| Abstract | NFIRAOS, the adaptive optics system of TMT, uses a pyramid wavefront sensor in addition to the 6 LGS WFSs. This pyramid WFS, referred to as Visible NGS WFS (VNW) reach its final design and is now ready for fabrication. One of the high-risk items is the custom achromatic double-pyramid prisms that are very challenging to manufacture. Fortunately, for NFIRAOS we were able to relax the angular tolerance of the pyramid to 2 arcmin. First, this paper lists the main optical specifications and tolerances of the double-pyramid. Then, we present the fabrication process developed by BMV Optical Technologies, our supplier, who successfully produced four double-pyramids. Finally, we describe the sequence of tests we performed at NRC during the incoming inspections; including a new optical test we developed to quickly verify the angular tolerance of the delivered double-pyramid prisms. This is a simple, accurate and robust test that doesn’t require any special lab equipment other than a pinhole source and a stock lens in front of the pyramid, and it is insensitive to lens imperfections and misalignment errors. We call this test the “Metafocus Test” as it makes use of a so-far unknown interesting optical property of the double-pyramid, for which the four exiting beams intercept into a single point. Any angular error on the faces of the pyramid will split the metafocus up to four spots. The as-built apex and azimuth angles, as well as assembly errors (wedge and clocking of front pyramid w.r.t. rear pyramid) can be derived from the measurement of the relative position of each spot. |
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