One end of this 2" diameter x 2" long extension tube fits into 2" focuser drawtubes. The opposite end is a 2" diameter eyepiece holder that uses a non-marring brass clamp ring to hold 2" eyepieces and accessories in place. The captive thumbscrew that tightens the clamp ring can't be lost in the field. This extension tube makes up for the loss of light path photographically when a 2" star diagonal . . .