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<div class="section" id="fits-distortion-proposal">
<h1>FITS Distortion Proposal<a class="headerlink" href="#fits-distortion-proposal" title="Permalink to this headline">ΒΆ</a></h1>
<p>The current FITS Distortion Paper conventions <a class="reference internal" href="appendix3.html#distortionpaper">[DistortionPaper]</a> provide a mechanism for specifying either a lookup table
or polynomial model for the distortion of each axis. The standard states in Section 2.1:</p>
<blockquote>
<div>Note that the prior distortion functions, <span class="math">\delta_p(p)</span>, operate on pixel coordinates (i.e.
<span class="math">p</span> rather than <span class="math">p-r</span>), and that the independent variables of the distortion functions
are the <em>uncorrected</em> pixel or intermediate pixel coordinates. That is, for example,
we do not allow the possibility of</div></blockquote>
<div class="math" id="equation-Equation 1">
<p><span class="eqno">(1)</span><span class="math">q'_{3} = q_{3} + \delta_{q_{3}}(q'_{1},q'_{2})</span></p>
</div><p>The keywords used for describing these corrections use the syntax given in Table 2 of the FITS Distortion Paper.
For our purposes, the keywords of interest are those related to lookup tables; namely,</p>
<div class="highlight-python"><pre>CPDISja string 2.4.1 distortion code new Prior distortion function type.
DPja record 2.4.2 distortion parameter new Parameter for a prior distortion
function, for use in an image header</pre>
</div>
<p>This syntax only provides the option to specify one correction at a time for each
axis of the image. This precludes being able to use this convention to specify both
a lookup table and a polynomial model at the same time for the same axis. It does not
state what should be done if the polynomial has been specified using a different
convention, for example, the SIP convention. Thus, SIP and FITS Distortion Paper should not be
seen as mutually exclusive. In fact, they may work together rather naturally since the
SIP and FITS Distortion Paper conventions both assume the corrections will work on the input pixel
and add to the output frame.</p>
<p>The sample header in <a class="reference internal" href="appendix1.html#appendix1"><em>Appendix 1 - Sample ACS/WFC Image</em></a> shows how these keywords get populated for
an actual reference file; specifically, an NPOLFILE as described in the next section.</p>
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