3.1	On The Fly Reprocessing (OTFR)

ACS data requests from the HST Archive are processed using the “On-the-Fly Reprocessing” (OTFR) System. It provides users with calibrated data that is created using the best available reference files and latest software upgrades.

STScI’s 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 the Goddard Space Flight Center are transmitted to STScI in the form of “POD” files which are stored in the HST Archive. When data is requested from the Archive, OTFR processing begins with the POD files when OPUS runs a step called Generic Conversion to extract and create uncalibrated “raw” data. During this step, the “Calibration Database System” (CDBS), where all calibration reference files are stored, is queried to determine the most current reference files for the observation’s configuration and date. OPUS then calibrates the raw data using calacs, with the best-available reference images and tables from the CDBS. These calibrated data, along with other requested data products, are sent to the user. (See Chapter 1 of the Introduction to the HST Data Handbooks for more information about retrieving data from the Archive.)

CCD dark reference files are frequently delivered to CDBS, and other types of bias and dark reference files are occasionally updated. Users who receive their data shortly after an observation was executed are advised to wait a few more weeks for the best-suited reference files (usually the darks) to become available in CDBS. At that point, the data could be re-retrieved from the Archive or manually recalibrated using the new reference files. To find out if reference files have been updated, compare the names of the reference files in the image header with those listed as the recommended reference files for a particular dataset in StarView. For dated ACS products, it is often more convenient to retrieve freshly-calibrated data from the Archive before beginning data analysis. Users who choose to rerun calacs on raw data should retrieve their reference files from the ACS Reference Files Web page (See “Manual Recalibration of ACS Data” in Section 3.5 for information about running calacs).