The Large Binocular Telescope (LBT) consists of two 8.4 m telescopes mounted on a common alt-az gimbal. The
telescope has various modes of operation, including prime-focus, bent- and direct-Gregorian modes. The telescopes can
feed independent instruments or their light can be combined in one of two interferometric instruments, giving an
interferometric baseline of over 22 m.
With all large telescopes, including the LBT, collimation models or modeled values for hexapod positions, are required
to maintain reasonable optical alignment over the working range of temperatures and telescope elevations. Unlike other
telescopes, the LBT has a highly asymmetric mechanical structure, and as a result the collimation models are required to
do a lot more "work", than on an equivalent aperture monocular telescope that are usually designed to incorporate a
Serurrier truss arrangement.
LBT has been phasing in science operations over the last 5 years, with first light on the prime-focus cameras in 2006,
and first light in Gregorian mode in 2008. In this time the generation of collimation models for LBT has proven to be
problematic, with large departures from a given model, and large changes in pointing, being the norm. A refined
approach to generating collimation models, "range balancing", has greatly improved this situation. The range-balancing
approach to generating collimation models has delivered reliable collimation and pointing in both prime focus and
Gregorian modes which has led to greatly increased operational efficiency. The details of the range-balancing approach,
involving the removal of pointing "contamination" from collimation data, are given in this paper.