Temporary sources and explosive celestial bodies in the universe can only be observed through their high-energy radiation, for example X-rays. The multilayer nested Wolter-I type focusing mirror is widely used in X-ray astronomical telescopes. The electroforming replication process can achieve batch high-precision production of Wolter-I type focusing mirrors, with the process flow of mold ultra-precision turning, ultra-smooth, coating, electroforming, and demolding. The fabrication of molds accounts for a large proportion in the process, and the molds accuracy and surface quality directly determine the optical performance of the mirrors. We built the DRL2000 ultra-precision lathe for mold turning of mission EP (Einstein Probe) and eXTP (enhanced X-ray Timing and Polarimetry mission). The swing straightness error of the guide rail was separated and compensated through trial cutting and reverse measurement methods to achieve a turning generatrix accuracy of peak-to-valley (PV)<0.2μm. An in-situ measurement device has been built based on the ultra-precision lathe. After adjustment the system measurement error can be controlled within 0.1μm. The corresponding spiral measurement path and coaxiality error separation method were studied and verified through experiments. An offline measurement device was built and based on it, an optical lever measurement method was preliminarily validated. Finally, a division method of medium-low frequency error and medium-high frequency error was proposed.
The removal function in machining is not an ideal impulse function, and the material removal at any point on the workpiece is a superposition of the removal at that point from the distribution of removal at surrounding points. When the residence time suddenly jumped to zero at the edge, the removal function and the residence time convolution results will gradually decrease at the edge, the removal matrix at the edge of the formation of the "collapse", after many convolution iterations, the edge of the face shape of the error convergence rate is lower than the other positions, resulting in edge effects. This paper firstly put forward an iterative algorithm based on the residence time of the cylindrical projection, and then analyze the edge effect in the iterative algorithm, and put forward two methods to suppress the edge effect, and select different removal functions and the form error on the algorithm to carry on the simulation calculation. Finally, the results of machining of mold #54 verifies the effective suppression of edge effect error by the method in this paper.
Based on chemical-mechanical polishing and combining mechanical and tribochemical polishing techniques, a precision lapping method suitable for parabolic polishing specified by grazing incident X-rays is introduced. Various factors affecting the polishing process are analyzed, and the results show that the proposed method is consistent with Preston's equation and Hertz contact principle. Therefore, this paper proposes a general material removal model based on the above two methods. The mid and high-spatial frequency errors are demanded to reach the requirements with an angular resolution consistently < 6 arcsec HEW and a roughness of 0.3 nm rms (between 1 mm and 0.002 mm spatial frequency range). To achieve the conformal ultra-smooth polishing of focusing mirrors, the process of full-aperture super-smooth pitch polishing is investigated. The influences of key polishing parameters are revealed. The evolution of the surface topology has been studied. A polishing setup is established to carry out experimental polishing to verify the optimum processing parameters obtained by simulations and previous polishing tests. Besides, the effect of abrasive particle size on the roughness is also verified. The roughness of the polished mandrel is measured at different positions, and the optimum roughness reaches Ra 0.359 nm. The polishing approach can significantly reduce the surface roughness of the replication mandrel, satisfying the low energy band focusing requirement of grazing incidence X-ray mirrors.
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