Energetic electron precipitation into the atmosphere is a primary electron loss mechanism for the outer radiation belt – despite being studied for decades, there are still many unknowns to be solved. With the increasing number of space mission concepts to address this question, less costly alternatives such as CubeSats and even newer concepts such as Thinsats will be used in the foreseeable future. Here we introduce the Dual Aperture Relativistic Telescope (DART), a miniaturized solid-state charged particle telescope whose objective is to measure differential energy of energetic electrons in two look directions, 180 degrees apart, in order to characterize and quantify particles precipitating into the atmosphere and those returning back up the magnetic field line. This work presents a description of the DART design including Geant4-based analysis to characterize the instrument response and instrument testing from a Sr-90/Y-90 radioactive source. Our Geant4 analysis shows that DART can measure ~0.5MeV - ~3.9MeV electrons in four differential energy channels and two look directions, and results from the radioactive source test have verified the instrument performance and the validity of the Geant4 simulations. These measurements will enable a new scientific understanding of radiation belt particles lost to the atmosphere that is crucial for better understanding of energetic electron precipitation and overall radiation belt dynamics.