Edge sensor for segmented mirror telescopes

Abstract

To address key research problems of astronomy, a large aperture telescope has become a need for every astronomical group around the globe. Optical telescopes with single monolithic primary mirror of 8m and above aperture are difficult to manufacture and maintain. The solution for such problem is to use small mirror segments and align them to function as a large aperture telescope. Once the mirror segments are aligned using telescope alignment and phasing system, then continuous monitoring of segment position is required to maintain the shape of the primary mirror. Edge sensors are used for aligning and stabilizing the relative position of mirror segments and play a vital role in the segmented mirror telescopes. Stability, precision, flexibility, resolution, immunity towards relative temperature and humidity changes, defines the performance of the edge sensor. Edge sensors are expected to resolve few nm relative displacement of mirror segment and should be stable over period of about a month. Capacitive and inductive edge sensors which can meet all required technical specifications are used in well known segmented mirror telescopes like KECK, HET, SALT, TMT, GTC and ELT. Inductive sensors are based on the principle of mutual inductance measurement between two planner coils. The mutual inductance of these sensors varies with overlapping area of the transmitter and receiver coils as well as gap between them which in turn cause change in induced EMF in receiver coil. These changes can be converted to the measurable quantities like amplitude, frequency or phase shift. Each sensor comprises of a transmitter on one segment and receiver on adjacent segment. These sensors are crafted on a very stable base usually made of same material used to make the mirror segment. Two plates of these sensors are placed beneath segments. To suppress the most of the external effects edge sensors are being used in differential mode. In the ITCC laboratory of IIA a project has been initiated to indigenously develop an inductive edge sensor which should not only full-fill all technical requirements, but also be very cost effective. This nine months M.Tech project is part of the work aimed to develop a robust edge senor which can be first used in prototype segmented mirror telescope and later with the NLOT. The inductive sensor modeled and analyzed using COMSOL multiphysics software, which helped to visualize the field strength and orientations. The coil geometry, as well as its densities are optimized in such way that it should work over long range and should have high and almost linear sensitivity. Then after Dip Trace software was used to design the planner square coils of desired shape and Gerber files were generated. With help of PCB manufacture, inductive coils were created on flexible polyimide kapton substrate of about 80 micron thick, then after the flexible coils were bonded on to a thermally stable fused silica glass substrate using silicone RTV structural glue. Parallely an FPGA based sensing electronics which works on the principal of amplitude modulation/demodulation has also been designed and simulated using OrCAD Pspice software. Subsequently a very stable and precise sensor board has been developed and tested independently. Finally, the full system was integrated with the sensor, sensing electronics and FPGA and tested for sensitivity, linearity, noise, and stability. The test results are very satisfactory and we could easily achieve sub-micron accuracy over a range of about 10mm. However, to achieve required performance of an edge sensor, lot more need to be done on the sensor as well as sensing electronics.