The biggest challenge in fabricating the segmented mirror is match each segment

that is match the radius of each segment

so they can equal to a single

continuous surface. For spherical segmented mirror

the radius of each segment must be the same. We put forward a new method which utilizes Shack-Hartmann sensor and a high precision spherometer to measure the relative radius of each spherical segment; also

we setup an experiment system. The method first use the co-focus technique to co-focus each segment; then use the S-H sensor to measure the defocus in the optical axis direction and employ the PZT actuator behind the segments to make the defocus near zero; after another co-focus process

the relative radius of each segments can be measured by high precision spherometer. In the experiment

the segmented mirror consists of three spherical hexagonal segments which are 300mm side by side

and the designed radius of curvature is 2000mm. The analysis and experiment show that the precision of the method is about

and the method is suitable for the measurement of relative radius of curvature of large spherical segmented mirrors.