Stray light analysis and reduction for IFU spectrograph LLAMAS

Abstract

The Large Lenslet Array Magellan Spectrograph (LLAMAS) is an Integral Field Unit (IFU) spectrograph under construction as a facility instrument for the 6.5-meter Magellan Telescopes. For each pointing, LLAMAS delivers 2400 optical spectra (λ =350-970nm) over a 37”x37” celestial solid angle with a resolution of 2000 through a densely packed microlens+fiber array and replicated low-cost spectrographs. One of our main science goals is to study circumgalactic gas through Lyα emission. To achieve the required signal-to-noise ratio for these observations, LLAMAS must minimize stray light reaching the detector: diffuse scattered light must stay below 0.25% of sky flux and ghost images must not exceed 0.1% of the source signal. We present a non-sequential ray tracing analysis of a simplified LLAMAS model using Photon Engineering’s FRED Optical Engineering Software. We focus on stray light resulting from the volume phase holographic grating and from focal ratio degradation of the fibers. The analysis feeds into a discussion of the design and fabrication of baffles to mask the primary sources of stray light. Additionally, we develop a backup system of mounting rings inside of the cameras where pre-made baffles can be quickly added as needed. Finally, we report on the laboratory performance of a 2-camera LLAMAS prototype featuring the aforementioned stray light interventions.