Index: src/sas/qtgui/Perspectives/Invariant/media/invariant_help.rst
===================================================================
--- src/sas/qtgui/Perspectives/Invariant/media/invariant_help.rst	(revision 417c03f075685f86777838c8c717dec8e0ed8965)
+++ src/sas/qtgui/Perspectives/Invariant/media/invariant_help.rst	(revision abcac7ac5d81eb0564d33fca7e2f342abba88a16)
@@ -10,42 +10,53 @@
 -----------
 
-The scattering, or Porod, invariant ($Q^*$) is a model-independent quantity that
+The scattering, or Porod, invariant (:math:`Q^*`) is a model-independent quantity that
 can be easily calculated from scattering data.
 
 For two phase systems, the scattering invariant is defined as the integral of
-the square of the wavevector transfer ($Q$) multiplied by the scattering cross section
-over the full range of $Q$ from zero to infinity, that is
+the square of the wavevector transfer (:math:`Q`) multiplied by the scattering cross section
+over the full range of :math:`Q` from zero to infinity, that is
+
 
 .. math::
 
-    Q^* = \int_0^\infty q^2I(q)\,dq
+    Q^* = \int_0^ \infty q^2 I(q)\,dq
 
-in the case of pinhole geometry. For slit geometry the invariant is given by
+
+in the case of pinhole geometry (SAS).
+
+
+For slit geometry (USAS) the invariant is given by
 
 .. math::
 
-    Q^* = \Delta q_v \int_0^\infty qI(q)\,dq
+    Q^* =  \int_0^\infty \Delta q_v \, qI(q)\,dq
 
-where $\Delta q_v$ is the slit height.
+where :math:`\Delta q_v` is the slit height.
 
-The worth of $Q^*$  is that it can be used to determine the volume fraction and
-the specific area of a sample. Whilst these quantities are useful in their own
-right they can also be used in further analysis.
+The worth of :math:`Q^*` is that it can be used to determine the volume fraction
+and the specific area of a sample. Whilst these quantities are useful in their
+own right, they can also be used in further analysis.
 
-The difficulty with using $Q^*$  arises from the fact that experimental data is
-never measured over the range $0 \le Q \le \infty$. At best, combining USAS and
-WAS data might cover the range $10^{-5} \le Q \le 10$ 1/\ |Ang| . Thus it is usually
-necessary to extrapolate the experimental data to low and high $Q$. For this
+The difficulty with using :math:`Q^*`  arises from the fact that experimental
+data is never measured over the range :math:`0 \le Q \le \infty`. At best,
+combining USAS and WAS data might cover the range
+:math:`10^{-5} \le Q \le 10`|Ang|:math:`^{-1}`. Thus it is usually
+necessary to extrapolate the experimental data to low and high :math:`Q`.
+For this
 
-High-\ $Q$ region (>= *Qmax* in data)
+High-\ :math:`Q` region (>= *Qmax* in data)
 
-*  The power law function $C/Q^4$ is used where the constant
-   $C = 2 \pi \Delta\rho S_v$ is to be found by fitting part of data
-   within the range $Q_{N-m}$ to $Q_N$ (where $m < N$).
+*  The power law function :math:`C/Q^4` is used where the constant
+   :math:`C = 2 \pi \Delta\rho\, S_v` with
+   :math:`\Delta\rho`, the scattering length density (SLD) contrast and
+   :math:`S_v`, the specific surface area. The value of :math:`C` is to be found
+   by fitting part of data within the range :math:`Q_{N-m}` to :math:`Q_N`
+   (where :math:`m < N`), .
 
-Low-\ $Q$ region (<= *Qmin* in data)
+Low-\ :math:`Q` region (<= *Qmin* in data)
 
-*  The Guinier function $I_0 exp(-R_g^2 Q^2/3)$ where $I_0$
-   and $R_g$ are obtained by fitting as for the high-\ $Q$ region above.
+*  The Guinier function :math:`I(Q)=I(0) \exp (-R_g^2 Q^2/3)` where :math:`R_g`
+   is the radius of gyration. The values of :math:`I(0)` and :math:`R_g` are
+   obtained by fitting as for the high-\ :math:`Q` region above.
    Alternatively a power law can be used.
 
@@ -62,28 +73,48 @@
    the dataset into the *Invariant* panel.
 
-4) Use the *Customised Input* boxes on the *Invariant* panel to subtract
-   any background, specify the contrast (i.e. difference in SLDs - this must be
-   specified for the eventual value of $Q^*$  to be on an absolute scale), or to
-   rescale the data.
+.. image:: image_invariant_load_data.png
 
-5) Adjust the extrapolation range as necessary. In most cases the default
-   values will suffice.
+4) Use the *Customised Input* box on the *Options* tab to subtract any
+   background, specify the contrast (i.e. difference in SLDs - this must be
+   specified for the eventual value of :math:`Q^*` to be on an absolute scale),
+   or to rescale the data.
 
-6) Click the *Compute* button.
+5) Adjust the extrapolation range in the *Options* tab as necessary. In most
+   cases the default values will suffice.
 
-7) To include a lower and/or higher $Q$ range, check the relevant *Enable
+
+6) Click the *Calculate* button.
+
+7) To include a lower and/or higher :math:`Q` range, check the relevant *Enable
    Extrapolate* check boxes.
+
+   .. figure:: image_invariant_option_tab.png
+
+       ..
+
+       *Option tab of the Invariant panel.*
+
 
    If power law extrapolations are chosen, the exponent can be either held
    fixed or fitted. The number of points, Npts, to be used for the basis of the
-   extrapolation can also be specified.
+   extrapolation can also be specified in the related *Power* box(es).
 
-8) If the value of $Q^*$  calculated with the extrapolated regions is invalid, a
-   red warning will appear at the top of the *Invariant* panel.
+   .. figure:: image_invariant_outplot_plot.png
+       :width: 300pt
 
-   The details of the calculation are available by clicking the *Details*
-   button in the middle of the panel.
+       ..
 
-.. image:: image005.png
+       *Output plot generated after calculations.*
+
+8) If the value of :math:`Q^*` calculated with the extrapolated regions is
+   invalid, the related box will be highlighted in red.
+
+   The details of the calculation are available by clicking the *Status*
+   button at the bottom of the panel.
+
+
+   .. image:: image_invariant_details.png
+      :width: 300pt
+
 
 .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
@@ -95,5 +126,5 @@
 ^^^^^^^^^^^^^^^
 
-The volume fraction $\phi$ is related to $Q^*$  by
+The volume fraction :math:`\phi` is related to :math:`Q^*`  by
 
 .. math::
@@ -101,5 +132,5 @@
     \phi(1 - \phi) = \frac{Q^*}{2\pi^2(\Delta\rho)^2} \equiv A
 
-where $\Delta\rho$ is the SLD contrast.
+where :math:`\Delta\rho` is the SLD contrast.
 
 .. math::
@@ -112,5 +143,5 @@
 ^^^^^^^^^^^^^^^^^^^^^
 
-The specific surface area $S_v$ is related to $Q^*$  by
+The specific surface area :math:`S_v` is related to :math:`Q^*`  by
 
 .. math::
@@ -118,5 +149,5 @@
     S_v = \frac{2\pi\phi(1-\phi)C_p}{Q^*} = \frac{2\pi A C_p}{Q^*}
 
-where $C_p$ is the Porod constant.
+where :math:`C_p` is the Porod constant.
 
 .. ZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZZ
