Use this female T-thread to male T-thread spacer to extend standard T-thread adapters to achieve the correct backfocus for your telescope. Many accessories are design for a 55mm backfocus (the standard for a DSLR and T-ring); Atik 4-series CCD cameras have a 13mm backfocus, so this 42mm spacer makes up the difference. Also available in 0.25", 0.5", 0.625", 0.75" and 1.00" lengths. Outside diameter is 1.90". Anodized black aluminum, internally threaded for light baffling. Made in USA.