Optical design of the KMOS slicer system

KMOS is a 2nd generation instrument in project for the VLT. It is an NIR multi-object integral-field spectrograph capturing 24 fields of 2.8” x 2.8” anywhere on a 7.2’ field. The 24 fields are fed to 24 IFUs with a resolution of 0.2”. KMOS is made of 3 stages: the pickoff system which captures the 24 images, the slicer system and the spectrographs. The slicer system is then the optical link between the 24 images captured by the pickoff system and the 3 slits in front of the 3 spectrographs. It is made of 24 Advanced Image Slicer systems similar to the GNIRS IFU except for 2 modifications: there are 3 mirrors with power in the fore-optics instead of 2 and there are 2 lines of pupil mirrors instead of one. The fore-optics of each system is made of 3 off-axis aspheric mirrors that accomplish a series of task: transfer the field image, position the pupil, magnify the image, give an anamorphic magnification to the image, and give a high image quality. The possibility of rotating the image with the fore-optics has also been studied.