VLT MAVIS: optical designs of the reflective IFU and transmissive spectrograph

MAVIS is the next instrument to go on the VLT. It is an imager and IFU spectrograph fed by Multi-Conjugate Adaptive Optics. It is presently in Phase-B (Preliminary Design Phase). The spectrograph will be preceded by an Integral Field Unit (IFU) with a choice of fields of 3.6 '' x2.5 '' with square spaxels 25 mas wide or 7.2 '' x5.0 '' with 50 mas spaxels. Two sets of interchangeable fore-optics permit the change of field size. The IFU will be based on the concept of Advanced Image Slicer present in many instruments as MUSE and KMOS on the VLT. 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. A final and third mirror array on the slit places the pupil of each slice image at the right place in the spectrograph. Toroidal surfaces in each foreoptics arm give the 2X magnification needed in the spectral direction. Each of the 2 subfields has its own lens spectrograph with a 9k x 9k detector and 4 interchangeable grisms giving a resolution from 5000 to 15000. High transmission glasses are used to ensure excellent throughput across the full wavelength range into the blue, covering 370-950 nm. The slice width being near the diffraction limit, special consideration was given to the diffraction focal ratio degradation.