PLANETS will be a 1.8-m off-axis telescope combined with contrast enhancement techniques, enabling us to observe faint emissions in the vicinity of bright objects. This "high dynamic-range"capability is largely dependent upon precision of telescope optics as well as atmospheric distortion. We present feasibility study of monitoring water plumes on Europa, neutral torus close to Enceladus, and ionosphere on Mars using PLANETS telescope. To test feasibility of high dynamic-range observation under actual conditions of wavefront error, we modeled propagation of light though the system based on Fraunhofer calculation taking into account for wavefront error made by atmospheric distortion and by primary mirror figure error. Then point spread function is calculated for several cases of figure errors under use of adaptive optics. The modeling result predicts that the moderate or high-precision primary mirror is mandatory to accomplish the high dynamic-range observation. We also present the latest design of PLANETS, especially focus on the support structures of primary mirror. We employ 36-point whiffletrees with 33 warping harnesses for axial support, and 24-point Schwesinger support for lateral support. The active support system is expected to reduce pre-polished RMS error from 1.51 μm to 0.66 μm corresponding to 70% reduction in total volume of final polish. The laboratory experiment using one third part of prototype whiffletrees shows supporting force RMS repeatability < 0.005 kgf, and drive hysteresis < 0.7% of load range, which are precise enough to control or to keep the primary mirror figure.