SPIFFI: a high-resolution near-infrared imaging spectrometer

Abstract

SPIFFI is an integral field spectrograph with an HAWAII array that enables us to simultaneously take near IR spectra of 1024 spatial pixels in a hexagonal field of view on the sky. It can be used on 4 to 8 meter class telescopes with a maximum pixel scale of 0.5 arcsec and with adaptive optics pixel scales, Nyquist sampling the point spread function of the telescope. A fiber bundle of 1024 silica/silica fibers rearranges the 2D field of view into the 1D entrance slit of the spectrometer. A novel technique involving flared fibers is used to achieve a high filling factor and coupling efficiency. Each fiber tip in the bundle is flared to increase the fiber core diameter by a factor of 15. The tapered end is polished to form a spherical micro-lens with a hexagonal cross-section to couple light into the fiber core. Apart from yielding a high coupling efficiency and a high geometrical filling factor, the monolithic micro- lens/fiber system can be used at a working temperature of 77K without loosing positioning accuracy. The spectrometer optics is achromatic from 1.1 to 2.5 microns and use four reflection gratings on a wheel as dispersing elements with a resolving power from 2000 to 4500. The fore-optics includes the filter wheel, the cold pupil stop and a scale changing mechanism to switch between three different image scales according to observing and seeing conditions. The spectrometer collimator is a f/4.3 three lens achromat, the spectrometer camera is a f/1.2 folded Schmidt camera. The optical design of the spectrometer is distortion free to get straight, equidistant spectra that match the columns of the detector, thus minimizing cross-talk form adjacent spectra to less than 5 percent.