MAVIS IFU with AO for VLT: image slicer concept and design

MAVIS is a future imaging spectrograph for the VLT in which the spectrograph is fed by an IFU. Imager and IFU are fed by Multi-Conjugate Adaptive Optics for the wavelength range 370 nm to 930 nm. The spectrograph will deliver a spectral resolution of more than 4000. The IFU field has a choice of 2 spaxel sizes. It is 9 arsec2 with 25 mas spaxels and 36 arsec2 with 50 mas. The design follows the proven concept of Advanced Image Slicer (AIS) as for Gemini NIFS and GNIRS, VLT MUSE and KMOS, JWST NIRSpec, and many others. In the present design, the field is first split in 2 and each subfield is imaged on a slicer mirror array made of long thin mirrors that slice the field into 50 images and send them in different directions to be reimaged side by side on the slit by another mirror array. Additional optics on the slit reimage the pupil at the right place in the spectrograph. Three different options are under study for the slit optics. One is a mirror array of considerably lower cost than the standard design in an AIS. At 25 mas, the spaxels are near the diffraction limit of the longest wavelength. This present challenges not present for seeing limited IFUs as focal ratio degradation due to diffraction by the slices. Another challenge comes from the short minimum wavelength. It is difficult to manufacture efficient reflection coatings for the whole wavelength range. Transmissive fore-optics were then also studied. The field splitter which sends half the field to each of the 2 arms was integrated into the fore-optics. This removes 2 air-glass surfaces from each arm.