Calibration of the KMOS Multi-Field Imaging Spectrometer

When it is delivered to the VLT KMOS, will be the first near infrared (0.8–2.5 μm) spectrograph to use multiple integral fields units. Pick-off arms provide the optical relay to image objects selected from the patrol field at the Nasmyth focus onto 24 image slicing IFUs. The output from 8 IFUs forms a single slit which feeds one of three identical spectrometers. The calibration of such an instrument presents a number of challenges. Flat field and wavelength calibration for the instrument will be provided by an internal calibration system of unique design. External light sources are fed via integrating spheres and high transmission light pipes to a calibration sphere with 24 output ports addressed by the 24 arms. Obtaining a high degree of field flatness is key to the sensitivity of NIR instruments. The KMOS calunit is predicted to deliver flatnesses of ∼0.1% over the KMOS IFU fields.