Element Wheel Assembly (EWA)The element wheel is a large disk made of lightweight metal placed in front of the detector array. It has slots around its perimeter that contain calibration hardware, filters for imaging, and the grism and prism for spectroscopy. The element wheel allows the Wide Field Instrument to both tune the wavelengths of light that reach the detectors and spread the light of astronomical objects into spectra. The wheel rotates at its central axis to bring different elements into the light path, changing the instrument's observations. The element wheel is in a housing that contains heaters to hold the wheel and various elements at a constant temperature and limit distortions in the optics.
