Manufacturing, testing, and metrology of axi-symmetric circular phase masks for stellar coronagraphy

Abstract

Stellar coronagraphs using circular phase masks are promising concepts dedicated to the image suppression of an observed bright star in order to enhance the substellar mass companions present in its vicinity, typically 2 λ/D angular separation. These concepts include a focal plane phase mask which introduces a phase delay on a part of the stellar image. With an adequate choice of the mask parameters (thickness, diameter), the light going through the mask and the light going outside the mask will interfere destructively inside the geometric pupil in the following pupil plane. The light rejected outside this re-imaged pupil will be blocked by a Lyot stop. Typically, the mask physical size is about λF, where F denotes the f-number of the optical system, and the mask thickness depends on the required phase shifting. The contrast provided by these concepts is highly related to the quality of thickness profile of the phase mask and therefore, severe manufacturing tolerances are necessary to reach the theoretical performance of the corresponding coronagraphic system. In 2007, we designed a Roddier & Roddier phase mask with a 65 μm diameter and ordered it to GEPI of Paris Observatory which manufactured it using ion etching process. A roughness of 0.8 nm rms and a transition width of 1% of the mask diameter were measured with a profilometer for this mask showing the good quality of the mask (N'Diaye et al. 2010). We pursue our efforts to design and manufacture high quality masks in collaboration with the firm SILIOS. Several tests of manufacturing procedures are currently realized to reach the best trade-off between mask roughness and mask transition width. These values, measured in our laboratory with a profilometer, allow us to determine the best configuration for fabrication. In addition, by knowing the mask profile, we can estimate theoretically the performance that can be reached.