| EP(Einstein Probe)and eXTP(enhanced X-ray Timing and Polarization satellite)are two of China’s current scientific missions for space X-ray astronomy.The eXTP is a science mission designed to study the state of matter under extreme conditions of density,gravity,and magnetism.The four payloads on the satellite include the Spectral Focusing Array(SFA)payload and the Polarimetric Focusing Array(PFA)payload,responsible by IHEP,composed of 13 x-ray focusing mirrors in total.The telescope payloads on both satellites are designed as the Wolter-I type,which are nested by a set of coaxial confocal mirror shells made by the nickel electroforming replica technology.The mirror shells f should have excellent performances,which is essential or the integration.Therefore,during this period,the focusing mirrors are tested using visible light instead of soft Xrays in a vacuum environment to evaluate the optical performance of the prepared mirror shells.In addition,the integration facility also needs a high accuracy to make the nested mirror module a high performance.In this paper,we design the facility and methods to meet the requirements of the high angular resolution(HPD)focusing mirrors for both EP and eXTP missions.The imaging characteristics of the Wolter-I X-ray focusing mirrors are studied in depth,and a visible light test system for the focusing mirrors is developed.The 21 st mirror shell of eXTP is tested by the 470 nm parallel lights by the integration facility.And the calculation and analysis algorithm of the angular resolution has been optimized.The error analysis in the visible light test is carried out with the influence of the diffraction effect and image acquisition position.Based on the Furlong and Fay diffraction theory of circular aperture,the formula of fine circular diffraction distribution with great coverage is derived.The angular resolution was measured as 32.0±0.4 " in the visible test using the 18 th EP-FXT mirror shell under 473 nm parallel laser,and the contribution of visible light diffraction was calculated to be 20.15".Except that,the result of the mirror shell is 24.9±1.6 ".When compared to the 100 m vacuum X-ray calibration facility measurements of 25.1±1.6" at the IHEP,these results are congruent within the margin of error.Based on the visible light test,we have completed the precision integration of the optical prototype for t eXTP.The precision integration of the optical prototype with nested four mirror shells was completed by the method of calculating the center pixels of the spots in different positions of the camera,and then tilt the axis of the mirror shell to a certain degree.At last,the angular resolution of the 4-layer mirror module is 45.3±0.4″HPD.The idea of the roundness correction of the mirror shells was proposed and experimentally verified by using EP-FXT #16 single-layer focusing lens for roundness correction.The angular resolution before the correction was 48.0±0.4″,and it is 37.4±0.4″after the correction. |
