In this paper, we demonstrate a compact and long focal length imaging system for building detection. This system is an all-spherical system. In order to avoid the use of aspheres, the system relies on two groups of correction elements to correct aberrations. This allows for short manufacturing cycles and simplified adjustments. The system works in the visible band from 460 nm to 750 nm. The focal length is 900 mm, and the optical length is only 280 mm. The designed modulation transfer function (MTF) is consistent with the diffraction limit. The MTF curve is 0.63 at the Nyquist frequency of 78 lp/mm. The optomechanical structure is developed based on the analysis consequence of tolerance and stray light, and a device is fabricated. We also propose a simplified method for measuring the space between the primary mirror and the secondary mirror in a compact coaxial optical system. According to the experimentation, the device is proven to meet the application requirement. Detailed descriptions of the design and implementation are provided.