The Anti-Electron Sub-Orbital Payload Low Energy (AESOP-Lite) is designed to explore the origin of the negative spectral index in the cosmic ray electron spectrum below 100 MeV through a series of balloon flights. The original entry telescope from the Classic LEE (Low Electron Energy) instrument has been directly integrated in AESOP-Lite. The instrument utilizes a gas Cherenkov and magnetic spectrometer configuration to identify particle type and determine the energy. The first flight took place May 15-21, 2018 from Kiruna, Sweden accumulating roughly 130 hours of exposure above 130,000 feet altitude before landing on Ellesmere Island, Canada. In this paper, we report on its design, calibration and performances. This includes the analyses of ground data taken during the integration of the detector before the flight. The observed muon charge separation from ground runs is discussed and compared to the expected performance of the spectrometer. The energy resolution from track reconstruction algorithms and the energy-dependent geometry factor are tested with Monte Carlo simulations. Finally, we present a brief overview of the 2018 flight and preliminary results of the detector performances in flight.