The determination of radius of curvature (ROC) of optics typically uses either a phase measuring interferometer on an adjustable stage to determine the position of the ROC and the optics surface under test. Alternatively, a spherometer or a profilometer are used for this measurement.

The difficulty of this approach is that for large optics, translation of the interferometer or optic under test is problematic because of the distance of translation required and the mass of the optic. Profilometry and spherometry are alternative techniques that can work, but require a profilometer or a measurement of subapertures of the optic. The proposed approach allows a measurement of the optic figure simultaneous with the full aperture radius of curvature.

The steps required for this measurement are:

  • Alignment of the phase measuring interferometer with the optic under test. For a spherical optic, a transmission sphere that overfills (faster f#) the optic is used.
  • The power is nulled by translating the optic under test (OUT) or the interferometer. At this point, the transmission sphere focus is the radius of curvature of the OUT.
  • An adjustable mount near the focus of the transmission sphere has a magnetic nest for placing a laser tracker retrotarget. This is a spherical target with a cube corner inset co-aligned with the center of the sphere. These devices are available commercially.
  • The laser tracker target is then placed in the calibration nest of the laser tracker, zeroed, and hand-carried to the position of the laser tracker nest near the interferometer. The laser tracker beam must be continuously locked to the tracker to stay in lock. This is a typical mode of the laser tracker used for absolute metrology.
  • The spherical laser tracker is positioned so that the shiny (non-retro) surface of the tracker target is aligned to the transmission sphere. It is then translated until a nulled interferogram is observed. At this point, the center of the laser tracker target is at the focus of the transmission sphere (and thereby at the ROC of the OUT.)
  • The x,y,z position of the tracker target is then acquired by the laser tracker.
  • The laser tracker target is then removed from the nest, and without losing lock to the tracker, is hand-carried to the OUT. It is then placed on the OUT at its center (which has a nest on its surface pre-aligned to the center).
  • The tracker then acquires the x,y,z position of the laser tracker target.
  • The data is reduced and the two positions calculated. The position at the ROC is directly at the ROC, while the position on the mirror is displaced by the radius of the laser tracker target.

This radius must be added to the distance measurement in the calculation.

This process is repeated to allow redundancy of the measurement.

This work was done by John Hagopian and Joseph Connelly of Goddard Space Flight Center. GSC-15941-1

Photonics Tech Briefs Magazine

This article first appeared in the April, 2011 issue of Photonics Tech Briefs Magazine.

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