Space Telescope Science Institute
Intro to HST Data Handbooks 8.0 May 2011
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Introduction to the HST Data Handbooks > Chapter 5: HST File Names > 5.3 Suffixes of Files Common to All Instruments

The suffix of an HST file identifies the type of data that it contains. Several types of file suffixes are common to all instruments. These common suffixes are briefly described below. Please refer to the appropriate instrument’s “Data Structures” chapters (in Part II) for the definitions of the instrument-specific suffixes.
Trailer Files
Trailer files (suffix trl) are FITS ASCII tables that log the processing of your data by the OPUS pipeline.
Support Files
Support files (suffix spt) contain information about the observation and engineering data from the instrument and spacecraft recorded at the time of the observation.
Observation Logs
Observation Logs (suffix ji* or cm*, aka “obslogs”) are FITS files that contain information describing how the HST spacecraft behaved during a given observation. More information on these files is given in Chapter 6.
Old Version Observation Logs
The engineering telemetry, which is the basis of the observation logs, does not get processed with OTFR. Thus any significant change in its processing results in a different type of observation log. Such a change occurred in February 2003, when the existing Observation Monitoring System (OMS) was streamlined to reduce the software maintenance costs. Obslogs produced prior to this time have cm* suffixes and contain science specific information, in addition to the pointing information. Obslogs produced after this date have ji* suffixes and do not contain any instrument specific information.
Obslogs headers, which you can read with the IRAF task imheader (see Section 2.3.3), are divided into groups of keywords that deal with particular topics such as spacecraft data, background light, pointing control data, and line-of-sight jitter summary. The headers themselves provide short descriptions of each keyword. Obs logs tables and images record spacecraft pointing information as a function of time. For more information on these files, consult Chapter 6 or the STScI Observation Logs Web pages at:
PDQ Files
The suffix pdq denotes Post Observation Summary and Data Quality files which contain predicted as well as actual observation parameters extracted from the standard header and science headers. These files may also contain comments on any obvious features in the spectrum or image, as noted in the OPUS data assessment, or automatically extracted information about problems or oddities encountered during the observation or data processing. These comments may include correction to the keywords automatically placed in the obs logs files. PDQ files were discontinued on May 9, 2002.
OCX Files
The suffix ocx denotes Observer Comment Files which were produced by STScI personnel to document the results of real-time commanding or monitoring of the observation, along with keywords and comments. Prior to April 17, 1992, OCX files were not always archived separately and, in some cases, were prepended to the trailer file.
After early February 1995, OCX files were produced only when an observation was used to locate the target for an Interactive Target Acquisition. At this time, mission and spacecraft information were moved to the PDQ reports and the Observation Logs (OMS jitter image and jitter table). OCX files were discontinued on May 9, 2002.

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