New High-Efficiency Grating Technology

A relatively new grating technology promises significant advances in the performance of the next generation of optical and near-IR astronomical spectrographs. Volume-phase holographic (VPH) gratings diffract light by refractive index modulations within a thin layer of material sandwiched between two glass substrates. VPH gratings can achieve higher diffractive efficiencies in many applications than currently available surface gratings, and offer improved longevity since the grating volume is actually encapsulated between two substrates.

NOAO has been developing the technology to design and build VPH gratings in collaboration with Kaiser Optical Systems Inc. These gratings have transmission efficiencies ; the highest efficiency grating tested to date has a transmission of . Figure 19 shows the predicted total system throughput for the telescope and spectrograph (ignoring slit losses) for the baseline concept described here and for the specific case of resolution R=5000. The model for the grating efficiency has been verified using actual laboratory measurements of VPH gratings. The improvement in efficiency over Keck+LRIS comes partly from the use of the higher efficiency grating, but also partly from the use of protected silver coatings (similar to that being developed for Gemini North) on the primary and secondary surfaces.

  

Figure 19: The anticipated total system efficiency for SWIFT, exclusive of the atmosphere and the slit losses, exceeds 50% across the optical (Top) and 40% across the near IR (Bottom). The CCD characteristics adopted are those of typical SITe 2Kx4K CCDs, such as those used in the NOAO Mosaic camera. The IR efficiency is lower relative to the optical efficiency because the optical coatings (simple SolGel/MgF₂) were optimized for 5000 Å in the scenarios represented here.