ACS data requests from the HST Archive are processed using the “
On-the-Fly Reprocessing”
(OTFR) System. It provides users with calibrated data that’s 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 observations’ configuration and date. OPUS then proceeds to run
calacs on the raw data using 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 reference files are occasionally updated. If you received your data shortly after an observation was executed, it may be best to wait a few more weeks for the best-suited reference files (usually the darks) to become available in CDBS. At that point, you could re-retrieve the data from the Archive or recalibrate the data yourself using the new reference file(s). To find out if a better-calibrated product is now available, compare the reference file names in the image header to
StarView’s listing of recommended reference files for your dataset. If your ACS data have been sitting on your disk for a long period of time, you would almost certainly benefit from retrieving freshly-calibrated data from the Archive before beginning data analysis. You may also choose to re-run
calacs on the raw data on your home machine; in that case, reference files can be obtained from the
ACS Reference Files Web page (See
“Manual Recalibration of ACS Data” in
Section 3.5 for information about running
calacs).
