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@@ -108,6 +108,7 @@ pyrateoptics.listOptimizableVariables(opticalsystem, max_line_width=75)
 pyrateoptics.draw(opticalsystem, rays)
 # This function can get a list of raypath argument to draw rays.
 ```
+# Some Impressions
 
 ![grin_1](https://user-images.githubusercontent.com/12564815/40586439-3adda2f0-61c2-11e8-89b7-ee88b96ff7d4.png)
 ![grin_2](https://user-images.githubusercontent.com/12564815/40586440-3b027a80-61c2-11e8-89b3-01572ace57e3.png)
@@ -144,7 +145,50 @@ Grin material with coordinate system tilted against global coordinate system.
 Water droplet which bends the light of different wavelengths differently in order to create a rainbow.
 The material data was obtained from https://refractiveindex.info by Mikhail N. Polyanskiy.
 
-## Formulas
+# Frequently Asked Questions
+
+## I know of another raytracer project, why not combining forces?
+
+Our goal is to provide a very general raytracer with arbitrary coordinate systems and nearly arbitrary materials and we want to see how far we could go this route.
+
+This implies also some conceptual problems on the road which are difficult to solve and therefore our development is far slower than it could be.
+
+This also implies that we cannot properly throw together code with projects where the goal is "to just get this
+raytracer done and useable". At the end of the day our raytracer should also be useful, but maybe our goal is not compatible with other raytracer projects.
+
+Nevertheless: we are always interested in contributions, questions, issues and if somebody implemented cool stuff or is interested in our stuff, we are happy to provide interfaces or code.
+
+https://github.com/mess42/pyrate/issues/142
+
+## What about the FreeCAD interface?
+
+The FreeCAD interface has more or less the status of a mockup and is broken since the last large refactoring in 2017. Until now there is no good idea about getting a not-so-tight coupling between FreeCAD classes and pyrate classes. Since the API is not quite settled it is really difficult to start with a full fledged FreeCAD interface without rewriting all the stuff when there is a slight API change appearing. All ideas here are appreciated. If you are interested in digging into the matter, please have a look in the `freecad/PyrateWorkbench` directory.
+
+https://github.com/mess42/pyrate/issues/136
+https://github.com/mess42/pyrate/issues/134
+https://github.com/mess42/pyrate/issues/91
+
+## How does the optimizer work?
+
+The optimizer decomposes in backend and frontend whereas the backend determines which optimization algorithm is used and the frontend determines which meritfunction or parameters to be used.
+
+There were several discussions on that in the following issues:
+
+https://github.com/mess42/pyrate/issues/140
+https://github.com/mess42/pyrate/issues/137
+
+## How to create a full sphere lens?
+
+Since the `Conic` surface solves the ray intersection equation analytically and the two intersection points in a sphere correspond to the two roots of the quadratic equation, one cannot properly use two subsequent `Conic`s and expect a proper raytracing. In issue XX and our own research, we came up with a few workarounds:
+
+* use a slightly deformed second surface (introduce radius deviation or slight conic const) -> this is no optimal solution when you need an exact sphere
+* use the `Asphere` type surface to avoid the quadratic analytical solution problem and use a numerical solver instead
+* use an intermediate plano surface (this is the most reliable solution without any downsides)
+
+https://github.com/mess42/pyrate/issues/149
+https://github.com/mess42/pyrate/issues/50
+
+# Formulas
 
 If you want to use formulas in issues or wikis, then the following example would be useful: