Instrument Support Structure
Instruments are mounted at the Cassegrain focus on the instrument support structure (ISS). The 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.
The 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 instrument specifications webpage provides detailed information relevant to ISS-mounted instruments including interfaces and environmental conditions.