1.25 inch calibration eyepiece is of short size, made from aluminum alloy and finished by CNC manufacturing. Used to calibrate the optical axis of a refractor, put the collimation eyepiecre into the tube. Make sure the black spot is in the center of 2 lines of collimation eyepiece. Has a 45 degree plate for easier visual accuracy. Collimation is the alignment of elements inside an optical system like a telescope. Suitable for refracting telescope