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We describe designs for spectrometers employing convex dispersers. The Offner spectrometer was the first such
instrument; it has almost exclusively been employed on satellite platforms, and has had little impact on ground-based
instruments.
We have learned how to fabricate curved Volume Phase Holographic (VPH) gratings and, in contrast to the planar gratings
of traditional spectrometers, describe how such devices can be used in optical/infrared spectrometers designed specifically
for curved diffraction gratings. Volume Phase Holographic gratings are highly efficient compared to conventional surface
relief gratings; they have become the disperser of choice in optical / NIR spectrometers.
The advantage of spectrometers with curved VPH dispersers is the very small number of optical elements used (the simplest
comprising a grating and a spherical mirror), as well as illumination of mirrors off axis, resulting in greater efficiency and
reduction in size. We describe a “Half Offner" spectrometer, an even simpler version of the Offner spectrometer. We
present an entirely novel design, the Spherical Transmission Grating Spectrometer (STGS), and discuss exemplary
applications, including a design for a double-beam spectrometer without any requirement for a dichroic.
This paradigm change in spectrometer design offers an alternative to all-refractive astronomical spectrometer designs,
using expensive, fragile lens elements fabricated from CaF2 or even more exotic materials. The unobscured mirror layout
avoids a major drawback of the previous generation of catadioptric spectrometer designs.
We describe laboratory measurements of the efficiency and image quality of a curved VPH grating in a STGS design,
demonstrating, simultaneously, efficiency comparable to planar VPH gratings along with good image quality. The stage
is now set for construction of a prototype instrument with impressive performance.
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