New "extremely large telescopes" are being designed by a small number of groups around the world. These ELTs will have primary mirrors of unprecedented sizes (25 to 42 meters!). In order to take advantage of the very high angular resolution and image clarity that these telescopes can deliver, it is necessary to make considerable upgrades to the technology of adaptive optics (AO). AO allows astronomers to correct distortions introduced when the light from distant objects in space passes through the earth's atmosphere. As ttelescopes get larger, the AO must correct over an increasingly large area. Furthermore, since the image resolving capabilities scale with the size of the mirror, the AO techniques must improve along with the mirror size if the best possible images are to be achieved.
Current AO technology has come a long way and is now well established and robust. But the challenges for AO with ELTs are considerable. One component of AO systems is a "deformable mirror" (DM) used to rapidly (a thousand times a second) correct the errors introduced by the atmosphere. DMs in use today cannot make the kinds of corrections needed for ELTs. Dr. Joel Kubby of the University of California - Santa Cruz is developing new, larger, and more responsive DMs to meet the coming needs of ELTs. Dr. Kubby's work is supported by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.
Monolithic fabrication of continuous facesheet high-aspect ratio gold Micro Electro Mechanical Systems (MEMS) deformable mirrors onto a thermally matched ceramic-glass substrate (WMS-15) has been performed. The monolithic process allows for thick layer deposition (tens of microns) of sacrificial and structural materials thus allowing high-stroke actuation to be achieved. The fabrication process does not require wafer bonding to achieve high aspect ratio 3-dimensional structures. An approximate 2 μm thick gold continuous facesheet mirror has been deposited on both 16×16 and 10×10 arrays of X-beam actuators. An example is shown in the accompanying image. This project resulted in the granting of a Ph.D. to an underrepresented engineer, and the development of high-stroke, high-order MEMS mirrors for the next generation of extremely large (30 meter) telescopes. The monolithic fabrication of the continuous facesheet deformable mirror was performed on optically flat glass-ceramic substrates that are used to make wavelength division multiplexing filters in the telecommunication industry.1 The substrates have a root mean square (RMS) roughness of <1nm and a coefficient of thermal expansion (CTE) of 11.4×10-6K-1 that is close to the CTE of gold, 14.2×10-6K-1. The process starts with a deposition, patterning and lift-off of a 0.5 μm counter electrode layer on to the glass ceramic substrate. The actuator anchors are then patterned and electroplated to a height of 22 μm. The copper sacrificial layer is then electroplated and both the sacrificial copper and gold anchor posts are planarized with a chemical mechanical planarization (CMP) method down to a height of ~20 μm. A 4 μm gold spring layer is then patterned and electroplated followed by copper electroplating up to the height of the spring layer. The top surfaces of these layers are then planarized with CMP. After the planarization of the spring layer, the 30 μm mirror support posts are patterned and electroplated. Another layer of sacrificial copper is electroplated up to the top of the post and both the copper and support post are CMPed. A final 2 μm gold mirror layer is then patterned, electroplated and CMPed to an optical surface. Chemical etching of the copper sacrificial layer is carried out, leaving behind a continuous facesheet attached to X-beam actuators. The X-beam actuators were designed to help prevent premature pull-in associated with unsupported actuator corners or edges, while at the same time allowing large displacements to be achieved.
