A T-Adapter is a device that adapts the telescope side connection to threads you can attach accessories to on the camera side. In most cases, you will need both a T-Adapter and a T-Ring to be able to attach your DSLR or Mirrorless camera to your telescope. A handful of telescopes, however, come with the right threads to attach a T-Ring already on the back so you don't need a T-Adapter. T-Adapters are often specific to your telescope, so be sure you get the right one.