In this paper, a wavefront curvature sensor is presented. This sensor is based on the measurements of the differentials of wavefront slopes, where the wavefront slope measurements can be achieved by a Shack-Hartmann sensor. A Shack-Hartmann sensor with three output collimated beams will be introduced with a lenslet array installed in each beam. By shifting two of the Shack-Hartmann grids in the x- and in the y- directions independently compared to the third grid, we can measure the slope differences and obtain the wavefront curvatures at each grid point. A wavefront reconstruction can be performed with the wavefront curvature measurements.
In this system, the segmented-mirror consists of three submirrors. They are hexagonal with 250 mm diagonal. The shape of each submirror is spherical with 3000 mm radius and the departure of each radius from the mean radius is less than 0.025 mm. A special S-H test device is used for co- focus measurement. It includes nine sub-apertures in three groups and each group is for a submirror. There are six displacement actuators in this system. The based unites of it are flexure hinges. The actuator is driven by a stepping motor with ten subdivisions. Six capacity displacement sensors are used in this experiment system. It is made in Tianjin University. A computer is used for data collecting, calculating and controlling. A special method for co-focus is developed in our work. By using this method the error of co-focus, i.e. the tilt error of submirror, is less than rms 0.035 arcseconds. The methods of calibration and maintaining for co-phase are also introduced in this paper. After once calibration, the diffraction limit image can be observed in about 220 mm aperture at (lambda) 650 nm, and it can be maintained about 20 minutes.
In this system, the experiment mirror has 500 mm aperture and 6 mm thickness. There are 58 actuators and three fixed points in it. A Shack-Hartmann test apparatus is used for the measurement of wavefront aberration. In this apparatus an ingenious equivalent of lenslet array is used. All image points formed by it appear very clear theoretical diffraction pattern. And a CCD from a TV camera is used. Like European Southern Observatory, we use quasi-Zernike polynomial to fit the wavefront aberration for correcting. But in our work correction is to the whole wavefront aberration (except lateral focus and longitudinal focus). In our work, another important character is that the damp least square method is used for determining the forces. The correction results are the root mean square of wavefront aberration about 0.02 - 0.04 micrometers . A circle including measuring and correcting the wavefront aberration takes about 3.3 minutes. A more precise algorithm proposed by us is used for calculating the wavefront aberration for checking.