In order to develope the three mirror telescope, a formula group of third order aberrations and a program had been developed. Two families of the aplanat with zero astigmatism or zero distortion were found. One of them is a new result. Based on the family 2, a 10 me-ter diameter telescope with a spherical primary mirror had been designed. The primary's F-number is 7/1.5 and the full field angle of view is 20 arc minutes. The RMS spot diameter (5 colours) is less than 0.313 arc second.