|
Andrew, the offset happens because the light cone from the primary mirror is getting smaller as it comes up the tube, and the top and bottom ends of the diagonal mirror intercept it at different points.
So, if you can imagine it, the light cone is larger where it hits the bottom of the diagonal than where it hits the top of the diagonal.
The offset moves the diagonal across and down a bit to correct for this, so that it intercepts the light cone in the right place to catch all of it evenly.
Draw it on a piece of graph paper and it'll make sense.
cheers, Bird
|