Index: trunk/doc/modules/ModulesSDRS.tex
===================================================================
--- trunk/doc/modules/ModulesSDRS.tex	(revision 2649)
+++ trunk/doc/modules/ModulesSDRS.tex	(revision 2818)
@@ -1,3 +1,3 @@
-%%% $Id: ModulesSDRS.tex,v 1.25 2004-12-07 21:29:43 price Exp $
+%%% $Id: ModulesSDRS.tex,v 1.26 2004-12-24 02:30:50 price Exp $
 \documentclass[panstarrs]{panstarrs}
 
@@ -992,5 +992,5 @@
 \code{inputs} before rejection is performed.  If \code{zero} is
 non-\code{NULL} and \code{applyZeroScale} is false, then the values
-shall only be used in calculating the noise.
+shall only be used in calculating the Poisson variances.
 
 If the \code{scale} vector is non-\code{NULL} and
@@ -998,5 +998,6 @@
 shall multiply the \code{inputs} before rejection is performed.  If
 \code{scale} is non-\code{NULL} and \code{applyZeroScale} is false,
-then the values shall only be used in calculating the noise.
+then the values shall only be used in calculating the Poisson
+variances.
 
 The purpose of \code{applyZeroScale} is to allow combination of fringe
@@ -1009,5 +1010,25 @@
 If the \code{gain} and \code{readnoise} are positive and non-negative
 (respectively), then these shall be used to provide weights for the
-combination using Poisson statistics.
+combination using Poisson statistics ($\sigma_i$ below).
+
+In summary, pixels corresponding to the same physical pixel are
+combined, having values $x_i \pm \sigma_i$.  In the case that
+\code{applyZeroScale} is \code{true}, then:
+\begin{eqnarray}
+x_i & = & s_i f_i + z_i \\
+\sigma_i & = & [g x_i + r^2]^{1/2} / g
+\end{eqnarray}
+Where $f_i$ is the value of the pixel in image $i$, $s_i$ is the scale
+applied to image $i$, $z_i$ is the zero offset applied to image $i$,
+$g$ is the gain, and $r$ is the read noise.  If scales are not
+provided, they are set to unity; if zero offsets are not provided,
+they are set to zero.
+
+If \code{applyZeroScale} is \code{false}, then the values are:
+\begin{eqnarray}
+x_i & = & f_i \\
+\sigma_i & = & [g (s_i f_i + z_i) + r^2]^{1/2} / g
+\end{eqnarray}
+where the same symbols are used as above.
 
 The \code{inputs, zero} and \code{scale} may be of S16, S32 and F32