The telescope uses what's known as a Naismith port, called NA-2, for mounting instruments. This requires a tertiary mirror to send the light from the normal Casegrain prime-focus through the pivot bearing to the instrument. The advantage is that the telescope operator can change the instrument in less than 15 minutes, so multiple instruments can be used in a night as needed by the observer.
This particular instrument was the Grissom Imager, known as Grimm. It has since been retired.