Index: trunk/doc/pslib/psLibSDRS_Astrom.tex
===================================================================
--- trunk/doc/pslib/psLibSDRS_Astrom.tex	(revision 2502)
+++ trunk/doc/pslib/psLibSDRS_Astrom.tex	(revision 3069)
@@ -694,2 +694,62 @@
 \end{verbatim}
 Calculate the parallax factor for the given exposure.  \tbd{Why do we need this?}.
+
+
+%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
+
+\subsection{Astrometry and World Coordinate System}
+
+The FITS World Coordinate System (WCS) headers are commonly employed
+with astronomical images in order to relate pixels to celestial (or
+otherwise) coordinates.  Since it is a FITS standard, we must be able
+to read and write from WCS into our internal format.  For the time
+being, we will consider only celestial WCS (i.e., no spectral
+wavelength calibrations, etc).  Because WCS does not support the
+multiple layers that we have built for \PS{}, we will use a simple
+internal representation: a transformation, which handles any
+distortions (i.e., goes directly from the coordinate frame of the
+image to the tangent plane); and the projection.
+
+The FITS WCS standard is described in Calabretta \& Greisen, A\&A,
+2002, 395, 1077.  An extension to the standard, the TNX projection, is
+described at \url{http://iraf.noao.edu/projects/ccdmosaic/tnx.html}.
+This extension is in widespread use and must also be supported for
+both reading and writing.
+
+\begin{verbatim}
+bool psAstrometryReadWCS(psPlaneTransform **transform, // Output transformation
+                         psProjection **projection, // Output projection
+                         psMetadata *header // Input FITS header
+                         );
+bool psAstrometryWriteWCS(psMetadata *header, // Output FITS header
+                          psPlaneTransform *transform, // Input transformation
+		          psProjection *projection // Input projection
+                          );
+bool psAstrometrySimplify(psPlaneTransform **transform, // Output transformation
+                          psProjection **projection, // Output projection
+			  psCell *cell // Cell for which to generate transform and projection
+                          );
+\end{verbatim}			
+
+\code{pmReadAstrometry} shall parse the specified FITS \code{header},
+returning new instances of the \code{transform} and \code{projection}
+that represent the WCS.  The function shall return \code{true} if it
+was able to successfully generate the outputs; otherwise it shall
+return \code{false}.
+
+\code{pmWriteAstrometry} shall add WCS keywords to the supplied FITS
+\code{header} that implement the given \code{transform} and
+\code{projection}.  The function shall return \code{true} if it was
+able to successfully generate the output; otherwise it shall return
+\code{false}.
+
+\code{pmSimplifyAstrometry} shall take a \code{cell} and simplify the
+internal astrometric representation (\code{cell->toFPA} or equivalent,
+\code{cell->parent->parent->toTangentPlane} and
+\code{cell->parent->parent->grommit}) to a single \code{transform} and
+\code{projection}.  This allows the subsequent use of
+\code{pmWriteAstrometry} in the case that we have only the
+multi-layered \PS{} internal astrometric representation.  The function
+shall return \code{true} if it was able to successfully generate the
+output; otherwise it shall return \code{false}.
+