This architecture features an active wavefront sensing and control scheme along with methods for measuring the relative positions of the primary to aft optics, such as the secondary mirror, and should enable larger and cheaper telescope architectures needed for future applications. This design overcomes the stability requirements of large telescope primary mirrors.
A wavefront source/sensor is placed at the center of curvature of the primary mirror. The system provides continuous light onto a primary mirror that is retro-reflected onto itself. This allows the wavefront controller to constantly update the positions of the primary mirror segments (or deformable mirror actuators). For spherical primaries (where replicated mirrors can be used), a spherical source is used. For aspheric primaries, a null is used. The return beam can be analyzed through focus by using established wavefront sensing and control techniques, including prisms for coarse alignment, multi-wavelength interferometry, or phase retrieval. The light can be monochromatic or white light. This same source and sensor can also be used to check out the system during assembly.
Another function of this innovation involves using a concave mirror on the back of the secondary mirror (or other aft optic) that has the same center-of-curvature location (in defocus) as the primary mirror. The two return beams can be aligned next to each other on a detector, or radially on top of each other. This provides a means with which to measure the relative position of the primary to the secondary (or other aft optics), thus allowing for the removal of misalignment of the center-of-curvature source/sensor (meaning it doesn't need precision placement) and also provides a means with which to monitor the relative alignment over time.
This innovation does not require extremely good thermal stability on the primary mirror and can thus be used in any thermal environment and with cheaper materials. This factor could be critical in enabling the construction of very large telescopes, and provides a means for testing a very large telescope as it is being assembled. In addition to this, the architecture lets one phase (or align) the primary mirror independent of whether a star or scene is in the field. The segmented, spherical primary allows for cost-effective three-meter class (e.g. Midex and Discovery) missions as well as enabling 30-meter telescope solutions that can be manufactured in a reasonable amount of time. The continuous wavefront sensing and control architecture enables missions for low-Earth-orbit.
This work was done by Lee Feinberg, John Hagopian, Bruce Dean, and Joe Howard for Goddard Space Flight Center.