10.1117/2.1201301.004565 Sliced pupil gratings: a compact solution for increased spectral resolution Maria Luisa Garc´ıa Vargas, Ernesto S´ anchez-Blanco Mancera, Manuel Maldonado Medina, Ana P ´ erez-Calpena, Ismael Mart´ınez-Delgado, Armando Gil de Paz, Jes ´ us Gallego-Maestro, and Jaime Zamorano-Calvo A novel astronomical spectrograph design enhances instruments with multiple resolutions, strong envelope constraints, or that are predesigned to use compact dispersive elements. Spectrographs are the main tool in astronomical instrumenta- tion, providing information about the physical and chemical processes of gas and stars. In recent decades, observational as- tronomers have wanted to improve the spectrographs built onto their telescopes, either by increasing the spectral resolution of their instruments or by combining different regimes of spec- tral resolution in the same instrument. But redesigning such systems while remaining within the spatial constraints of already-built instruments has proven difficult. We developed a spectrograph design that bypasses problems with more conven- tional approaches to increasing spectral resolution. A spectrograph is composed of a slit (at the entrance focal plane) followed by collimating optics, then a pupil where the dispersive elements are placed. At this point, the light is sep- arated by wavelength then passed through camera optics that focus the dispersed beam onto the detector. The optical design of a spectrograph is driven by the spectral resolution require- ment, which determines the instrument’s ability to distinguish spectral lines. It is set by the resolving power, R, which is the ratio between the wavelength (/ and the spectral resolution el- ement (/, which in turn is related to the number of pixels on the detector on which the image of the entrance slit is projected with the required image quality. The design parameters to be considered are the entrance aperture (slit width), the pupil size, and the geometry required to operate at the optimum configu- ration, such as the Bragg angle for Littrow-based spectrographs. Maintaining the Bragg angle is especially important to optimize Figure 1. Sliced pupil grating for the Elmer spectrograph. Left: Optical layout at pupil with three slices. At the pupil, prisms break the beam into three slices, which are then diffracted by the volume phase holo- gram (VPH) grating. Right: Vignetted area at pupil. Some light is lost due to slicing the beam, but 62% is transmitted in this particular case. efficiency when using volume phase holograms (VPHs), because although VPH gratings are more efficient than ruled gratings at the Bragg angle, they lose efficiency very quickly away from it. 1 Increasing the spectral resolution in an already-built instrument implies either operating with a smaller slit width or changing the angle of light at the pupil. The first approach was used during the last 20 years but has a major drawback: decreasing the slit width reduces the amount of light that enters the spectrograph, in a situation where light is already limited. To avoid loss of light flux, optical designers used different techniques for dividing the focal plane object (slit) into smaller sub-slits, for which individual images were produced on the detector and later combined by software, producing the final target spectrum. This division at the entrance focal plane has been implemented in two ways: one could use either im- age slicers—a set of adjustable mirrors that cut the object image in sub-images that then become the new slits—or, alternatively, optical fibers whose cores limit the slit width. Continued on next page