Instruments are mounted at the cassegrain focus on the instrument support structure (ISS). Each ISS is a stiff cube that itself houses the Acquisition and Guidance Unit which contains the Acquisition Camera and Peripheral Wavefront Sensors (guiders). A science fold mirror mounted inside the ISS can be rotated to send the light from the telescope to any of four side-looking ports, or can be retracted so that the light goes to the up-looking port.

Each ISS is attached to the telescope via the cassegrain rotator, which may be oriented at any desired position angle and which normally rotates during observations to compensate for rotation of the field of the alt-az telescope.

The current assignment of instruments to ISS ports is: