The Optic Spacer is the interchangeable core of the On Glass system - each size snaps into the adapter ring to match your specific eyepiece. Grab an extra to run our system on a second optic without buying another full kit. FeaturesSnaps into the On Glass adapter ring - no tools requiredLocks your optic's eyepiece into the exact position for auto-alignment with the MagTab8 sizes available (Alpha through Hotel) to cover the majority of spotting scopes and binoculars Sizing Guide:Know your eyepiece's outside diameter (in inches)? Match it here to find your spacer size.Alpha 1.55- 1.64"Bravo 1.65- 1.74"Charlie 1.75- 1.84"Delta 1.85- 1.94"Echo 1.95- 2.04"Foxtrot 2.05- 2.14"Gulf 2.15- 2.24"Hotel 2.25- 2.34"