The extension tube set is used to obtain the correct focal length or exact spacing required,therebysolving the spacing problem in astrophotography. By combining multiple extenders of different lengths,you get the right focal length to focuswith the camera or eyepiece. The M42x0.75 threadson both ends make it versatile. External thread M42x 0.75;internal thread M42x0.75; All metal construction and anodizing aluminium process;durable for long time use. The required spacing can be achieved by combining several T2-Extension Tube of different length;you can use 1 piece;2 pieces;3 pieces ;4 piece;5 piece;6 piece of the extension tubes as you like.