Proceedings PaperConcept for a multispectral imager with a continuously rotating filter wheel and multiple 2-D focal plane arrays
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Large 2-D focal plane arrays (FPAs) can be used for multispectral imaging by sequentially viewing a scene through multiple discrete narrow-band filters on a continuously rotating filter wheel. The constant angular velocity of the wheel minimizes momentum disturbances, jitter and power consumption and maximizes reliability, but the optical beam straddles two adjacent filters during each transition between them. The duty cycle of the imager can be maximized by separating these narrow band filters into two equal groups: long-wavelength and short-wavelength. The narrow-band filters can then be mounted contiguous to one another on a ring of the filter wheel, alternating between long and short wavelength filters. The optical beam transmitted by the filter wheel can then be divided into a long-wavelength beam and a short-wavelength beam by a dichroic beamsplitter that transmits the output of one group of filters and reflects the output of the other group. Each beam can then be transmitted through an additional broadband spectral filter and can be imaged on a separate FPA. When the optical beam straddles two narrow-band filters on the wheel, the dichroic beamsplitter and the broadband filters prevent out-of-band radiation from reaching either FPA, allowing FPA to integrate a spectrally-pure 2D image (although the flux will decrease and the diffraction will increase as the beam is vignetted.) When the incident beam passes through a long-wavelength filter on the wheel, the short wavelength FPA can be read our without requiring a shutter, and vice versa.