Precision optical alignment and bonding of Volume Phase Holographic (VPH) grisms for SAMOS

Developed largely for LSST follow-up, the SOAR Adaptive Module Optical Spectrograph (SAMOS) is a multi-object spectrograph that utilizes a Digital Micromirror Device (DMD) as a programmable slit mask for ground layer adaptive optics (GLAO) corrected observations. The SAMOS optical system is highly folded and very compact, using a grism exchange mechanism to switch between dispersion modes and resolving powers. To conserve space SAMOS uses round grisms, 70 mm in diameter. With no straight references edges on either the prisms or the VPH grating, the grism alignment and bonding proved to be challenging. This paper presents the method used to align and bond the SAMOS grisms. This method utilizes an elegant optical alignment technique to precisely align the VPH grating fringes to the prism facets during the bonding process, with minimal adhesive cleanup, zero bubbles in the clear aperture, and in the end producing exceptionally well-aligned spectra at the detector.