The Optics Free Spectrometer analyzes the energy of ionized rare gas electrons. In this recently developed spectrometer the energy spectrum of incoming EUV photons is transformed directly into an electron energy spectrum by taking advantage of the photoelectric effect in one or several rare gases at low pressure (1 to 10 mTorr). This instrument has no degradable optical elements, a problem common to conventional optical spectometers, and it also eliminates the problem of multiple order separation.

The sounding rocket OFS is a long tube mounted to the metal plate. While the OFS can be made much smaller and lighter the short duration of a sounding rocket flight and their large vibrations call for extra hardware. To protect against the Earth's magnetic field, magnetic shielding is placed around the instrument.