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.
Highlights
The field of astronomical spectroscopy is rapidly approaching an impasse: the size and cost of instruments for extremely large telescopes (ELTs) is pushing the limits of what is feasible as they require optical components that are at the very edge of the physical size that can be achieved
The experiments were conducted with the aid of the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument, which is based at the 8-m Subaru Telescope, on Maunakea, Hawaii
All throughputs for the pre-injection elements including the sky, telescope, ADC, AO188, SCExAO optics and apodization optics were taken from [22]
Summary
The field of astronomical spectroscopy is rapidly approaching an impasse: the size and cost of instruments for extremely large telescopes (ELTs) is pushing the limits of what is feasible as they require optical components that are at the very edge of the physical size that can be achieved For this reason astronomers are constantly looking to expand their arsenal by embracing new technologies. Another key motivator is provided by spatial filtering This property ensures that the output beam from a single-mode fiber (SMF) is temporally invariant in regards to its shape (the amplitude can vary), which means the point-spread function (PSF) inside the instrument is well understood and can be calibrated out. For both of these two reasons, several groups have recently deployed diffraction limited spectrographs on 8-m class telescopes [11, 12]
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