Title: Demonstration of an efficient, photonic-based astronomical spectrograph on an 8-m telescope,
Abstract: We demonstrate for the first time an efficient, photonic-based astronomical
spectrograph on the 8-m Subaru Telescope. An extreme adaptive optics system is
combined with pupil apodiziation optics to efficiently inject light directly
into a single-mode fiber, which feeds a compact cross-dispersed spectrograph
based on array waveguide grating technology. The instrument currently offers a
throughput of 5% from sky-to-detector which we outline could easily be upgraded
to ~13% (assuming a coupling efficiency of 50%). The isolated spectrograph
throughput from the single-mode fiber to detector was 42% at 1550 nm. The
coupling efficiency into the single-mode fiber was limited by the achievable
Strehl ratio on a given night. A coupling efficiency of 47% has been achieved
with ~60% Strehl ratio on-sky to date. Improvements to the adaptive optics
system will enable 90% Strehl ratio and a coupling of up to 67% eventually.
This work demonstrates that the unique combination of advanced technologies
enables the realization of a compact and highly efficient spectrograph, setting
a precedent for future instrument design on very-large and extremely-large
telescopes.