Laboratory evaluation of a pre-spectrograph wavelength splitter and pupil slicer for fiber optic-based multiobject spectroscopy

Fiber optic based multi-object spectrographs have been in use for astronomical surveys since the 1980's. Towards the end of the 1990's and the early 2000's the multiplex capability grew to allow hundreds of astronomical targets to be observed simultaneously within the same telescope field of view (e.g. 2dF[1], Sloan Digital Sky Survey[2]). Additionally, recent instruments have been developed with considerably higher target counts and with implementation on larger aperture telescopes (LAMOST[3], HETDEX[4], DESI[5], 4MOST[6], WEAVE[7], PFS[8], MUST[9]. Design studies are currently in progress for more massively multiplexed spectrographs on telescopes with apertures of 10 to 20 meters (MSE[10], WST[11], Chinese 12m[12]) with target counts approaching 20000 and larger. The increasing number of spectrographs needed for these facilities are ever larger in size with correspondingly difficult, risky, and costly designs. We present a design and laboratory evaluation of a prototype pre-spectrograph wavelength splitter and pupil slicer (WSPS). The goal is to remove the need for the large wavelength splitters that are currently implemented within the multiwavelength channel spectrograph optics and to achieve a more efficient packaging of the resultant single channel spectrographs. The WSPS is an optical assembly that interfaces to a subset of the incoming fiber optics (~100 per unit), splits the light into multiple wavelength channels (Blue, Green, Red, J, and H), and allows implementation of an arrayed output configuration to slice up the pupil. Performance of the WSPS must achieve excellent light coupling of the incoming light to the output channels and suppress scattered light cross talk so that bright targets illuminating the WSPS don't contaminate the spectra of the fainter targets that illuminate that same WSPS unit. Nano-structured etching (TelAzTec) is used on the Fused Silica lenses that clad the optical assemblies to suppress optical ghosting.