| Absolute testing is a high-precision optical component testing method.In the large-aperture interferometry,the two-flat absolute test avoids the problem of flat crystal flipping in the three-flat absolute test,but this method will introduce tilt errors in the translational interferometry process,and make the quadratic term of the restored wave surface deviate.The liquid level can always be kept level before and after translation,which can replace the flat crystal as the measuring element,avoiding the problem of introducing tilt error.Rotation and translation absolute testing is a two-flat absolute testing method.This article will study the absolute testing of liquid surface rotation and translation based on a Φ 300 mm large-diameter vertical Fizeau interferometer.First of all,the principle of absolute verification of rotation and translation is studied,and the difference Zernike polynomial restoration method is theoretically deduced and simulated,with emphasis on the wavefront decomposition restoration method.When using differential Zernike polynomials to restore the wavefront,the restoration accuracy is limited by the number of fitting items.When the wavefront decomposition method is used to restore the wavefront,the image N-fold rotation method or iterative method is used to restore the wavefront rotation change term,and the polynomial fitting method is used to restore the wavefront rotation invariant term,with an accuracy of 0.12 nm.Based on this method,the optimal translational shear ratio is 1.58%,and the optimal number of polynomial fitting terms is 25 terms.Secondly,a technique of regional wavefront reconstruction is studied.This method rounds the discrete wavefront to the nearest integer after rotating it,without relying on a special rotation angle.The simulation analyzes the influence of translational shear ratio and rotation angle on the restoration accuracy,and analyzes the influence of translation distance error,rotation angle error and rotation eccentricity error on the restoration result,which proves that the restoration accuracy of this method is high,and the error is within a certain range.,Has little effect on wave surface restoration.The influence of the tilt error introduced by the interferometric process on the wavefront restoration is analyzed: the defocus term of the restored wavefront is linearly related to the tilt error along the translation direction introduced by the translation process.Third,analyze the influence of external factors on the experimental measurement,including temperature,liquid pan movement,air flow and vibration.In the same horizontal plane,the temperature of the interference cavity was controlled at 20±0.1℃,and the liquid level interference experiment was carried out.The result showed that the deviation of the RMS value of the two measurement results was less than 0.1nm.By adjusting the counterweight of the liquid pan,the center of gravity of the interferometer is stabilized before and after the movement of the liquid pan,which avoids the tilt error caused by the change of the attitude of the reference flat crystal when the center of gravity is shifted.Through experimental analysis,the isolation gas hood and vibration damping device can reduce the impact of airflow and vibration.Finally,an experimental study on the absolute testing of liquid surface rotation and translation was carried out.The liquid level can always be kept level before and after translation without introducing tilt error.A liquid level rotational and translational absolute test system was constructed on a Φ 300 mm vertical phaeau interferometer.The restored PV value of the reference flat crystal face shape is 29.2nm,and the RMS value is 4.5nm.Comparing the experimental results of liquid level rotation and translation with the traditional three-side mutual testing method,the y-axis profile is basically the same,the deviation of PV value is 2.1nm,and the deviation of RMS value is 0.3nm.Experiments show that the liquid level rotation and translation method has good repeatability,the deviation of y-axis profile RMS value is less than 1.7nm,and at the same time,the repeatability is good,and the y-axis profile RMS value deviation is less than 2.5nm.Finally,the accuracy of the liquid surface rotation and translation method is confirmed. |
