Focal plane position : The power in the derotator ensures that the
focal plane position is close to that of Nasmyth focus direct.
The range of focus position is the same as for the Nasmyth focus direct,
however the derotator optics causes vignetting on-axis if the telescope
focus is moved more than 360 mm beyond the nominal position.
On the GHRIL optical table, the nominal focal plane is 150 mm from the
of the table with the optical axis 300 mm in from the side and at a height
of 150 mm above the table.