Details from the web below:
A piggyback bracket is a simple astronomical accessory that can help you produce stunning photographs of the night sky. It allows you to attach a DSLR camera and lens to any Celestron SCT optical tube assembly and take advantage of the fact that your telescope has a dual-axis motor drive. When your telescope is running with the camera attached, you can take longer exposures without star trails, and capture details in objects that would have been impossible had your camera been attached to a photographic tripod or a telescope without a drive system.
Experiment with different camera lenses and subjects. Some photographers love to capture comets with a relatively long, fast lens, and the Milky Way, as well as a myriad combination of terrestrial and celestial scenes, are yours for the taking when you own a Celestron SCT, a piggyback mount, a DSLR camera, and a couple of lenses.
The Piggyback adapter mounts to the rear cell of all Celestron 5, 6, 8, 11, 9.25 and 11-inch SCT & edgeHD
Mounting holes in the base of the piggyback allow it to be installed in pre-drilled threaded holes in the rear cell of any of the above Celestron telescopes, using the supplied mounting hardware. The camera platform of the piggyback has a standard 1/4" bolt with a hand tighten knob in the platform to hold your camera in place.