Efficiency Measurements and Installation of a New Grating for the OSIRIS Spectrograph at Keck Observatory

摘要

OSIRIS is a near-infrared integral field spectrograph operating behind theadaptive optics system at W. M. Keck Observatory. While OSIRIS has been ascientifically productive instrument to date, its sensitivity has been limitedby a grating efficiency that is less than half of what was expected. Thespatially averaged efficiency of the old grating, weighted by error, ismeasured to be 39.5 +/- 0.8 % at {\lambda} = 1.310 {\mu}m, with large fielddependent variation of 11.7 % due to efficiency variation across the gratingsurface. Working with a new vendor, we developed a more efficient and uniformgrating with a weighted average efficiency at {\lambda} = 1.310 {\mu}m of 78.0+/- 1.6 %, with field variation of only 2.2 %. This is close to double theaverage efficiency and five times less variation across the field. The newgrating was installed in December 2012, and on- sky OSIRIS throughput shows anaverage factor of 1.83 improvement in sensitivity between 1 and 2.4 microns. Wepresent the development history, testing, and implementation of this newnear-infrared grating for OSIRIS and report the comparison with thepredecessors. The higher sensitivities are already having a large impact onscientific studies with OSIRIS.