The design of ERIS for the VLT

The Enhanced Resolution Imager and Spectrograph (ERIS) is the next-generation instrument planned for the Very Large Telescope (VLT) and the Adaptive Optics Facility (AOF) 1 . It is an AO assisted instrument that will make use of the Deformable Secondary Mirror and the new Laser Guide Star Facility (4LGSF), and it is designed for the Cassegrain focus of the telescope UT4. The project just concluded its conceptual design phase and is awaiting formal approval to continue to the next phase. ERIS will offer 1-5 μm imaging and 1-2.5 μm integral field spectroscopic capabilities with high Strehl performance. As such it will replace, with much improved single conjugated AO correction, the most scientifically important and popular observing capabilities currently offered by NACO 2 (diffraction limited imaging in JM band, Sparse Aperture Masking and APP coronagraphy) and by SINFONI 3 , whose instrumental module, SPIFFI, will be re-used in ERIS. The Cassegrain location and the performance requirements impose challenging demands on the project, from opto-mechanical design to cryogenics to the operational concept. In this paper we describe the baseline design proposed for ERIS and discuss these technical challenges, with particular emphasis on the trade-offs and the novel solutions proposed for building ERIS.