Featuring bak 4 prisms and a multi coated optical system that allows for 92 light transmission and distortion free images the helios apollo 28x110 ms observation binoculars are high quality waterproof nitrogen filled binoculars suitable for medium to long range terrestrial and astronomical observations built into the binoculars is a tripod mounting bush and threaded eyepiece units to allow for filters the long eye relief and fold down rubber eyecups make these binoculars suitable for spec