A self-developed large aperture near-infrared phase-shifting flat interferometer was used to perform an absolute test for a large aperture SiC flat mirror in an oblique incidence condition. Firstly,the cavity wavefront data were obtained in a standard Fizeau configuration, and then the test flat was put into the cavity in its normal axis with an angle a to the optical axis of the interferometer to obtain the second wavefront data set. After processing the two data sets, the absolute vertical profile across the center of SiC mirror was also obtained. Finally, several absolute vertical profiles of the φ630 mm SiC mirror were measured and the results indicate that PV and RMS of the center absolute vertical profiles are 0.061λ and 0. 014λ, respectively. The experimental results show that this measurement device can achieve the absolute tests of vertical direction of flat mirrors with the effective apertures larger than that of the interferometer, and it is especially applicable for the absolute tests of optical surfaces coated with high-reflection films or metal surfaces.%采用自行研制的口径为600 mm的近红外相移平面干涉仪在斜入射条件下对大口径碳化硅(SiC)平面反射镜进行了绝对测量.首先,在一个标准的斐索干涉测试结构中测出空腔波面数据;然后,将被测平面置于干涉光路中,使被测件光轴与干涉仪光轴成α角,测得第二组波面数据.对两组波面数据处理后得到SiC平面反射镜中心垂线方向的绝对面形分布.最后,测量了630 mm口径SiC反射镜多条垂线方向的绝对面形.结果显示,中心垂线处的绝对检验PV值为0.061λ,RMS为0.014λ.实验结果表明,该测量装置可以实现比干涉仪有效口径大的光学平面垂线方向的绝对面形检测,尤其适用于镀有高反膜的光学表面或者金属表面等面形的绝对测量.
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