A second set of ZnSe grisms for the Rapid Infrared Imager/Spectrograph (RIMAS)

The Rapid Infrared Imager/Spectrograph (RIMAS) is an instrument designed to observe gamma ray burst afterglows in the near infrared between 0.9 and 2.4 μm. Dispersion in the moderate resolution mode (R~4000) is provided by a pair of ZnSe grisms: one covering the Y and J bands and the other covering the H and K. LLNL diamond machined the original pair of grisms several years ago, which are currently installed in the RIMAS instrument. We were tasked to fabricate a second pair of grisms and incorporated several improvements developed during the fabrication of the first set. The second HK grism showed some chipping near the end of the cut, but overall was much higher quality than the first HK grism. The rms surface error was reduced by a factor of 3 over the first HK to 0.085l at 633 nm and the wavefront error in the transmitted beam is well below the diffraction limit in the H and K bands. We measured a diffraction efficiency of 47% at 633 nm. The second YJ grism is currently being machined.