Method for figure control of optical surfaces

A method for figuring an optical surface of an optical element to achieve a target profile for the optical surface, the method comprising:

• a. applying a removal process to an extended region of the optical surface extending along a first direction to remove material from the extended region of the optical surface;
• and
• b. adjusting a position of the optical surface relative to the removal process along a second direction perpendicular to the first direction to remove material from additional extended regions of the optical surface extending along the first direction at each of different positions of the optical surface along the second direction,
• c. wherein, during the applying of the removal process and the adjusting of the optical surface relative to the removal process, the optical surface has a first rotational orientation about a third direction perpendicular to the first and second directions, and
• d. wherein the method further comprises:
• repeating the applying of the removal process and the adjusting of the optical surface relative to the removal process for each of one or more additional rotational orientations of the optical surface about the third direction to achieve the target profile of the optical surface,
• wherein the removal process is an etching process wherein at least part of the optical surface is immersed into an etching bath,
• wherein the adjusting of the optical surface relative to the removal process comprises immersing the optical surface into the etching bath along the second direction,
• wherein immersing the optical surface into the etching bath along the second direction comprises varying a speed of the immersion of the optical surface into the etching bath along the second direction to cause a rate at which the material at the additional extended regions of the optical surface extending along the first direction are removed at each of the different positions of the optical surface along the second direction to vary nonlinearly with respect to distance along the second direction.