Double-pyramid wavefront sensors: Tolerance relaxation and cheaper alternatives using achromatic double-roof prisms

Abstract

We present the current design of the double-pyramid planned for the TMT-NFIRAOS Visible Natural Guide Star WFS and its manufacture challenges to meet all the requirements in terms of tip size, pupil image quality and mapping on the detector pixels. We show that the angular tolerance of the pyramid can be relaxed from ±6 arcsec. To ±2 arcmin. Without significant impact on the AO performance, which is within the capabilities of most optical suppliers and reduces the fabrication cost by a factor 10 or more. Then we compare the pyramid design to a new concept using two double-roof prisms, optically equivalent to an achromatic double-pyramid and offering very similar performance. Roof prisms are easier to manufacture to tolerance than pyramids since there is no tip. We also present an adjustable version of the achromatic double-roof prism allowing very fine adjustments of the positions of the four pupils onto the detector pixels, which can relax the manufacture tolerance of the prisms even more.