A dual-channel telescope allows for a wide-field telescope design with a good, narrow field channel of fewer surfaces for shorter-wavelength or planet-finding applications. The design starts with a Korsch three-mirror-anastigmat (TMA) telescope that meets the mission criteria for image quality over a wide field of view. The internal image at the Cassegrain focus is typically blurry due to the aberration balancing among the three mirrors. The Cassegrain focus is then re-optimized on the axis of the system where the narrow field channel instrument is picked off by bending the primary mirror. This now makes the wide-field channel blurry (i.e., the TMA image), and it must be re-optimized while holding the fore-optics fixed. This leaves the tertiary mirror as a variable, as well as a fold mirror strategically placed at the image of the primary mirror (i.e., exit pupil of the telescope). This fold mirror can then be used to compensate for the departure in primary mirror figure used to optimize the narrow field channel. As such, only an aspheric term is needed for this final optimization on this “corrector” fold mirror.
This work was done by Joseph M. Howard and David Content of Goddard Space Flight Center. For further information, contact the Goddard Innovative Partnerships Office at (301) 286-5810. GSC-15574-1