  A prototype particle tracking telescope has been constructed using Timepix
and Medipix ASIC hybrid pixel assemblies as the six sensing planes. Each
telescope plane consisted of one 1.4 cm2 assembly, providing a 256x256 array of
55 micron square pixels. The telescope achieved a pointing resolution of 2.3
micron at the position of the device under test. During a beam test in 2009 the
telescope was used to evaluate in detail the performance of two Timepix hybrid
pixel assemblies; a standard planar 300 micron thick sensor, and 285 micron
thick double sided 3D sensor. This paper describes a detailed charge
calibration study of the pixel devices, which allows the true charge to be
extracted, and reports on measurements of the charge collection characteristics
and Landau distributions. The planar sensor achieved a best resolution of 4.0
micron for angled tracks, and resolutions of between 4.4 and 11 micron for
perpendicular tracks, depending on the applied bias voltage. The double sided
3D sensor, which has significantly less charge sharing, was found to have an
optimal resolution of 9.0 micron for angled tracks, and a resolution of 16.0
micron for perpendicular tracks. Based on these studies it is concluded that
the Timepix ASIC shows an excellent performance when used as a device for
charged particle tracking.
