Image reorganization in an integral field unit using the two-mirror optical system

Abstract

Many astronomical studies require obtaining detailed information about the spectrum of a celestial object, for which various types of spectrometers are effectively used. In the case of extended fields, it becomes necessary to optimally match the field image of an objective and the entrance slit of a spectrometer. For that, an optical scanning system is used. However, it becomes impossible to capture different events in real time this way. For the simultaneous capturing of a wide field, it is efficient to use an integral field unit (IFU). IFU is the optical system that allows slicing and transforming of a field into a set of long slits that are fed to a spectrometer. The design of the IFU optical system for the solar telescope-coronagraph (LST-3) is presented. LST-3 is the telescope with the main mirror of 3m, and it should operate in the wide spectral range, 0.4-1.6 μm. The telescope's objective field of rectangular shape, (1.1 x 2.2) mm, is transformed in two steps: slicing the image and reorganization of sub-images into a set of 8 long slits, (0.018 x 18.6) mm. The reorganization is performed using the system of two flat mirrors. The mirrors are rotated around two axes, which entails the accumulating error of a sub-image position in the image plane. For studying the offset error in an IFU, a simulation was performed. The expression of the error function makes it possible to consider it when using an IFU, while maintaining high spatial resolution and co-directionality of sub-image apertures in the slit plane. As a result of the work, it was confirmed that the use of two flat mirrors for geometric image transformation is possible when designing an IFU.