What is Optical Metrology?

Due to the mirror arrangement in the GMT, each mirror has to be highly aspheric–that is, the mirror’s face has a steeply curved slope. One side of the mirror is thicker than the other, differing by as much as 14 mm.

The difficulty of shaping each mirror segment is compounded by the fact that these large segments must have the exact same curvature in order to perform together. To achieve the stringent demand for accuracy, each mirror is periodically taken off of the polishing machine and carefully measured. The results of these measurements, in turn, guide the polishing program as it progresses.

An 82-foot high optical tower is used to test each mirror to extremely high precision.

Technician inside optical tower at Mirror Lab.

Computer holograms help measure complex optical surfaces.

The principal test to compensate for the unprecedented asymmetry of the mirror's surface is conducted using a laser interferometer. Instead of measuring the mirror directly, a beam is first bounced off of two mirrors at oblique angles, and then passed through a computer generated hologram. With the aspheric departure removed, interferometric surface measurements may be taken.

In theory, this is complicated, and in practice it is even more difficult. One of the bounce-off mirrors, called fold spheres, must be quite large: 3.8 meters in diameter. And, it must be positioned a considerable distance away from the primary mirror segment. The Test Tower protruding above the mirror lab actually had to be rebuilt to support this large fold sphere and accommodate the width of the interferometer beam.

Three additional methods are used to verify the mirror's shape and direct the polishing activity:

For information regarding the science and implementation of mirror metrology, refer to the following publications: