IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308 _______________________________________________________________________________________ Volume: 05 Issue: 04 | Apr-2016, Available @ http://ijret.esatjournals.org 107 DESIGN STUDY OF STRESSED MIRROR POLISHING (SMP) FIXTURE FOR SEGMENTED MIRROR TELESCOPES Alikhan Basheer 1 , T. Krishna Murthy 2 1 M-Tech Student, Optical Engineering,Department of Physics, Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram ,Kerala, India 2 Dr. Brahma Prakash Scientist, Applied Optics Area, Laboratory for Electro-Optics Systems (LEOS-ISRO), Bangalore, India Abstract Stressed Mirror Polishing (SMP) is a technique developed for the fabrication of off-axis aspheric mirror segments in a rapid and cost effective manner. In SMP, the polishing of the mirror is carried out along with specific loads which are capable of deforming a spherical mirror into any desired aspherical shape and thus reducing the difficulty of aspheric mirror polishing. When the blank is removed from SMP after spherical polish and metrology, the blank will relax to the required aspherical surface with-in desired PV & RMS surface figure accuracies. This paper discusses the different FEM approaches that can be adopted for simulating the deformations in the mirror blank. For preliminary study purpose, a 36-cm diameter off-axis, meniscus shaped parabolic roundel and a Spherical roundel mirror blanks are taken and required deformations are achieved through various FEM approaches. The net deformed surface is validated and compared with a best fit near spherical surface. Additionally the reversibility of the technique is also proved with-in required Surface accuracies. It is felt necessary that these preliminary simulations should form the basis in understanding the warping process and their locations, so that full SMP fixture can be designed which is capable of warping the mirrors. Keywords: Stressed Mirror Polishing, Segmented Primary Mirror, Aspheric Segment Fabrication, Bending Fixture --------------------------------------------------------------------***---------------------------------------------------------------------- 1. INTRODUCTION It is proved that SMP technique is one of the optimum approaches for bulk production of thin mirrors, by warping them and polishing them as spherical blanks. In SMP, the polishing of the mirror is carried out along with specific loads which are capable of deforming a spherical mirror into any desired aspherical shape and thus reducing the difficulty of aspheric mirror polishing. Thus the SMP fixture can be used to deform a mirror blank, polish the surface as a spherical surface, and carry out metrology for validation. When the blank is removed from SMP fixture after spherical polish and metrology, the blank should relax to a required aspherical surface with-in desired PV & RMS surface figure accuracies. Typically the SMP fixture may have to produce warping capability up to 185μm - 215μm, yet producing elastic behavior in the glass blank to be polished and in the relaxed state the aspherical surface must be with-in 2 micron (rms) surface. If the deforming range is made large but with- in limits of elastic behavior, then the same SMP fixture can be tuned to all type of segments with different off axis distances and different aspheric coefficients of a given primary mirror assembly. The forces and bending moments required in the SMP to warp a given glass blank can be calculated through closed form solutions developed for plate bending theories or more precisely through FEM (Finite Element Modeling) approaches. The SMP can be configured with force / displacement actuators located optimally at the rear side or at the edges of the glass blank so that required warping can be achieved. 2. WARPING THEORY OF SMP The method to represent each of the segment surfaces in local coordinate system(x,y,z) with respect to the global coordinate system(X,Y,Z) is given by Jerry Nelson and Temple-Raston [3].The segment coordinate system can be represented in the global coordinate system by a translation ΔZ and a rotation φ as shown in Figure 1. Fig-1: Local coordinate system within the Global coordinate system In this orientation Z-axis is normal to the Parent mirror surface and z-axis normal to the corresponding segment surface. By considering the segments along radial axis, we can make the orthogonal terms to vanish; so that the general form to represent a surface is reduced in such a way that each term of the segment shape can be represented by a single coefficient termed as polar monomial coefficient (SEG). The segment surface can be represent in the segment coordinate system by [5]: