The introduction of volume phase holographic (VPH) gratings into astronomy over a decade ago opened new
possibilities for instrument designers. In this paper we describe an extension of VPH grating technology that will have
applications in astronomy and beyond: curved VPH gratings. These devices can disperse light while simultaneously
correcting aberrations. We have designed and manufactured two different kinds of convex VPH grating prototypes for
use in off-axis reflecting spectrographs. One type functions in transmission and the other in reflection, enabling Offnerstyle
spectrographs with the high-efficiency and low-cost advantages of VPH gratings. We will discuss the design
process and the tools required for modelling these gratings along with the recording layout and process steps required to
fabricate them. We will present performance data for the first convex VPH grating produced for an astronomical
The Visible Integral-field Replicable Unit Spectrograph (VIRUS) is a baseline array of 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET). VIRUS is the first optical astronomical instrument to be replicated on an industrial scale, and represents a relatively inexpensive solution for carrying out large-area spectroscopic surveys, such as the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic (VPH) grating with a 138 mm diameter clear aperture as its dispersing element. The instrument utilizes the grating in first-order for 350 < λ (nm) < 550. Including witness samples, a suite of 170 VPH gratings has been mass produced for VIRUS. Here, we present the design of the VIRUS VPH gratings and a discussion of their mass production. We additionally present the design and functionality of a custom apparatus that has been used to rapidly test the first-order diffraction efficiency of the gratings for various discrete wavelengths within the VIRUS spectral range. This device has been used to perform both in-situ tests to monitor the effects of adjustments to the production prescription as well as to carry out the final acceptance tests of the gratings' diffraction efficiency. Finally, we present the as-built performance results
for the entire suite of VPH gratings.
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
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.
The Robert Stobie Spectrograph Near Infrared Instrument (RSS-NIR), a prime focus facility instrument for the 11-meter
Southern African Large Telescope (SALT), is well into its laboratory integration and testing phase. RSS-NIR will
initially provide imaging and single or multi-object medium resolution spectroscopy in an 8 arcmin field of view at
wavelengths of 0.9 - 1.7 μm. Future modes, including tunable Fabry-Perot spectral imaging and polarimetry, have been
designed in and can be easily added later. RSS-NIR will mate to the existing visible wavelength RSS-VIS via a dichroic
beamsplitter, allowing simultaneous operation of the two instruments in all modes. Multi-object spectroscopy covering a
wavelength range of 0.32 - 1.7 μm on 10-meter class telescopes is a rare capability and once all the existing VIS modes
are incorporated into the NIR, the combined RSS will provide observational modes that are completely unique.
The VIS and NIR instruments share a common telescope focal plane, and slit mask for spectroscopic modes, and
collimator optics that operate at ambient observatory temperature. Beyond the dichroic beamsplitter, RSS-NIR is
enclosed in a pre-dewar box operating at -40 °C, and within that is a cryogenic dewar operating at 120 K housing the
detector and final camera optics and filters. This semi-warm configuration with compartments at multiple operating
temperatures poses a number of design and implementation challenges. In this paper we present overviews of the RSSNIR
instrument design and solutions to design challenges, measured performance of optical components, detector
system optimization results, and an update on the overall project status.
The Visible Integral field Replicable Unit Spectrograph (VIRUS) is an array of at least 150 copies of a simple, fiber-fed integral field spectrograph that will be deployed on the Hobby-Eberly Telescope (HET) to carry out the HET Dark Energy Experiment (HETDEX). Each spectrograph contains a volume phase holographic grating as its dispersing element that is used in first order for 350 < λ(nm) < 550. We discuss the test methods used to evaluate the performance of the prototype gratings, which have aided in modifying the fabrication prescription for achieving the specified batch diffraction efficiency required for HETDEX. In particular, we discuss tests in which we measure the diffraction efficiency at the nominal grating angle of incidence in VIRUS for all orders accessible to our test bench that are allowed by the grating equation. For select gratings, these tests have allowed us to account for < 90% of the incident light for wavelengths within the spectral coverage of VIRUS. The remaining light that is unaccounted for is likely being diffracted into reflective orders or being absorbed or scattered within the grating layer (for bluer wavelengths especially, the latter term may dominate the others). Finally, we discuss an apparatus that will be used to quickly verify the first order diffraction efficiency specification for the batch of at least 150 VIRUS production gratings.
Volume Phase Holographic Gratings (VPHG) provide unique advantages over traditional dispersive elements and are
being considered for instruments on many large telescopes, including the Wide Field Optical Spectrograph (WFOS) for
the Thirty Meter Telescope (TMT). In this paper we review the properties of VPHG particularly with regard to their use
in large multi-object spectrographs such as WFOS. Design considerations include optimal sizes and working angles, and
variations in blaze efficiencies as a function of grating and field angles. For instruments like WFOS, a gratings mosaic is
a promising solution to meet the size requirements. The methodologies of mosaics and the required tolerances are
evaluated. VPH gratings may also be used in echelette mode with significant advantages, although more lab tests should
be carried out to explore and optimize performance. A brief status report on the VPHG development activities in the
Goodman lab is included, with a plan for future development.
Construction of the Southern African Large Telescope (SALT) was largely completed by the end of 2005 and since then
it has been in intensive commissioning. This has now almost been completed except for the telescope's image quality
which shows optical aberrations, chiefly a focus gradient across the focal plane, along with astigmatism and other less
significant aberrations. This paper describes the optical systems engineering investigation that has been conducted since
early 2006 to diagnose the problem. A rigorous approach has been followed which has entailed breaking down the
system into the major sub-systems and subjecting them to testing on an individual basis. Significant progress has been
achieved with many components of the optical system shown to be operating correctly. The fault has been isolated to a
major optical sub-system. We present the results obtained so far, and discuss what remains to be done.
The Goodman Spectrograph is an imaging, multi-object spectrograph for the SOuthern Astrophysical Research telescope (SOAR). It is one of the first designed to take advantage of Volume Phase Holographic (VPH) gratings by employing an articulated camera. This aspect of the mechanical design has had complicating effects on a number of usually simple systems, and has led to some unorthodox solutions. The spectrograph is also highly optimized for efficiency from 320 to 850 nm, and designed for rapid configuration changes, so that its throughput makes it competitive with instruments on larger telescopes. We present the high level requirements that have driven the mechanical and electronic systems, and show their implementation in the completed instrument. It is too early to assess the overall system performance, but tests of the modular subsystems show promising results. We discuss the expected overall performance and discuss mitigation strategies should that performance fall short of our goals.
The Goodman spectrograph is an all-refracting articulated-camera high-throughput imaging spectrograph for the SOuthern Astrophysical Research telescope (SOAR). It is designed to take advantage of Volume Phase Holographic (VPH) gratings. Due to the high level of mechanical complexity, a fully graphical control system with parallel motor control was developed. We have developed a software solution in LabVIEW that functions as a control system, component management tool, and engineering platform. A modular software design allows other instrument projects to easily adopt our approach. Distinguishing features of the control system include automated configuration changes, remote capability, and PDA control for component swaps.
We describe a novel optical design for a low-resolution imaging spectrograph that incorprates volume phase holographic (VPH) gratings. This spectrograph will provide imaging over a 5 foot square field of view, and single or multi-object spectroscopy with resolutions between 1000 and 8000. Our design choices have been dominated by a desire to preserve the superb image quality the SOAR telescope is expected to deliver, and to maximize throughput over the wavelength range of 320 to 850 nm. The resulting design is unusual in two respects: the angle between the collimated beam and camera optical axis is mechanically variable to exploit the efficiency advantage of volume holographic gratings, and all the optical elements are refracting, to maintain high throughput in the UV. In addition to the pre- construction collimator and camera design for the spectrograph, we also present our evaluation of sample volume phase holographic gratings to illustrate the advantages and difficulties they present for astronomical spectroscopy.