We propose a method for a differential imager that makes use of a variable channeled spectrum. The channeled spectrum is an oscillating spectrum with respect to wavelength, generated by two polarizers with a retarder between them. A variable retarder, inserted between these polarizers, modulates the channeled spectrum. By restricting a spectral band with a bandpass filter, the whole optical system acts as a common-path variable bandpass filter with two specific central wavelengths. Provided that the temporal modulation of the channeled spectrum is fast enough compared to that of the evolution of phase aberrations, it is possible to obtain spectral differential images free from non-common-path aberrations. Furthermore, polarization differential images are also available by inserting another variable retarder in front of this optical system. Thus, both common-path spectral and polarization differential images can be obtained. We propose using a liquid-crystal variable retarder, which has been successfully demonstrated in a polarization differential imager equipped with a four-quadrant polarization mask coronagraph. In order to achieve a higher system throughput, we suggest using Wollaston prisms instead of polarizers. In this paper, we describe the principle of this technique. Results of a preliminary laboratory experiment and a numerical simulation of the spectral differential imager are also given.
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