For Pro Zoom Spotters.
A T-Adapter allows you to attach your 35mm SLR camera to the prime focus of your telescope or spotting scope. This arrangement is used for terrestrial photography and short exposure lunar and planetary photography. It can also be used for long exposure deep-sky photography when using a separate guidescope.
Both a T-Adapter and T-Ring are required to mount a 35mm SLR camera to your instrument