Combined narrowband imager-spectrograph with volume-phase holographic gratings

摘要

In the present work we discuss a possibility to build an instrument with twooperation modes - spectral and imaging ones. The key element of such instrumentis a dispersive and filtering unit consisting of two narrowband volume-phaseholographic gratings. Each of them provides high diffraction efficiency in arelatively narrow spectral range of a few tens of nanometers. Besides, theposition of this working band is highly dependent on the angle of incidence. Sowe propose to use a couple of such gratings to implement the two operationalmodes. The gratings are mounted in a collimated beam one after another. In thespectroscopic mode the gratings are turned on such angle that the diffractionefficiency curves coincide, thus the beams diffracted on the first grating arediffracted twice on the second one and a high-dispersion spectrum in a narrowrange is formed. If the collimating and camera lenses are corrected for a widefield it is possible to use a long slit and register the spectra from itsdifferent points separately. In the imaging mode the gratings are turned tosuch angle that the efficiency curves intersect in a very narrow wavelengthrange. So the beams diffracted on the first grating are filtered out by thesecond one except of the spectral component, which forms the image. In thiscase the instrument works without slit diaphragm on the entrance. We provide anexample design to illustrate the proposed concept. This optical scheme works inthe region around 656 nm with F/# of 6.3. In the spectroscopic mode it providesa spectrum for the region from 641 to 671 nm with reciprocal linear dispersionof 1.4 nm/mm and the spectral resolving power higher than 14000. In the imagingmode it covers linear 12mm x 12mm field of view with spatial resolution of15-30 lines/mm.