Introduction
============
The Advanced Camera for Surveys Wide Field Camera (ACS WFC) is one of the
currently operating cameras on Hubble. The camera consists of two CCDs,
each with 2048 rows and 4096 columns. Due to damage to the camera's CCD its
images now show streaks or trails coming off of sources in the image. The
streaks are caused by charge getting trapped and later released as the
CCD is being read out. The technical term for this behavior is "charge transfer
inefficiency" (CTI), though often the complementary term "charge transfer
efficiency" (CTE) is used instead.
Correcting for this smearing effect is important for both scientific and
aesthetic uses of ACS WFC data. This software is designed to perform that
correction. A description of the correction can be found here:
http://blogs.stsci.edu/newsletter/2011/02/07/correcting-pixels-for-cti-in-acs%E2%80%99s-wfc/
One interesting aspect of CTI is that in terms of correcting the streaks, any
given column of data is completely independent of the others and all of the
8192 columns can be processed separate from each other, making the process
conceptually easy to parallelize. Each column of data is corrected by a
computationally intensive algorithm that requires a large number of iterations.
About the Data
--------------
The data we've included here are in FITS format, a file type common for
astronomy data. You can read more about FITS here: http://en.wikipedia.org/wiki/FITS
ACS WFC data files have a primary header and six image data extensions. The first
three extensions are related to one ACS WFC CCD and the last three are for the
other CCD. Of primary interest are extensions 1 and 4 since they hold the actual
image data.
We've included two data files because the CTE correction algorithm is sensitive
to the overall level of signal in the images. Images with high average levels
of signal require more iterations, and more time, to correct than lower signal
images. The image of spiral galaxy NGC 4258 has a high background level and
requires significantly more processing than the image of globular cluster
NGC 104 (also known as 47 Tucanae).
The data files can be found in the directories ``ngc104`` and ``ngc4258``,
respectively.
File Names
^^^^^^^^^^
The data have file names consisting of 9 characters/numbers followed by an
underscore and a suffix, plus the .fits extension. For example:
``jb1f89eoq_flt.fits``. Different file types are distinguishable by their
suffixes.
The ``flt`` suffix is for data which has been processed through
our calibration pipeline but has not been corrected for CTE. These will be your
inputs to the acstools correction software.
We also have included files with the suffix ``cte_ref`` which are files we have
processed that you can use as test comparison data.
Viewing the Data
^^^^^^^^^^^^^^^^
`DS9 `_ is the tool most commonly used to
view FITS images. It is available at http://hea-www.harvard.edu/RD/ds9/.
About the Acstools Software
---------------------------
Acstools is a Python package containing the module PixCteCorr. The task
YCte in PixCteCorr is the primary function used to perform the CTE correction.
The actual work of calculating the corrections is done in C extensions called
by the Python routine. Python performs the file IO operations using
`pyfits `_.
For information about installing acstools see :ref:`installation_label`.
For information about running acstools see :ref:`run_label`.
Parallel Acstools
^^^^^^^^^^^^^^^^^
Because each column of data can be processed independent of the others it makes
sense to parallelize the correction so that it can be applied to many columns
at a time. The software currently applies the correction to each column one
at a time in a loop in the function ``FixYCte`` in the source file
``src/FixYCte.c``. The outer-most loop in the function loops over columns of
data.