  We report final results from our 2.5 year infrared parallax program carried
out with the European Southern Observatory 3.5m New Technology Telescope and
the SOFI infrared camera. Our program targeted precision astrometric
observations of ten T type brown dwarfs in the J band. Full astrometric
solutions (including trigonometric parallaxes) for nine T dwarfs are provided
along with proper motion solutions for a further object. We find that
HgCdTe-based infrared cameras are capable of delivering precision differential
astrometry. For T dwarfs, infrared observations are to be greatly preferred
over the optical, both because they are so much brighter in the infrared, and
because their prominent methane absorptions lead to similar effective
wavelengths through the J-filter for both target and reference stars, which in
turn results in a dramatic reduction in differential colour refraction effects.
We describe a technique for robust bias estimation and linearity correction
with the SOFI camera, along with an upper limit to the astrometric distortion
of the SOFI optical train. Colour-magnitude and spectral-type-magnitude
diagrams for both L and T dwarfs are presented which show complex and
significant structure, with major import for luminosity function and mass
function work on T dwarfs. Based on the width of the early L dwarf and late T
dwarf colour magnitude diagrams, we conclude the brightening of early T dwarfs
in the J passband (the "early T hump") is not an age effect, but due to the
complexity of brown dwarf cooling curves. Finally, empirical estimates of the
"turn on" magnitudes for methane absorption in field T dwarfs and in young
stars clusters are provided. These make the interpretation of the T6 dwarf
sigma Ori J053810.1-023626 as a sigma Ori member problematic.
