VPH grating technology for the Thirty Meter Telescope instrumentation program

Abstract

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.

Roadmap on holography

“…Different technologies exist for making diffraction gratings, some of them already exploited in the astronomical field, some others that come from different technological fields. The dominant and baseline technology for Vis-NIR spectral range is the one based on volume phase holographic gratings (VPHGs) (figure 26(a)) based on a light induced periodic modulation of the refractive index (∆n) into a thin layer of a photosensitive material [223,224]. They have been applied in astronomical spectrographs since 2000 [225].…”

A New Photopolymer-based VPHG for Astronomy: The Case of SN 2013fj

“…One of the most important element along with the detector is the dispersing element. In the last 15 years, the Volume Phase Holographic Grating (VPHG) technology has gained a lot of interest in astronomical field and it has been used in some spectrographs (Baldry et al 2004;Bianco et al 2012;Barden et al 2000;Pazder & Clemens 2008). The reasons for such interest stem from the fact that these gratings show unique features, such as i) the high peak efficiency (up to 100% theoretically) both at low and large dispersion; ii) the ease of performance tuning and customisation (each VPHG is a master grating).…”