Advanced Geosynchronous Studies Imager: an all-reflective off-center off-axis focal telescope for Earth observation

摘要

Abstract: An unobscured, f/4, Earth scanning telescope was designed by optimizing an off-center, off-axis Cassegrain telescope with a unit magnification spherical camera mirror. A flat scan mirror in front of the 30-cm diameter telescope directs the 1-degree field-of-view. Observation form geosynchronous orbit results in sunlight illumination of the scanning, primary, and secondary mirrors near satellite local midnight. The 'all-reflective' design has a primary- secondary spacing of 87-cm. A field stop helps control stay sunlight illumination of the camera mirror and beam division optics. Exit pupil stops further control stray illumination. Sub-satellite pixel image scale is 1/2-, 1-, and 2-km over the spectral range 0.4- to 13.5-microns. The general requirements are for the first diffraction dark ring to be imaged on the pixels. The flat image plane is separated into four bands using dichroic beamsplitters and flat mirrors to accommodate linear array detector technology. Sub-band channels are defined with filters in near contact with the detectors. All but the visible detectors are enclosed behind transmission windows so that all bands can be operated without thermal vacuum facilities during ground testing. All transmission elements are 'wedged' to correct for astigmatism introduced by using them in converging light. The powered mirrors and scan flat are to be silicon carbide. The mechanical structure will be a near-zero expansion graphite fiber reinforced plastic. Tolerance analysis and thermal modeling show optical element displacements well below requirements. Instrument volume corresponds approximately to that of the current GOES Imager. !4