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<li><a class="reference internal" href="#">Non-polynomial Residual Correction</a><ul>
<li><a class="reference internal" href="#npolfile-reference-file-format">NPOLFILE reference File Format</a></li>
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<div class="section" id="non-polynomial-residual-correction">
<h1>Non-polynomial Residual Correction<a class="headerlink" href="#non-polynomial-residual-correction" title="Permalink to this headline">¶</a></h1>
<p>ACS and WFPC2 images used the DGEOFILE reference file to specify the residual
correction in X and Y for each and every pixel in each chip of the observation. These
DGEOFILE reference fiels required up to 168Mb each to cover all chips of each camera
for ACS/WFC images.
Distortion residuals have been always been calibrated for ACS by looking at the average
correction that needs to be applied over each 64x64 pixel section of each chip after applying
the polynomial coefficients correction. This would normally result in a 64 x 32 array of
residuals for each 4096 x 2048 chip. These arrays get expanded by one value in each
dimension to support interpolation all the way to the edge of each chip resulting in 65 x 33 arrays of
distortion correction data.</p>
<div class="figure align-center" id="figure1">
<img alt="ACS/WFC F475W NPOLFILE corrections" src="_images/npol_vector_text.png" style="width: 95%;" />
<p class="caption">This figure illustrates the corrections included in the ACS/WFC F475W non-polynomial
distortion correction included in the new NPOLFILE reference file. Each vector represents
the correction for a 64x64 pixel section of each chip.</p>
</div>
<p>These look-up tables follow the conventions
in the WCS FITS Distortion Paper <a class="reference internal" href="appendix3.html#distortionpaper">[DistortionPaper]</a>.
Record-valued keywords are used to map an image in the science extension
to a distortion array in the <tt class="docutils literal"><span class="pre">WCSDVAR</span> <span class="pre">extension</span></tt>. This new type of FITS keywords has been
implemented in PyFITS and is fully described in <a class="reference internal" href="appendix3.html#distortionpaper">[DistortionPaper]</a>. Specifically, <tt class="docutils literal"><span class="pre">DPj.EXTVER</span></tt> in the science
extension header maps the science image to the correct <tt class="docutils literal"><span class="pre">WCSDVAR</span></tt> extension. The dimensionality
of the distortion array is defined by <tt class="docutils literal"><span class="pre">DPj.NAXES</span></tt>. Keywords <tt class="docutils literal"><span class="pre">DPj.AXIS.j</span></tt> in the <tt class="docutils literal"><span class="pre">SCI</span></tt> extension
header are used for mapping image array axis to distortion array axis. In the keywords above j
is an integer and denotes the axis number. For example, if distortion array axis 1 corresponds
to image array axis 1 of a <tt class="docutils literal"><span class="pre">SCI</span></tt> extension, then <tt class="docutils literal"><span class="pre">DP.1.AXIS.1</span></tt> = 1.
A full example of the keywords added to a <tt class="docutils literal"><span class="pre">SCI</span></tt> extension header is presented in <a class="reference internal" href="appendix1.html#appendix1"><em>Appendix 1 - Sample ACS/WFC Image</em></a>.</p>
<p>A complete description of the conversion of the DGEOFILE reference data into NPOLFILE reference
files can be found in the report on the <tt class="docutils literal"><span class="pre">npolfile-tsr</span></tt>.</p>
<div class="section" id="npolfile-reference-file-format">
<h2>NPOLFILE reference File Format<a class="headerlink" href="#npolfile-reference-file-format" title="Permalink to this headline">¶</a></h2>
<p>With the goal of including all distortion reference information directly in the
science image’s FITS file, including the full 168Mb DGEOFILE for ACS/WFC images
would more than double the size of each input image. A new reference
file based on the sub-sampled calibrations, though, would be small enough to serve as the
basis for a new reference file while also being a more direct use of the calibration
data. This new reference file has been called <strong>NPOLFILE</strong> in the FITS image header,
so that any original DGEOFILE reference filename can be retained in parallel for
backwards compatibility with the current software. This reference file also
has a unique suffix, <strong>_npl.fits</strong>, as another means of identifying it as a new
reference file separate from the current DGEOFILE files. The header for this new
reference file also remains very simple, as illustrated in <a class="reference internal" href="appendix2.html#appendix2"><em>Appendix 2 - NPOLFILE Example</em></a>.</p>
<p>Applying these corrections starts by reading the two 65 x 33
arrays into memory with each input ACS/WFC chip WCS (one for
X offsets and one for Y offsets). Bi-linear interpolation based on the input pixel
position then gets used on-the-fly to extract the final offset from this reference
file. Initial versions of these sub-sampled NPOLFILE reference files for ACS have
been derived from the current full-size DGEOFILEs, and testing indicates residuals
only on the order of 0.02 pixels or less remain when compared to the original calibration.</p>
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