VATT (starting summer of 2017) Uses a PI hexapod to precisely position the secondary mirror.

A full description of the coordinate system can be found here. I will summarize below.

The PI coordinate systems for the hexapod are created in a hierarchy. Each higher level adding to the lower level. THe lower levels should be permanent when the mount and orientation has been properly characterized.

Each level also has different possible types which can be user defined or left as the PI default. You can read the details of those types in the coordinate system manual

the hierarcies are: {lower} → {higher} Leveling → Orientational → Operating

Permanent corrections to the alignment of the hexapod

permanent Rotation of the XYZ axes if necessary

Described in detail below with sub types.

The center of rotation always lies at the origin of the tool coordinate system.

  X, Y and Z directions are always spatially fixed

  X, Y and Z directions are permanently connected to the moving
  platform of the hexapod ie it moves with the platform. 

  The operating coordinate system is the "highest level"
  meaning it inherits properties from the lower
  level coordinate systems ie Orientational and Leveling
  The operating coordinate system has two Types KSD and KSF

      The work and the Tool coordinate systems are aligned
      at ZERO position only

      The Work and the tool coordinate systems are aligned
      at the current position of the platform. Work Coordinate
      system stays at that position for any future moves ie
      you can move the work coordinate system away from
      the zero point to the current position.