Please note: This text was taken directly from the manufacturer.
For those observing targets where you need more power than your eyepieces alone can provide, we offer the Tri-Mag. It gives a big 3x magnification boost to any 1.25" eyepiece it's used with. This is desirable for lunar and planetary observing and for extracting higher powers from "fast" telescopes such as f/5 and f/4 refractors and reflectors. The Tri-Mag utilizes a multi-coated achromatic "negative" lens that effectively triples the focal length of the telescope's optics. The housing is precision machined from aluminum stock and black anodized to a hard finish. Both the housing and the chrome portion of the barrel are glare threaded and blackened inside to provide maximum contrast. The lens end of the barrel is threaded to accept standard filters. Length, 5". Weighs 5.8 oz. One-year limited warranty.