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We report in this paper the test of a plane holographic optical element to beused as an aberration-corrected grating for a slitless spectrograph, insertedin a convergent telescope beam. Our long term objective is the optimisation ofa specific hologram to switch the auxiliary telescope imager of the Vera RubinObservatory into an accurate slitless spectrograph, dedicated to theatmospheric transmission measurement. We present and discuss here the promisingresults of tests performed with prototype holograms at the CTIO $0.9\,$mtelescope during a run of 17 nights in May-June 2017. After their on-skygeometrical characterisation, the performances of the holograms asaberration-balanced dispersive optical elements have been established byanalysing spectra obtained from spectrophotometric standard stars andnarrow-band emitter planetary nebulae. Thanks to their additional opticalfunction, our holographic disperser prototypes allow to produce significantlybetter focused spectra within the full visible wavelength domain$[370,1050]\,$nm than a regular grating, which suffers from strong defocusingand aberrations when used in similar conditions. We show that the resolution ofour slitless on-axis spectrograph equipped with the hologram approaches itstheoretical performance. While estimating the benefits of an hologram for thespectrum resolution, the roadmap to produce a competitive holographic elementfor the Vera Rubin Observatory auxiliary telescope has been established.

A transmission hologram for slitless spectrophotometry on a convergent telescope beam. 1. Focus and resolution