All instruments on UKIRT (except the future facility widefield imager WFCAM) are mounted at broken Cassegrain foci facing one of four exit ports of the Instrument Support Unit (ISU), which is attached to the bottom of the mirror cell. The ISU contains a rotating tertiary mirror with an IR-reflecting dichroic coating, which directs the IR beam to the to the selected port, while allowing visible light to continue downwards to the acquisition and guiding systems.

Each port on the Instrument Support Unit (ISU) is fitted with an adjustable alignment flange, which can directly support instruments up to about 50 kg in mass. These are not often used now: only two facility instruments are now supported from the ISU. All other instruments are now supported directly from the primary mirror cell.

The ultimate size limitation is probably space (which must also be found for any electronics requiring to be located near the instrument) and energy dissipation (no more than a couple of hundred W may be added to the power being dissipated at the mirror cell) rather than mass.

Users proposing to bring their own instruments to UKIRT should note:

1. Drawings of bolt circles, flanges, mounting platforms and space constraints are available on request, from either the UK ATC (Astronomy Technology Centre) or the UKIRT group at the JAC, Hawaii.
2. Details of proposed visiting instruments must be discussed with engineering and science staff at UKIRT (Hawaii), at least 3, preferably 6 months in advance of the PATT meeting for the semster in which it is proposed to employ the new instrument. UKIRT engineers will need to know the masss and its distribution, and will need drawings of the proposed mounting flange. (It is the responsibility of the user to ensure that his or her instrument interfaces to the telescope, but UKIRT staff must be consulted.)
3. For a large instrument (over 100 kg) at least 6 months should be allowed for JAC staff to prepare for its reception at UKIRT.
4. PIs of new instruments should request an allocation of engineering time for checkout, testing and shakedown before a science observing run is planned.