Prusa Mendel Adjustable Z-opto end stop

I found that it was quite difficult to accurately adjust the Z-opto end stop on my Prusa. Once I got to the point of having to move it up or down by 0.1mm I was oscillating between the correct point for quite a while before I got it right.

So this design is for an end stop with an adjusting screw. Now I can get the setting almost right, then use the screw to fine tune it.