SBIG accessory that allows you to more firmly attach the camera to the telescope instead of using one set screw. Comes with parfocal eyepiece as well to aid in focusing. List price is $395. First reasonable offer takes it.
From SBIG Website:
The T-THREAD VISUAL BACK allows users to rigidly mount the CCD imaging camera head to the rear of an SCT. The adaptor screws onto the rear bulkhead of the SCT and then screws directly onto the front of the CCD imaging camera head. This is a prefered method of mounting over inserting the CCD imaging camera into a draw tube with a single locking screw to support it