Add text about mixing models

docs/conf.py

@@ -139,7 +139,7 @@
 #html_split_index = False
 
 # If true, the reST sources are included in the HTML build as _sources/<name>.
-#html_copy_source = True
+html_copy_source = False
 
 # If true, an OpenSearch description file will be output, and all pages will
 # contain a <link> tag referring to it.  The value of this option must be the

docs/mixing.txt

@@ -0,0 +1,56 @@
+Mixing models
+----------------------
+The source model created in :mod:`deproject` is just a special case of the
+general class of mixing models where::
+
+ source_model[i] = Sum_j(M[i,j] * model_component[j])
+ M[i,j] = mixing matrix
+
+The Sherpa model language implementation allows definition of models using
+normal python addition and multiplication operators (+ and *) on model
+component objects.  This is very powerful because it means that 
+complex models can be created using normal programming syntax.  For example::
+
+ create_model_component('xswabs', 'gal_abs')  # galactic absorption
+ create_model_component('xszwabs', 'zabs')  # galactic absorption
+ create_model_component('xsmekal', 'mekal')  # galactic absorption
+ create_model_component('gauss1d', 'line')  # galactic absorption
+ create_model_component('gauss1d', 'line2')  # galactic absorption
+
+ source_intr = zabs * mekal + line
+ source_obs = gal_abs * source_intr
+
+Expanding on this and allowing for mixing via matrix multiplication, one could
+generate a cluster deprojection model that allows for variable absorption in
+each annulus with something like the following::
+
+ dep = Deproject(radii)
+ n_annuli = len(radii)-1
+ n_shell = dep.nshell
+
+ # Make a normal Sherpa absorber model called gal_abs
+ create_model_component('xswabs', 'gal_abs')  
+
+ # Make a stack of redshifted absorption models for each annulus
+ cluster_abs = ModelStack('xszwabs', n_annuli)
+ cluster_abs.redshift = 0.5
+
+ # Make a stack of thermal emission models for each shell
+ cluster_mekal = ModelStack('xsmekal', n_shell)
+
+ # Make the mixing model as volume normalization matrix * emission model stack
+ cluster_emission = dep.vol_norm * cluster_mekal
+
+ # Account for different absorption in each annulus by multiplying
+ # (element by element) the two ModelStacks
+ cluster_emission_absorbed = cluster_abs * cluster_emission
+
+ # Create final observed model after galactic absorption.  
+ cluster_observed = gal_abs * cluster_emission_absorbed
+
+This would make a stack of source models that can then be used to fit a stack
+of data.
+
+Full design and implementation of this concept is in work.  Interested parties
+should contact the author for updates or to provide comments.
+