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24 October 2000 Double-pass Cassegrain revisited: a compact optic for space missions
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If intended to operate outside the Earth's atmosphere, the imaging optics demanded of a designer become critical. A typical specification would be diffraction-limited imaging over a usefully wide field angle with a broad spectral bandpass, high transmittance, no distortion and no vignetting. When combined with demands for a light and compact structure, such a specification calls for some trade-offs if a real system is to be devised. This paper describes an exceptionally compact imaging system in which the only significant trade-off is the 21% transmittance loss caused by the central obstruction of a Cassegrain-like optic. The entire 1° field angle is diffraction limited for all pupil diameters up to 1m (which would give 0.2 arcsec resolution), while providing a flat, distortionless, unvignetted, f/2.4 image, bandpass-limited only by the reflective coatings of the totally catoptric design. For fields less than 1 degree(s), scaling is possible to very large pupil diameters while maintaining full-field diffraction-limited imaging. By employing a single-axis, double-pass, four-reflection format, compactness is such that the overall length is less than the pupil diameter. The primary and tertiary mirror surfaces are formed from a single substrate. The primary, secondary and tertiary mirrors are hyperboloids of increasing eccentricity, and the quaternary is a weak aspheric zonal corrector. The geometry is such that stray light exclusion is simply achieved without occluding the optical path. The rearward propagating final beam provides significant useful free surrounding volume for cryostat structures, filters and shutters. A further example is given of a scalable f/1.25 version of the system.
© (2000) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
David A. Beach "Double-pass Cassegrain revisited: a compact optic for space missions", Proc. SPIE 4093, Current Developments in Lens Design and Optical Systems Engineering, (24 October 2000);


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