Index: /trunk/doc/psphot/sdss.summary.txt
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+
+The Legacy survey is restricted
+to high latitudes, and \photo\  is adequate everywhere
+there.  But at lower latitudes, when the density of stars brighter
+than $r=21$ grows above 5000 deg$^{-2}$, the pipeline
+is known to fail, due to a combination of being unable to find
+sufficiently isolated stars to measure an accurate PSF and limitations
+of the deblender.   Many of the SEGUE scans probe these low
+latitudes (Figure~\ref{fig:skydist}), and we therefore adapted an
+alternative stellar photometry code developed by the Pan-STARRS team
+(Kaiser 2002) to be used for these runs.
+
+Magnier (2006) has developed this code, PSPhot, to be used for the
+Pan-STARRS project; like, e.g., DOPHOT {\bf ref}, it begins with the
+assumption that every object is unresolved, and therefore does a
+better job in crowded stellar regions.  It uses an analytical model as
+to describe the basic PSF shape, with parameters which may vary across
+the field of the image to follow the PSF variations.  The software may
+use any of several functions to describe the radial profile (Gaussian;
+polynomial Gaussian approximation; and other power-law models), all of
+which are implemented as a 2D model using an elliptical contour.  The
+{\bf ??} model was used for the SEGUE scans.
+
+In addition to the analytical model, PSPhot uses a pixel-based
+representation of the residuals between the PSF objects and the
+analytical model, with the residual pixel contribution also varying
+across the field of the image.
+
+Candidate PSF stars are selected from the collection of bright objects
+in the frame by examining the distribution of the second moments of
+the objects and searching for a tight clump representing the PSF.  The
+candidate stars are fitted independently to the PSF model function,
+with all of the parameters fitted.  Poorly fitting objects are then
+excluded, and the fit parameters for the remaining objects are used to
+constrain the 2D variations in the PSF model.
+
+Note that unlike \photo, the code processes each frame separately
+(without any requirement of continuity of PSF estimation across frame
+boundaries), and each filter separately (without any requirement that
+the list of objects between the separate filters agree).  The pipeline
+outputs positions and PSF magnitudes (and errors) for each detected
+object.  The resulting photometry is then matched between filters
+using a $1^{\prime\prime}$ {\bf ??} radius.