Ion-beam polishing is a high-precision surface shape-modification technique that requires multiple iterations to ensure surface accuracy. Currently, it is widely used for the surface processing of precision optical flats. However, for transmission flats supported by an adhesive, the temperature generated during polishing may exceed the tolerance temperature of the silicone rubber, which may cause the detachment of the silicone. This changes the uniform support state of the transmission flat, which consequently introduces additional deformation to the surface morphology of the flat.

The detachment of adhesive spots occurs primarily in two forms: internal bubbles and flow tendencies. Adhesive spots with bubbles are primarily characterized by a decrease in the contact area with the transmission flat, whereas adhesive spots with flow tendencies primarily exhibit a displacement of the mass center position. We constructed a physical model of a non-uniform support by analyzing the change in force on the transmission flat after debonding. Based on the distance between the adhesive spots and the operating surface of the transmission flat, we classified the adhesive spots into three groups and selected one for analysis. The surface error caused by changes in the adhesive spots was simulated via COMSOL Multiphysics using the finite-element method. The direct result of the finite-element simulation is the global topography of the operating surface of the transmission flat, and the error surface was obtained by subtracting the surface topography under a uniform distribution of the adhesive spots. The results show that the adhesive spots nearest to the operating surface of the flat exert the most significant effect on their surface topography (Figs. 4 and 7). Additionally, we investigated the variation in the low-frequency morphology under different degrees of adhesive-spot detachment. The results indicate that adhesive-spot detachment exerts the most significant effect on astigmatism (Fig. 16).

The liquid-reference method was employed to monitor the iterative ion-beam polishing process of the transmission flat on a Φ300 mm vertical Fizeau interferometer. The initial peak valley (PV) value of the flat surface before polishing is 122.4 nm. After three rounds of iterative polishing, some adhesive spots around the transmission flat are degummed. An appropriate digital image-processing algorithm was applied to the captured onsite images to calculate the reduction in the area and the displacement of the mass center of the detached adhesive spots. The global surface error was estimated based on the condition of the adhesive spots, and the corrected surface was obtained by subtracting the 300-mm-aperture error surface from the absolute test result via the liquid reference. Further ion-beam polishing was performed based on the corrected surface. Finally, the accuracy of the transmission flat reaches 20.45 nm, which is better than λ/30, thus validating the effectiveness of the error correction (Fig. 14). The results show that the proposed debonding error-correction method can ensure the convergence of ion-beam polishing (Fig. 15).

The Φ300 mm vertical Fizeau interferometer adopted in this study was equipped with a precise temperature control system and an air-floating structure, thus ensuring a constant temperature and vibration isolation in the test environment. Using this device, the repeatability of the liquid surface interference test can reach 0.005 nm (Fig. 17). After assembling the polished transmission flat and considering other error sources, such as detection and light sources, the comprehensive uncertainty of the vertical interferometer is 0.99 nm.