PEPSI prisms and cross dispersers for LBT

PEPSI prisms and cross dispersers for LBT

pepsi_thales00Under a contract issued by the AIP, Thales SESO has manufactured a set of 6 Cross Dispersers (one for spectral band in-between 365nm-950nm) used in the PEPSI spectrograph installed at focal plane of the Large Binocular Telescope (LBT).

The technical challenges of these cross-dispersers were the following:

• Large dimension: each prism of the disperse is more than 200mmx200mm size
• Cemented concept including a pair of entrance and exit prism and a Volume-Phase Holographic (VPH) grating in the middle


• Tight cementing tolerance (less than 1mrad around optical axis) in order to align the lines of the VPH and the optical dihedral angles of the prisms
• Optical antireflection coatings with wide range of incidence angles