Using aspherical optical components in advanced optical systems not only reduces the system complexity, but also improves the performance remarkably. Compared with traditional spherical optical components, large aspherical surfaces are difficult to fabricate, which limits broad application in advanced systems.

Because of their complexity, so far there is no general high-accuracy aspherical manufacturing equipment. This kind of equipment is illegal to sell to China in the context of some important technologies, so it should be researched and developed in China.

Traditional optical fabrication technology is contact fabrication with pressure, which is readily influenced by the manufacturing environment and difficult to control, such as the polishing fluid (slurry) concentration, the temperature and chemical characteristics (and so on), the physical characteristics of the polishing pad and the workpiece, the pressure of the polishing load, and the relative velocity. In traditional polishing technology, subsurface damage will occur on the mirror surface, attributable to the polishing pressure. Because of this pressure, the polishing pad will be worn during the process, which means that the pad contact surface changes frequently. Consequently, the removal function of the technology is unstable. If one uses this kind of polishing pad, the workpiece surface error has very low repeatability and predictability. In contrast, noncontact polishing technology will have a very stable removal function in the process because of the absence of the working pressure. However, its biggest disadvantage is the low removal rate, which can only be used in the final finishing procedure.

Magnetorheological finishing (MRF) technology, developed in the 1990s, is a very useful contact polishing technology that works with normal levels of pressure, and uses tangential velocity to remove the mirror material. Not only does it have a high removal rate similarly to contact technology, but it also has a stable removal function similarly to noncontact fabrication technology^[1–5]. The advantage of MRF has helped it become a potential option to fabricate large ultra-thin mirrors^[6].

Based on the aforementioned characteristics, if MRF technology is assembled on a large computerized numerical control (CNC) to fabricate a large mirror, the mid and low spacial frequency will be removed quickly^[7]. In the meantime, the mirror stress introduced by the polishing pressure will be eliminated to the maximum extent.

A MRF polishing platform with a 400 mm diameter wheel is applied towards fast fabrication of a large aspherial mirror whose diameter is over 1 m-class. The 1 m-class mirror is usually fabricated by stressed lap polishing (SLP) or computer-controlled optical surfacing (CCOS). Its biggest disadvantage is the low fabrication efficiency in the fine polishing procedure, which causes the manufacturing period to be too long. Because of the high removal rate and stable removal function, MRF technology with a large polishing wheel is assembled on a large CNC, which combines both advantages and is suitable to fabricate large aspherical mirrors. The advantage of this assembly is embodied as the high fabrication efficiency of the large mirror and reducing the fabrication duration. The parameters of the large aspherical MRF CNC are described in Table 1.

To examine the applicability of MRF-2500, anther 200 mm diameter MRF polishing wheel is assembled to the Z1 manufacturing axes, where a 400 mm MRF polishing wheel is mounted. This is called a “dual MRF wheel manufacturing center;” it is displayed in Fig. 1(a). The traditional technology SLP or CCOS is assembled on the Z2 manufacturing axes and acts cooperatively with MRF to efficiently eliminate the middle and high spacial frequency surface error during the process.

The main polishing wheel diameter of a MRF-2500 is 400 mm, and the assistant wheel diameter is 200 mm, which is aimed at modifying the different spatial frequency surface error introduced by the former polishing procedure. Additionally, the dual MRF polishing system can effectively eliminate the “edge effect” of CCOS fabrication.

This Letter focuses on the removal function characteristics of a 400 mm polishing wheel assembled on a dual-wheel manufacturing center and simulates the fabrication result with the obtained removal function on a large concave fused silica aspherical optical component.

The removal function characteristic of a 400 mm polishing wheel is examined in this work. The parameters are reported in Table 2; the removal function test is carried out on a 100 mm fused silica flat mirror. The fabricated mirror is tested by using a GPI interferometer from Zygo Company. Figures 2(a)–2(d) show the removal function results under working distance of 1, 1.2, 1.5, and 2 mm, respectively.

To quantitatively analyze the removal function, Matlab is used to compute the results to obtain the peak to valley (PV) removal rate, volume removal rate, and the length and the width of the removal function. All the data are displayed in Table 3.

To observe the changing trend on an intuitive level, the data in Table 3 are shown in Figs. 3(a)–3(d).

Because the MRF removal function is nonaxial symmetrical (D shape), the relationship between the fabrication parameters and the working distance is nonlinear, which can be fit with the allometric function. The fitting equation is displayed in Eq. (1); the fitting parameters are reported in Table 4y=axb.(1)where x represents the working distance, y represents the fabrication parameters, such as PV removal rate, volume removal rate, etc. a and b represent the fitting parameters.

As displayed in Fig. 3, the optimized working distance area is between 1 and 2 mm. The data, except the volume removal rate, is stable in this region. If the working distance is bigger than 2 mm, the volume removal rate decreases too much according to the changing trend in Fig. 3(b). The lower volume removal rate is not advantageous for improving the fabrication efficiency of a large mirror, which will consequently increase the fabrication cost. If the working distance is less than 1 mm, the volume removal rate is much higher and the removal area is larger, but the damage potential to the optical component will increase remarkably, because the larger polishing wheel diameter easily leads to misalignment between the wheel axes and the CNC axes and has a slight influence on the CNC fabrication stability during the process of the optical component.

To examine the fabrication convergence of the 400 mm wheel, the removal functions in Figs. 2(a) and 2(d) are used to fabricate a 1450 mm diameter concave fused silica aspheical optical mirror in simulation^[8–10]. Before the virtual fabrication, the PV value of the surface error is 4.7287λ, and the rms value is 0.5513λ. The test result before simulation is shown in Fig. 4. The raster track is adopted in the virtual fabrication. The total fabrication time is 110 h in Fig. 2(a) and 309 h in Fig. 2(d). The simulation results are displayed in Figs. 4(b) and 4(c). The surface error data is reported in Table 5. The power spectrum density (PSD) is then used to analyze the surface map of Figs. 4(a)–4(c), and the analysis result is reported in Fig. 5. The red curve (original PSD) in Fig. 5 is the PSD of the map in Fig. 4(a); the green one (BIG PSD) is the PSD of Fig. 4(b), and the blue one (SMALL PSD) is the PSD of Fig. 4(c).

According to the virtual fabrication results in Figs. 4(b) and 4(c), we can come to a conclusion that a better middle and low spacial frequency surface error can be obtained with a wider removal function in Fig. 2(a). In the simulation, the high-frequency surface error decreases slightly but not remarkably. Analyzed in the context of spatial frequency, the wider removal function has a better ability to control the surface error. To obtain the wider removal function, the former fabrication parameters must be optimized further, such as increasing the MRF fluid flux and reducing the rotation speed of the polishing wheel, or increasing the flux and decreasing the working distance. The further detailed experiment will be carried out in the next step.

This Letter examines the removal function characteristics of a 400 mm MRF polishing wheel which is assembled on a large CNC. The removal functions with different parameters (two typical parameters) are used to fabricate a 1450 mm concave mirror virtually. The final surface errors were eventually reduced to 0.014λ and 0.024λ, and the convergence rates are 97.46% and 95.65%, respectively. The results show the fast fabrication ability of the 400 mm MRF and at the same time, the spacial frequency of the surface error has been controlled effectively. According to the simulation results, if a wider removal function is obtained through optimizing the wheel parameters, the 400 mm MRF polishing wheel will result in greater applications in large mirror fabrication.