Characterization of the Microlensed Hyperspectral Imager prototype

Abstract

Context. The Microlensed Hyperspectral Imager (MiHI) prototype is an integral field spectrograph based on a double-sided microlens array (MLA), installed as an extension to the TRIPPEL spectrograph at the Swedish Solar Telescope (SST).Aims. Due to the mixing of spatial and spectral information in the focal plane, the data are mapped in an interleaved way onto the image sensor. Mapping the information back into its original spatial and spectral dimensions renders the data reduction more complex than usual, and requires the development of a new reduction procedure.Methods. The mapping of the data onto the detector is calculated using a simplified model of the image formation process. Since the moiré fringes that are formed due to the interference of the pixel grid and the MLA grid are a natural consequence of this formation process, the extraction of the data using such a model should eliminate them from the data cubes, thereby eliminating the principal source of instrumentally induced artifacts. In addition, any change in the model caused by small movements of the raw image on the detector can be fitted and included in the model.Results. An effective model of the instrument was fitted using a combination of the numerical results obtained for the propagation of light through an ideal dual microlens system, complemented with an ad hoc fit of the optical performance of the instrument and the individual elements in the MLA. The model includes individual fits for the position, focus, focus gradient, coma, and a few high-order symmetric modes, which are required to account for the spectral crosstalk within each image row. The model is able to accurately reproduce the raw flat-field data from a hyperspectral cube that is virtually free of moiré fringes, and it represents a critical first step in a new hyperspectral data reduction procedure.