The 4DAD, Four Dimensions Alignment Device was developed in the frame of the project 4LGSF (4 Laser Guide Star
Facility) for the AOF (Adaptive Optics Facility) on the ESO VLT (Very Large Telescope). It is used to accurately
align with respect to kinematic interfaces and mirror alignment targets, a 20 W-class laser source in lateral and angular
directions. 4DAD is of size 25x25x15cm and is based on a commercial CCD beam profiler.
The incoming beam is highly attenuated at 589 nm and split in two parts through a set of beam conditioning optics. One
beam is directed onto the detector revealing lateral movements whereas the other part is refocused to reflect angular
changes in the incoming beam. Both lateral and angular beam positions are simultaneously recorded as pixel coordinates
on the single CCD.
The distinctive feature of 4DAD is its metrology, i.e. the pixel reference coordinates are recorded through the cross
projection from a conventional sighting telescope. The device is capable of being used with a low-power white light
sighting telescope (alignment reference) as well as with a high power laser beam (to be aligned) without any optical
Mounted on a reference frame, it can align all 4LGSF laser units beams, giving them equal optical interface and
The sighting telescope is the main alignment equipment for the 4LGSF, thus all sub-systems, including 4DAD are
aligned, merged, using a simple, single, practical and well-known alignment instrument.
The alignment strategy, the design, and results of 4DAD are presented.