Technology Nugget—APS
  Gary Chanan

The Alignment and Phasing System, or APS, is the TMT instrument that is responsible for the precision optical alignment of the telescope. Perhaps its most challenging task is the proper phasing of the segments: in order for the TMT to achieve its ultimate theoretical resolution, the 492 segments of its primary mirror must form a continuous optical surface, with the steps between adjacent segments no larger than 6 nanometers (nm) high, or less than one one-hundred-millionth of a segment diameter. By contrast, a poorly phased TMT would have an angular resolution no better than that of a 1.2 meter telescope!

The phasing technique that the APS will use is a variation of Young's famous two slit diffraction experiment, familiar to elementary physics students. In this case the two "slits" are the two halves of a small circular aperture which straddles two adjacent segments. If the segments are in phase, one obtains a circular diffraction pattern, but if there is a phase difference between the two segments, the pattern is more complicated. The figure shows simulated APS diffraction images. In the upper left panel the segments are in phase, and in each successive panel the step between segments has been increased by an additional 40 nm, or λ/22, where the wavelength λ is 891 nm. Note that in general the simple in-phase pattern splits and forms two separate peaks whose relative intensity varies with the phase difference. The seventh picture (not shown) is the mirror image of the fifth; the eighth is a mirror image of the fourth, and so on. After 11 steps, the segments are half a wave apart, so the optical path difference, which doubles on reflection, is a full wave, and the pattern repeats. The practiced eye can easily estimate the phase to the nearest half-picture, or about 20 nm. Of course the computer can do this correlation much faster and better than the practiced eye, and we are confident that the phasing goal of 6 nm can be achieved.

The APS will contain a "fly's eye" lens—actually an array of tiny lenslets, which will enable it to record the diffraction patterns corresponding to all 1386 intersegment edges simultaneously, so that all segments can be phased in parallel. Based on experience with the 10-meter segmented Keck Telescopes in Hawaii, we expect be able to phase the TMT in about an hour.