Eyepiece Orion Sirius Plossl 4mm Orion Telescopes & Binoculars · Sirius Plossl 4mm Key specifications Focal length4 mm Apparent FOV52 ° Barrel size1.25" All specifications Elements4 About 4mm 52-degree Plossl eyepiece; Orion's standard 4-element Plossl line.