A T-Adapter means that you can attach your 35mm SLR camera to the prime focus of your telescope or spotting scope. This arrangement
...The Celestron Trail Seeker 100 Straight Spotting Scope has a rugged & durable rubber armored housing that is fully waterproof
...The Celestron Telescope Camera T Adaptor with 2x Barlow Lens means that you can attach your 35mm SLR camera to the prime focus of your telescope or spotting scope.