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| Part II: ACS Data Handbook | |||||
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3.2 Pipeline Overview
Pipeline processing is carried out by 2 separate packages: CALACS, which corrects for instrumental effects and produces calibrated products, and MultiDrizzle, which corrects for distortion, performs cosmic ray rejection, and uses Drizzle/PyDrizzle to combine associated dithered data.
3.2.1 CALACS: Image Calibration
The CALACS software consists of a series of individual tasks that:
- Orchestrate the flow of data through the pipeline.
- Perform the initial tasks of basic two-dimensional image reduction (e.g., overscan subtraction, bias subtraction) for CCD data.
- Reject cosmic rays from CR-SPLIT CCD data.
- Perform the remaining tasks of basic two-dimensional image reduction (e.g., dark subtraction, flat fielding) for CCD and MAMA data.
- Sum any REPEAT-OBS exposures.
The calibrated products from the pipeline may still contain artifacts such as hot pixels and cosmic-rays. If hot pixels are to be detected and flagged by CALACS, dithered exposures are required. Similarly, cosmic ray rejection by the pipeline requires the specification of CR-SPLIT exposures in the observing program. Finally, the definition of associations has been expanded to include exposures that use REPEAT-OBS and POS TARG offsets, thereby allowing these data to be automatically corrected in the pipeline for cosmic-rays during subsequent processing with MultiDrizzle.
While intermediate steps in CALACS make use of sky subtraction, for example for identifying cosmic-rays, all data products created by the pipeline will not be sky subtracted. Fully calibrated data products (with suffixes FLT, CRJ, SFL) will be in units of electrons.
CALACS and Single exposures
A single raw ACS exposure is processed by CALACS following the steps outlined in Section 3.3. These steps include the standard detector calibrations: bias subtraction, dark subtraction, flat fielding, etc. When CALACS processes a single raw exposure, the calibrated product is given the FLT suffix. The data in the calibrated SCI and ERR extensions are in units of electrons.
CALACS and Multiple exposures
The same processing steps performed on single images are performed for multiple images which are part of an association. The result is a single calibrated product in units of electrons.
CALACS will recognize and correctly process
CR-SPLITorREPEAT-OBSexposures by interpreting the association table and determining which exposures should be combined during calibration.REPEAT-OBSexposures are individually run through all the calibration steps to create calibrated, flat-fielded (FLT) product for each input file. The FLT images are then summed to create the summed flat-fielded (SFL) product. No cosmic-ray rejection is performed forREPEAT-OBSimages, until subsequent processing by MultiDrizzle.Observations taken as part of a CR-SPLIT association, on the other hand, are combined during cosmic-ray rejection. First, the bias and dark subtraction is performed. Then, the images are combined while at the same time rejecting cosmic rays. The product is then flat fielded to create a single calibrated, cosmic-ray rejected, flat-fielded (CRJ) image.
CALACS and Dithered exposures
Observations which use the dither patterns provided in the proposal instructions will be automatically associated for combining in the pipeline. As of Cycle 14, pointing patterns specified with POS-TARGs are also automatically combined in the pipeline. An effort is under way to retroactively create POS-TARG associations for pre-Cycle 14 datasets. Any pattern designed with POS-TARGs will not be associated. When processing observations which are part of a dither pattern, CALACS will produce a calibrated FLT, CRJ, or SFL file at each dither position. It will not, however, combine images from multiple positions. Further pipeline processing by MultiDrizzle will correct for geometric distortion and combine dithered images.
Table 3.1: The input and output image suffixes from CALACS and MultiDrizzle are given for various observing modes.
3.2.2 MultiDrizzle and PyDrizzle: Distortion Correction and Dither Combination
All ACS data will be automatically corrected for distortion during pipeline processing using the MultiDrizzle task which automatically performs image registration, cosmic ray rejection and final drizzle combination. This task relies on the IDCTAB reference file for a description of the distortion model. MultiDrizzle understands the ACS association tables which allow the pipeline to combine dithered observations. Observations which use the dither patterns provided in the proposal instructions, as well as exposures that rely on explicit POS-TARG offsets, are automatically associated for combining in the pipeline. MultiDrizzle will automatically produce images which are both astrometrically and photometrically accurate regardless of whether they were taken as a single exposure or a set of dithered exposures.
The flat-fielded, calibrated products from CALACS are listed in Table 3.1. The calibrated product of a single ACS exposure is an FLT file. The product of a CR-SPLIT or REPEAT-OBS association is a CRJ or SFL file, respectively. The product of a dither pattern is an FLT, CRJ or SFL file at each dither position. Using the FLT files as input, MultiDrizzle performs the geometric correction on all data (dithered or not), carries out cosmic ray rejection, and combines multiple dithered images into a single output image with the DRZ suffix. For WFC observations, both chips are combined into a single extension. MultiDrizzle also converts the data to units of count rate (electrons/sec).
It is important to recognize that MultiDrizzle is built around an earlier script, PyDrizzle, which was capable of aligning the images and correcting for the geometric distortion but did not remove cosmic-rays, instead relying on the CRJ products as input. The use of MultiDrizzle supersedes this, and uses the original FLT files directly as input to perform cosmic ray rejection and produce a final drizzled output image. This has significant advantages in cases where small numbers of CR-SPLIT images were obtained at a small number of different dither positions, as MultiDrizzle will use all the information form the FLT files to produce the best cosmic ray rejection. The resulting drizzled images should be generally useful for science as-is, although subsequent reprocessing off-line with MultiDrizzle may be desirable to optimize the data for some specific scientific applications. For information on obtaining this software, we refer the reader to Section 3.5.1.
3.2.3 When is OTFR not Appropriate?
The goal of the ACS pipeline is to provide data calibrated to a level suitable for initial evaluation and analysis for all users. Observers frequently require a detailed understanding of the calibrations applied to their data and the ability to repeat, often with improved products, the calibration process at their home institution. There are several occasions when OTFR is not ideal and when off-line interactive processing by the user is required:
- when running CALACS with personal versions of reference files,
- when running CALACS with non-default calibration switch values,
- when images must be cleaned of artifacts such as new hot pixels or cosmic-rays.
While the final drizzled images from MultiDrizzle have been produced using parameters that are suitable for the widest range of scientific applications, there may remain some specific datasets that would benefit significantly from further processing off-line. For example, a different pixel scale or orientation may be desired, or cosmic ray rejection parameters might need to be slightly modified for some datasets. Therefore, the same version of MultiDrizzle script that is in the pipeline is also available to users in STSDAS for off-line processing of data that have been retrieved from the pipeline. MultiDrizzle provides a single-step interface to the complex suite of tasks in the STSDAS dither package. It is built around the PyDrizzle software and needs to be executed within the PyRAF environment. For more information, we refer the reader to Chapter 4.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |