Space Telescope Science Institute
COS Data Handbook 2.00
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COS Data Handbook > Chapter 3: COS Calibration > 3.1 Raw Data Compilation

The basic input to calcos is raw science data files. This section provides a brief overview of how these files are generated from raw spacecraft telemetry.
Telemetry containing COS science data is downlinked from the HST satellite through a Tracking and Delay Relay Satellite System (TDRSS) satellite to a ground station in White Sands, NM. From there it is sent to Goddard Space Flight Center where the data capture facility packet processor (PACOR) collects the downlinked science data and places them into telemetry “pod files”. These pod files are then transmitted to STScI where they are saved to a permanent storage medium. The STScI ingest pipeline, OPUS, then unpacks the data, populates keyword values extracted from the telemetry stream, reformats the data, and repackages them into raw, uncalibrated, but scientifically interpretable data files.
The raw files are then processed by the calcos software to produce a variety of calibrated data files. The results of these procedures are used to populate the databases that form the searchable archive catalog at STScI describing the individual instrument exposures. At this point, the raw and calibrated data files generated from the pod files are normally discarded. Only the pod files and the information placed in the archive databases are preserved. Each time a user requests data from the Hubble Data Archive via the “On The Fly Reprocessing” (OTFR) system, the raw files are regenerated from the original pod files, and then recalibrated with the latest reference files. A more detailed description of the OTFR system as it applies to COS and other HST instruments can be found in Swade et al, available on-line at
Although the OTFR system uses pod files for input, calcos uses the raw files (together with their association files) as its initial input. Figure 1.7 shows a raw image of one segment of the COS FUV XDL detector.

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