Ultraviolet beam splitter characterization for use in a CubeSat optical system

The U.S. Naval Research Laboratory (NRL) has developed the Triple Tiny Ionospheric Photometer (Tri-TIP), an ultraviolet remote-sensing instrument based on the TIP. Tri-TIP measures emissions of atomic oxygen (Oi 135.6 nm) to determine plasma density in the nighttime ionosphere. The Tri-TIP design shrinks TIP to a 1U CubeSat form-factor and simplifies the mechanical design with a three-channel photometer system to isolate the target wavelength without a filter wheel. A heated strontium fluoride (SrF2) filter eliminates incoming light at wavelengths shorter than 135.6 nm. The filtered light is divided between two matched photometers by a beam splitter with a magnesium fluoride coating over aluminum (AlMgF2) deposited on 50% of the surface in a polka-dot pattern. The third photometer monitors dark count noise for later subtraction. One Tri-TIP configuration uses a beam splitter with a sapphire (Al2O3) substrate, which is opaque to wavelengths shorter than ∼140  nm, to later subtract contaminating emissions at wavelengths longer than 140 nm. A second Tri-TIP configuration uses a MgF2 substrate beam splitter to simultaneously measure Oi 135.6 nm from two adjacent fields-of-view. The performance of both beam splitters has been tested at NRL, and the results are presented.