What is the principle behind schuster focussing? Schuster Focussing is often used to focus the collimator and the telescope for a parallel beam of light. What is the underlying principle behind the procedure? How is it related to the fact that 2 angle of incidences correspond to the same angle of deviation?
 A: Let the angle of incidence at minimum deviation be $i_{\rm min}$.  
For a prism the graph of angle of deviation against angle of incidence looks something like this.  
 
A key feature of this graph is that it is asymmetric about the angle of incidence for minimum deviation.  
If the collimator is not set to produce a parallel beam of light then when the light hits the prism it will do so with a range of angles of incidence  $\Delta i$.
If the angles of incidence are less than $i_{\rm min}$ then the beam which emerges from the prism is more divergent than the incident beam.
Whereas if the angles of incidence are greater than $i_{\rm min}$ then the beam which emerges from the prism is less divergent than the incident beam.  
Consider the difference in focussing the telescope on a beam which is more divergent than the incident beam (angles of incidence less than $i_{\rm min}$ and focussing the telescope on a beam which is less divergent than the incident beam (angles of incidence greater than $i_{\rm min}$.
Which moves the telescope to being focussed closer to receiving parallel light?
It must be the second case where the angles of incidence are greater $i_{\rm min}$ than on a beam which is more divergent than when the incident beam (angles of incidence less than $i_{\rm min}$.  
So that is where you start from.  
Set the prism so that the angle of incidence is greater than $i_{\rm min}$.  
This will cause the light which has come through the collimator and passed through the prism to diverge less than the light that entered the prism ie it is closer to being parallel.  
Adjust the telescope to focus this divergent beam of light so that the image of the collimator slit is sharp.
You are actually focussing on an image of the collimator slit where the diverging rays meet when back produced.  
Now move the prism so that the angle of incidence is less than $i_{\rm min}$.  
This will cause the light which has come through the collimator and passed through the prism to diverge more ie be further from being parallel.  
Adjust the collimator so that the image of the collimator slit is sharp.
This will adjust the collimator to producing light which is closer to being parallel.  
Repeating the procedure will adjust the telescope to being focussed to receive light which is nearer and nearer to being parallel and the collimator to be adjusted to produce light which is closer and closer to being parallel.
