5.1 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 MAST, 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 2 for more information 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.

5.1.1 Jitter File Contents

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:

• ipppssooj_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 "ASSOCATION KEYWORDS" block. Each of the header values for extensions 1 and beyond represent a single exposure in the association, and will contain all the group 0 keywords as well as additional blocks of parameters relating to that particular exposure.
• ipppssooj_jit.fits: This is the 3-second average jitter table. It contains the reconstructed pointing, guide star coordinates, derived jitter at the science instrument aperture, pertinent guiding-related flags, orbital data (e.g., latitude, longitude, limb angle, magnitude field values, etc.) and fine guidance sensor flags.

5.1.2 Retrieving Observation Log Files

Observation logs can be retrieved via the MAST Portal by deselecting the "Minimum Recommended Products" option in the download basket and then selecting the "jit" and "jif" options in the "Group" panel.