A tunable achromatic phase mask for a stellar coronagraph

摘要

A nulling Phase Mask has been designed that is achromatic over a relatively wide wavelength range. The mask is made using a combination of Fused Silica and Silicon, resulting in a device that matches the needed dispersion to achieve a pi phase shift over a range of wavelengths. Used in a stellar coronagraph, this device could possibly produce on-axis nulls in excess of 60dB with a bandwidth of over 132nm, or 50dB nulls with a bandwidth of over 230nm. The design integrates a simple method of tuning the depth of the null to match observing conditions and bandwidth requirements, while also moving the center of the nulling band slightly. Such tuning capability allows for a dramatic simplification of the manufacturing tolerances, thereby substantially reducing the likely cost to manufacture. Tuning is accomplished by placing the mask in a chamber with a gas that has an accurately controllable pressure. This mask works in a similar manner whether the geometry is of the four-quadrant type or half-half type, but does not apply to circular phase masks.