SPECTRAL RESOLUTION IMPROVEMENT TECHNIQUE FOR A SPECTROGRAPH MOUNTINGA DISCRETE ARRAY DETECTOR

A new technique has been developed for improving the spectral resoluti on of spectrographs mounting discrete array detectors. The basic conce pt is to acquire various spectroscopic images, each one being shifted on the detector by a fraction of the pixel (i.e., the detector resolut ion element), and to apply suitable numerical procedures in order to e xtract a spectral profile with subpixel resolution. This technique has been applied to a vacuum spectrograph, adopting the Johnson-Onaka con figuration. The dispersion element is a concave toroidal grating that can rotate around a pivot axis displaced from its vertex, in order to maintain a good spectral focus on the detector. The latter is a multia node microchannel-plate array (MAMA) and operates in the photon-counti ng mode. Several stigmatic images of the H I Ly-alpha line at 1216 Ang strom emitted by a D-2 lamp have been acquired for various rotation of the grating. The results of the application of this new technique and the numerical algorithm are presented and discussed in terms of poten tialities and limitations due to the signal-to-noise ratio and intrins ic spectral broadening of the signals. (C) 1996 Society of Photo-Optic al instrumentation Engineers.