Christine,
Tomorrow I will reverse a 50 onto another 50 and 105 to see the level of magnification and if there is vignetting how much. So hold on until then.
As I discovered this evening for an experiment of Daves set up, 50 reversed on a 200 gives huge magnifiction. 5 1/2 mm is all you will see in the long side of the viewfinder.
The pogram works well but I have only done staight stacks. I will post screen grabs of the route I took.
The prog is quite complicated so I just took the easy way out once I had found it.
Don