通光口径2.16m天文望远镜凹双曲面主镜的磨制工艺

The technology of the concave hyperbolic primary mirror of the effective aperture 2.16 m astronomical telescope

2 16m光学望远镜是目前我国研制的口径最大的反射式天文望远镜 ,主镜通光口径为 2 16m ,边厚为330mm ,重量约 2 2 0 0kg ,顶点曲率半径R0 =12 960mm ,偏心率平方e2 =1 0 95 134 7[1] ,相对口径为 1/3,最大非球面度δ0max≈ 2 1μm。由于所用玻璃毛坯为原苏联制造 ,质量极差 ,通体充满气泡、结石、折叠 ,是块等外品。更致命的是磨出的表面各处硬度不均匀 ,出现大面积、形状不规则的高、低区 ,不得不用手持小抛光盘进行手修 ,像雕刻一样去掉那些不规则形状的硬的局部高 ,保留不规则的软的局部低 (所谓修光程 ) ,并把它拼凑成一个较为接近的理想双曲面。可以想像得出 ,这样做会遇到多麽大的困难。在大家的努力下 ,终于用手把它磨修到尽可能完善。最后望远镜在由该主镜、凸双曲面副镜及熔石英像场改正镜组成的R C卡塞格林 (Ritchey ChretienCassegrain)光学系统的焦面上拍摄了星团底片。经鉴定委员会测试组专家测量后认为 ,在全视场 ( 30 0mm× 30 0mm )内 ,不管是边、角还是中心像均很圆 ,暗星像直径达0 18mm。说明主镜的加工工艺是成功的 ,同时也说明凸双曲面副镜的加工、检验[2 ] ,熔石英像场改正镜的设计[1] 、选料、加工[3] 及光学系统的调整[4] 也是成功的。

The 2.16 m optical telescope is the largest astronomical reflecting telescope that we have researched and manufactured so far in China. The effective aperture of the primary mirror is 2.16 m, the edge thickness is 330 mm, the weight about 2.2 t, the vertex radius of curvature R 0=12960 mm , the square of eccentricity e 2=1.0951347, the relative aperture 1/3 and the maximum aspheric departure δ 0 max =21μm. The optical blank was made in the late U.S.S.R., the quality is very bad. The whole blank is full of bubbles, stones and fold. It is a substandard product. The fatal defect is the different hardness on the working surface that resulted in some large hollows with irregular shapes distributing on the surface after grinding. That forced us to use a small polishing tool to figure the surface by hand, something like carver to remove the high region and keep the low region unremoving (so-called figuring optical path difference). Finally turned it into a nearly ideal hyperbolic surface. You can imagine that a great difficulty it is! We have been working hard using the hand of ordinary person to figure the surface as perfect as possible. Finally, at Ritchey-Chretien Cassegrain optical system's (The optical system consists of the primary mirror, the second mirror and the fused quartz field corrector) focal plane takes a star cluster photograph. A member of the appraisement committee measured the linear size. In the whole field of view (30×30 cm) from center to edge and corner, the images are round. The diameters of dim stars are 0.18 mm. These show that the technology of primary mirror, the fabricate on and test of the second mirror , the design of the fused quartz field corrector , the select of the fused quartz material and its fabricating method, the alignment of the optical system are successful.