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J. A. Arns, W. S. Colburn (Kaiser Optical Systems, Inc.), S. C. Barden, J. B. Williams (N.O.A.O.)
Volume-phase holographic (VPH) gratings show great promise as dispersing elements in the next generation of optical, and possibly near-IR, astronomical spectrographs. These gratings have high diffraction efficiencies (sometimes approaching 100%), can be tuned to peak up the efficiency at a desired wavelength, can provide very high dispersive power, are encapsulated so that anti-reflection coatings can be applied and the surfaces can be cleaned, can be fabricated with complex grating configurations, and are more easily customized than traditional surface-relief gratings. The transmissive nature of VPH gratings allows one the option of always working in a Littrow mode, resulting in simplified camera optics due to the lack of anamorphic magnification. The VPH technology is also in a state in which very large grating formats are possible (approaching meter sized elements).
We present results from an NSF funded program in which eight VPH gratings are being fabricated and evaluated. At the end of the study, NOAO will make many of the gratings available to the US astronomical community through an NSF distribution for their utilization into novel astronomical instruments. Most of the gratings are designed around a 75 mm diameter beam size. The gratings include a 300 l/mm grating with good performance across the full optical spectrum, a 1200 l/mm grating with optimal performance at 532 nm, a 2400 l/mm grating designed for performance at 532 nm, a 2400 l/mm grating with prisms designed for optimal performance at 1064 nm, a 4800 l/mm grating with prisms for operation at 532 nm, a dual H-alpha/H-beta multiplex grating (1200 l/mm and 1620 l/mm), a 1200 l/mm reflection grating with peak performance at 532 nm, and an attempt to make a high order (about m=10 at 532 nm) grating with 300 l/mm.
There are presently several spectrograph concepts being pursued around the community which plan to utilize this grating technology on 4-meter class telescopes (including SOAR, the AAT, and KPNO).
This project is supported under Cooperative Agreement AST-9613615 awarded by the National Science Foundation.
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