English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

We report the discovery of optical ghosts generated when using Volume PhaseHolographic (VPH) gratings in spectrographs employing the Littrowconfiguration. The ghost is caused by light reflected off the detector surface,recollimated by the camera, recombined by, and reflected from, the grating andreimaged by the camera onto the detector. This recombination can occur in twodifferent ways. We observe this ghost in two spectrographs being developed bythe University of Wisconsin - Madison: the Robert Stobie Spectrograph for theSouthern African Large Telescope and the Bench Spectrograph for the WIYN 3.5mtelescope. The typical ratio of the brightness of the ghost relative to theintegrated flux of the spectrum is of order 10^-4, implying a recombinationefficiency of the VPH gratings of order 10^-3 or higher, consistent with theoutput of rigorous coupled wave analysis. Any spectrograph employing VPHgratings, including grisms, in Littrow configuration will suffer from thisghost, though the general effect is not intrinsic to VPH gratings themselvesand has been observed in systems with conventional gratings in non-Littrowconfigurations. We explain the geometric configurations that can result in theghost as well as a more general prescription for predicting its position andbrightness on the detector. We make recommendations for mitigating the ghosteffects for spectrographs and gratings currently built. We further suggestdesign modifications for future VPH gratings to eliminate the problem entirely,including tilted fringes and/or prismatic substrates. We discuss the resultantimplications on the spectrograph performance metrics.

Recombination Ghosts in Littrow Configuration: Implications for Spectrographs Using Volume Phase Holographic Gratings