In the present work the design of aplanatic and achromatic telephoto type telescope objective is developed. A telephoto lens has the property that the total length from the front vertex to the image plane (overall length) is less than the focal length. Telephoto lenses are used when the overall length of the lens system is a critical limit. The objective lens of the telephoto type telescope(TTT) has the configuration of a positive doublet in front of a negative doublet. For the objective lens of TTT, the correction of chromatic aberration, spherical aberration, and coma is of primary importance. On the other hand astigmatism, curvature of field, and distortion of the system are not so critical because the field angle is very small. From the Seidel's first order aberration theory, the aberration coefficients for the spherical aberration and coma are derived analytically in this work. By using the Gaussian bracket, the aberration coefficients are compactly expressed and the dependence of aberrations on the system parameters is expressed clearly. The analytic solution of the system satisfying the aplanatic condition were obtained for each module. The system is also corrected for the longitudinal chromatic aberration to be zero. From the set of initial solutions with three design parameters the solution at the center of the set of solutions is taken, and then the system is optimized with respect to the finite aberration.
The objective lens that is optimized in this work has f/8.5 with the half field angle of 0.3˚, and the telephoto ratio is 0.843 with the focal length of 30 cm.
