science data. The Observation Log files contain a specialized set of pointing and engineering data associated with each science exposure (including the jitter information). The calibration pipeline began routinely producing these files Oct 20 1994. They are produced by the Observatory Monitoring System, an automated software system which interrogates the HST engineering telemetry and correlates the time-tagged engineering stream with the scheduled events as determined from the Mission Schedule, the seven-day command and event list which drives all HST activities. The Observatory Monitoring System reports the status of the instruments and observatory and flags discrepancies between planned and executed actions. The Observation Log files have the same first 8 characters as the rootname of the science exposure to which they pertain, and are composed of a header (with extension CMH) and two FITS tables (extensions CMJ and CMI). The data are in FITS Format and may be converted to STSDAS binary table format using IRAF STRFITS and manipulated using the IRAF table tools (TTOOLS). The Observation Log Files will not be written to the GO data tapes until February of 1995. However, GOs whose data was taken after October 20, 1994 can retrieve the Observation Log Files from the HST Archive, using StarView.

The Observation Log contains a plethora of interesting information, a necessity in evaluating and reconstructing pointing stability and environment during a given exposure. The target recipients are General Observers and STScI Scientists and Engineers as well. The Observation Log header file (CMH) contains, among other things, keywords describing the actual (not requested but executed) tracking mode achieved at the end of the guide star acquisition process, the rms and maximum of the jitter (motion of the Fine Guidance Sensors during the observation), the number of occurences of loss of lock and recenterings, estimated Zodiacal light, stray Earth and Moon light, reconstructed absolute pointing, and various angles (target to earth limb, target to ram direction and others). The header also alerts the observer to slews occurring during an observation and reports anomalies which can compromise the integrity of the science data (degraded guide star acquisitions, losses of lock) or result in a failed observation (instrument or vehicle safings, failed target acquisitions).

The tables contain pointing data and other parameters which are traced as a function of time during an exposure. The engineering data are transmitted in a variety of formats, the formats differing in their rates of update and information content. Because of the potential for large amounts of data in the highest rate formats, we have included in the Observation Logs two tables, the CMJ table which reports the parameters at the highest time resolution, and the CMI table which averages each parameter over a 3 seconds interval, a more manageable data set for quick look analysis. The CMJ table contains the vehicle coordinates of the guide stars during an exposure and the vehicle and absolute coordinates (ra, dec, and roll) of the science aperture (i.e., the jitter) as calculated from the guide star activity. It also traces the day/night flag, recentering and loss of locks as a function of time, and telescope slews. The CMI table contains these same parameters averaged over 3 seconds, and also includes "environmental" parameters such as the target-to-earth limb angle, terminator angle, estimated stray earthlight, orbital information (latitude, longitude, zenith angle, magnetic field), and a science-instrument specific column. Future enhancements of the Log will include a time trace of telescope "breathing" affects and estimated model backgrounds given in instrument counts (these are now provided in V mags/arsec**2).

Figure 1 is an example of the jitter at the position of the science aperture during an exposure. This jitter is calculated from the combined motion of the two guide stars.

Figure 2 shows a time trace of the recentering flag activity during an exposure.

Figure 3 displays the Ra/Dec of the center of the aperture position as during a Target Acquisition Peak-Up for the FOS.

We emphasize, to all users of these logs, the importance of understanding the origin of the parameters and their associated error bars before interpreting the results. For example, the absolute pointing provided in the Log is reconstructed from the guide star data and the guide star absolute coordinates whose catalog errors are of order several tenths of arcseconds. When the tracking mode uses the gyros only, the absolute pointing is reconstructed from the gyro data which has time-dependent errors (due to gyro drift). Documentation describing the details and accuracy of the Observation Logs is available on line through the STScI home page of the World Wide Web - simply click on the documents button. Hard copies of the OMS documentation or help retrieving these files from the HST archive can be requested from the Archive Hotseat (archive@stsci.edu). Enjoy!

kimball@stsci.edu