Space Telescope Science Institute
Intro to HST Data Handbooks 8.0 May 2011
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Introduction to the HST Data Handbooks > Chapter 6: Observation Logs > 6.1 Observation Log Files

Observation Log Files, also known as jitter files, record pointing, jitter, and other Pointing Control System (PCS) data taken during an HST observation. You can use them to assess the behavior of the HST spacecraft during your observation, and in particular, to evaluate the jitter of the spacecraft while it was taking data. Here we describe the contents and structure of the observation log files, how to retrieve them from the Archive, and how to work with the data they contain.
These data files are produced by the Engineering Data Processing System (EDPS), 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. The EDPS replaced the Observatory Monitoring System (OMS) in February 2003. EDPS 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 Chapter 5 for more on the names of HST data files). The jit table accompanies the jif header. The jit table is the three-second average pointing data. The jif header is a two-dimensional histogram of jitter excursions during the observation which includes the header plus some image related keywords.
Table 6.1: OMS Observation Log Files

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
The EDPS jitter files are limited to the engineering data that describe the performance of the Pointing Control System (PCS) including the Fine Guidance Sensors that are used to control the vehicle pointing. The jitter files report on PCS engineering data for the duration of the observation. The EDPS jitter files contain:
rootnamej_jif.fits: The FITS header contains keywords providing information regarding the file structure, observation details, modeled background light, point control system, jitter summary, orbital geometry, and problem flags and warnings. The extension 0 header will contain parameters relating to the entire association or dataset, ending at the “ASSOCIATION KEYWORDS” block. The header values for extensions 1 and beyond will contain all the group 0 keywords as well as additional blocks of parameters relating to that particular exposure in the association.
rootnamej_jit.fits: This is the 3-second average jitter table. It contains the reconstructed pointing, guide star coordinates, derived jitter at the SI aperture, pertinent guiding-related flags, orbital data (e.g., latitude, longitude, limb angle, magnetic field values, etc.) and FGS_flags. There are examples of headers and tables in the online Observation Logs documentation located at:

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