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| Part II: ACS Data Handbook | |||||
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3.1 On The Fly Reprocessing (OTFR)
OTFR is now the standard way of processing data which has been requested by the user from the STScI archive. It provides the best calibrated products by reprocessing the raw telemetry files "on-the-fly" for distribution each time data is requested. Each observation requires calibrations suited to the particular mode used for taking data. Calibration data specific to the various modes of ACS operation are prepared and archived in the Calibration Data Base System (CDBS).
The STScI calibration pipeline consists of two main software systems: the Operations Pipeline Unified System (OPUS) and the Data Archive and Distribution System (DADS). Raw spacecraft telemetry data from Goddard Spaceflight Center are transmitted to STScI in the form of POD files. When a user requests data from the HST Archive, OTFR uses the POD files as input to the OPUS step called Generic Conversion and generates the uncalibrated "raw" data. At the same time, Generic Conversion queries the CDBS to determine the most up-to-date reference files for the observation configuration. OPUS then runs CALACS to process the uncalibrated data, using specific ACS reference images and tables from the CDBS. With these data, DADS populates a database which is accessible to users via StarView. DADS then distributes any data (both calibrated and uncalibrated) requested for download to the user. (See Chapter 1 of the HST Introduction for more information.)
The most current ACS reference files are used by CALACS each time OTFR is run. The calibration reference files (e.g. flat fields, bad pixel tables) are also available from the HST Archive. Since reference files such as CCD biases and darks are frequently updated, OTFR may use different reference files depending on the date of reprocessing. Previously, the archived data from DADS had to be manually recalibrated by running the calibration software on the user's home workstation with the updated reference files. OTFR replaces this step by automatically recalibrating with the most current reference data available. The user simply waits until the contemporaneous reference files are in place and requests the data via StarView or the HST web-based archive request form. The image header keywords are then updated with the names of the reference files used during that OTFR run. The PROCTIME keyword is updated to reflect the pipeline processing date and time in MJD.
OTFR allows the user to avoid downloading archived data which is outdated because of software changes made for bug fixes, improved algorithms, new capabilities, or header keyword changes. An archive request for calibrated data will result in OTFR reprocessing and distributing data which uses the latest software versions available.
Currently OTFR will distribute all files associated with an ACS observation, including both the uncalibrated and calibrated files. Future versions could enable users to select only certain parts of the dataset, for example the final calibrated image. Future versions could also enable users to set calibration parameters for a particular OTFR run. Until then, OTFR will process with default values. Of course, the option of recalibrating aside from OTFR always exists. (See Manual Recalibration of ACS Data in Section 3.5.)
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |