The LUCI instruments have dedicated calibration units that can be moved in front of their entrance windows when required. Three arc lamps (Neon, Argon and Xenon) are provided for wavelength calibration, in addition to three halogen lamps (listed as Halo1, Halo2, and Halo3) for flat fields.
The Calibration Units are articulated, the units pivot out over the primary mirrors and then bend down to position their exit ports in the beam. Moving the calibration unit in front of LUCI obstructs the light coming from the telescope.
When deployed the calibration unit also occupies the same space as the ARGOS dichroic. There is an interlock to prevent the calibration unit from moving into the beam when the ARGOS dichroic is deployed, and vice versa.
The CUs do not provide perfectly even illumination to the instrument, although they are not bad. For the arc lamps this does not matter, but for flatfields you should be aware of this, how it can affect your data, and take the necessary additional steps to correct it if it matter to your science. Recommended exposure times for the various combinations of camera, filter, and grating are discussed in more detail in the Calibration pages.
The calibration unit can be commanded in place with Calibration Scripts, which also can turn on and off desired lamps. Or the unit can be brought in and lamps manipulated using the Instrument GUI (left).