Hexapod actuated focal plane for high-resolution suborbital and ground-based exploration

This paper outlines development efforts to produce an imaging system, known as the HExapod Resolution Enhancement SYstem (HERESY), that can be interchangeably used aboard balloon-based and ground-based observing platforms. The instrument is a cryogenic hexapod system that accomplishes image stabilization similar to a tip-tilt mirror but by actuating the focal plane rather than the incoming optical beam. In its balloon configuration, HERESY is not a full-gimbaled pointing instrument but is rather a high-precision, highfrequency image stabilization instrument that removes image blurring caused by jitter. The pointing error signal (collected by star tracker) is fed to the hexapod at a high frequency to drive positional corrections in close to real-time. 1 arcsecond sustained pointing has already been demonstrated by missions such as NASA's STO-2 Antarctic balloon mission, and HERESY would improve STO-like gondola's pointing by an order of magnitude (0.1") bringing the imaging capability of the platform down to the 300nm diffraction limit. These balloon imaging capabilities have caught the interest of the planetary community since a long-duration mission would enable persistent diffraction-limited imaging for applications such as Gas Giant storm tracking, small body remote sensing, and exoplanet detections. The HERESY instrument is transportable and interchangeable since different detector configurations can be readily interchanged on the hexapod's mounting surface. HERESY can also be plugged into the focal point of any telescope system without introducing the need for any additional optics of its own. Therefore, it is straight-forward to reconfigure HERESY from a balloon-based instrument to a ground-based instrument. In the ground-based configuration an additional fast-read CMOS detector is co-mounted next to the primary science detector and acts as a star tracker. Once the imaging targets are lined up properly, the CMOS tracks the center-point of the guide-star at a rate of 100-200fps and feeds the positional corrections to the hexapod while the primary detector can take a long exposure simultaneously. Using this technique, the hexapod can remove X-Y blurring error in an image caused by atmospheric turbulence. In this configuration, HERESY can be installed at the focal plane of any optical telescope and immediately provide a working image stabilization system. Engineering testing of this prototype instrument have been completed at the 61" Kuiper observatory in Tucson, AZ, but more refinement of the pointing algorithm is needed before this instrument can collect publishable science data. A known limitation of the instrument is that a bright star must be in the FOV of the CMOS while the science target is in the FOV of the primary detector, so future modifications of the ground-based version of HERESY will likely include the addition of several more fast-read CMOS star trackers to broaden the star tracker field of view.