The TSA-120 is equipped with an air-spaced ED triplet apochromatic lens with 120 mm diameter (900 mm focal length and F/D 7.5), mounted in a 3-element barrel. The latter ensures good stability of the optomechanical chain in the face of usual transport constraints and significant variations in operating temperature. It has a retractable dew shield and the length of the tube does not exceed 990 mm for a moderate weight of 6.7 kg.

The lens has been designed to achieve a very high level of performance in observation and photography. The chromatic aberration in short wavelengths (blue and violet) is reduced by one third compared to the old FS-102/FS-128 with Fluorite lens. The Strehl ratio at optimal wavelengths of human vision is almost perfect, at 99.2%, equivalent to that of the TOA series.

The TSA-120 is therefore a lightweight, compact and economical refractor, whose performance is similar to that of the TOA, Takahashis flagship models and recognized as among the best in the world.

The addition of the optional field corrector allows native focus imaging. The TOA-35 reducer (F/5.25) makes it more comfortable in deep sky photography. All assemblies meet the rigidity and flexibility requirements of new imaging devices, such as digital cameras and the latest generation of CCD cameras.