Machining a YJ band grism in ZnSe for the Rapid Infrared Imager Spectrograph (RIMAS): improvements in technique

摘要

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. The quality of the HK grism was less than desired, suffering from moderate to severe chipping of the grooves and mediocre wavefront error. As a result, several improvements in technique were implemented during the machining of the YJ grism. Groove chipping results from cumulative tool wear, which was reduced when the crystal axes of the diamond tool were aligned with the cutting edges. Minimizing the total length of cut also reduces tool wear. We created a raised elliptical mesa on the substrate corresponding to the optical footprint of the beam, so that grooves are machined only within the optical footprint. Finally, we modified the machining fixture to prevent oil build up beneath the grism blank. Removing the buoyancy effect of the oil eliminated astigmatism in the finished grating and reduced the rms wavefront error by nearly a factor of 3.