Pipeline processing is carried out by two separate image processing packages: calacs
corrects for instrumental effects to produce calibrated products. multidrizzle
corrects for geometric distortion, performs cosmic ray rejection on combined images, and for dithered images, also removes hot pixels.
controls the image calibration steps based on the type of images and/or associations:
|The task acs2d
continues with routine image reductions; MAMA images are flat-fielded. CCD images—single images and images combined with acsrej
—are flat-fielded and, in most cases, dark current is subtracted.
Fully calibrated data products from calacs
(with suffixes flt.fits
) are in units of electrons
While intermediate steps in calacs
make use of sky subtraction values to perform certain steps, such as in identifying cosmic rays, all data products created by the pipeline will not
be sky subtracted.
Calibrated products from the pipeline may still contain some artifacts such as hot
pixels, cosmic rays, amplifier crosstalk, and in the case of post-SM4 WFC images, bias shift and bias striping.
Hot pixels and cosmic rays can be removed from dithered single images (with
after calibration by calacs
) using multidrizzle
to process associations created from “POS TARG” or dither “PATTERN” special requirements in Phase II proposals.
As of spring 2011, calacs
software does not correct for amplifier crosstalk, bias shift, or bias striping. To correct these artifacts, it is currently necessary to use stand-alone STSDAS routines outside calacs
. These are described in Section 4.2.1
Each single-exposure raw image in an association undergoes standard detector
calibrations in calacs
, such as bias subtraction, dark subtraction, and flat-fielding (see Section 3.3
), to create an flt.fits
image. This is done regardless of whether those single images will be combined in later calacs
steps. Data in the “SCI” (science image) and “ERR” (error image) extensions of a calibrated flt.fits
image are in units of electrons
, whereas the raw ACS images are in units of counts
images are summed to create an image with the suffix sfl.fits
Note that each single exposure image from a “CR-SPLIT” or repeated
sub-exposures set will also be calibrated individually to produce a flt.fits
image for later use in multidrizzle
if the header value EXPSCORR=“PERFORM”
which is currently the default.)
produces a calibrated flt.fits
file for each single-exposure image in an association, including those created from using dither “PATTERN” and “POS TARG” special requirements in the Phase II proposal.
If more than one exposure was taken at a pointing using “CR-SPLIT” (as is
sometimes the case for CCD observations), calacs
produces a cosmic ray-rejected combined image, crj.fits
, for that pointing.
If there were two or more repeated sub-exposures at a pointing, calacs
produces a cosmic ray-rejected combined image, crj.fits
, for CCD data. For SBC MAMA data, a summed image is created with the suffix sfl.fits
will not combine images from multiple positions. Later in the pipeline, after calacs
processing is completed, flt.fits
images will be corrected for geometric distortion and combined, with cosmic ray and hot pixel removal, by multidrizzle
files are not used in multidrizzle
All ACS data are automatically corrected for distortion during pipeline processing
using the multidrizzle
task which automatically performs image registration, cosmic ray rejection and final Drizzle image combination. This task relies on the IDCTAB
reference table for a description of the distortion model.
Observations that use dither “PATTERN” and “POS TARG” offsets in a single visit
are described in the pipeline-created association table. If the association table is present in the working directory and provided as input to multidrizzle
, the task will process the flt.fits
files specified in it. multidrizzle
also accepts, as input, wildcard-specified filenames, a comma-separated list of images, an ascii file containing a list of images, or just one image name.
performs geometric distortion corrections on all flt.fits
input images. Using WCS information in the image headers, the images are registered and combined with cosmic ray removal. For dithered images, hot pixels are also removed. The resulting combined image, in units2
, is written to a file with the suffix drz.fits
. (For WFC, data from the two chips are mosaicked together as one image.)
It is important to recognize that multidrizzle
is built around the pydrizzle
task, which was capable of aligning the images and correcting for the geometric distortion but did not
remove cosmic rays. Instead, pydrizzle
relied on crj.fits
products as input to create cosmic ray-free images.
The use of multidrizzle
, and uses the original flt.fits
files directly as input to combine images with cosmic ray rejection, and for dithered images, also performs hot pixel rejection. This has significant advantages for observations with one exposure, or very few sub-exposures at each pointing, taken at few dither positions. In these cases, multidrizzle
will use the information from overlapping fields in all flt.fits
images to produce the best possible cosmic ray and hot pixel rejection for the output combined image. The resulting drizzled images should generally be 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 more information, please see Section 3.5.1
The goal of the ACS pipeline is to provide data calibrated to a level suitable for
initial evaluation and analysis for all users. Observers 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 data processed via OTFR from the Archive is not ideal, requiring off-line interactive processing:
with different reference files than those specified in the image header.
with non-default calibration switch values.
Images combined with MultiDrizzle in the pipeline were produced using
parameters that are suitable for the widest range of scientific applications. Some datasets, however, 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. The same muultidrizzle
task used in the pipeline is also available to users in STSDAS for off-line processing of flt.fits
images retrieved from the Archive. multidrizzle
provides a single-step interface to a complex suite of tasks in the STSDAS dither
package. It is built around the pydrizzle
task and needs to be executed within the PyRAF environment. For more information, please refer to the Multidrizzle Handbook