A telescopic imaging system that includes an optomechanical scanning subsystem is undergoing development. The system is designed for use in scientific observation of the Earth from aboard a satellite in geostationary orbit. The basic principles of the optomechanical scanning subsystem may also be applicable to special-purpose terrestrial optical instruments.
The principal optical component of the optomechanical subsystem is the scanning mirror - a flat mirror with an elliptical cross section. The mirror is rotated about two mutually perpendicular axes to scan the focal plane of the telescope in a raster pattern that defines a field of regard that extends across the surface of the Earth and into adjacent areas of dark space (observations of adjacent dark space provide instrument background readings that can be subtracted from Earth-surface readings). The use of the scanning mirror makes it unnecessary to rotate the relatively massive telescope to scan this field of regard, which is wider than the field of view of the telescope. The use of a scanning mirror for such a purpose is not new or unique; instead, the unique aspect of the optomechanical subsystem lies in some of the particulars of its design.
The system (see Figure 1) is most readily characterized by reference to a Cartesian coordinate system. The central line of sight of the field of regard is along the nadir (the z axis). In the first version of the system, the optical axis of the telescope is parallel to the south (y) axis; in the second version, the optical axis of the telescope is tilted 30° from the yaxis in the y,z plane. The image is detected on a focal-plane array (FPA) of photodetectors arranged in rows of pixels along the x axis. The along-scan (east-west rapid-scan) component of the raster pattern corresponds to motion of the focal spot along a row of pixels in the x direction. The FPA-readout scheme is based partly on time delay and integration (TDI); this produces a requirement that the optomechanical-scanning velocity be constant and equal to the electronic-scanning velocity so that the packet of charge from each pixel on the FPA will correspond to a line of sight to a single spot in the scene.
The scanning mirror is mounted on a gimbal with two perpendicular axles. The outer axle is attached to the telescope body and is parallel to the xaxis; this axle is used to generate the cross-scan component of the raster pattern (north-to-south steps between scanning arcs). The mirror is mounted on the inner axle, which is perpendicular to the outer axle and is used to generate the rapid east-west scan component. In the first version of the system, the angle, Θ, between the telescope's optical axis and the inner axis of the scan mirror is 45° at the center of the raster patten, and decreases from 49.35 to 40.65° as the line of sight (LOS) is scanned from the North Pole to the South Pole (from +8.7 to -8.7° elevation at geosynchronous altitude). In the second version of the system, the angle Θ is 60° at the center of the raster pattern and decreases from 64.35 to 55.65°.
The along-scan components of the raster pattern can be described as a series of predominantly east-west arcs separated by small cross-scan north-south intervals (see Figure 2). Each arc is scanned by rotation of the inner axle at constant angular speed to obtain the required constant speed of scanning of the focal spot along a row of pixels. The outer axle is held stationary during the motion along each arc. At the end of each arc, the outer axle is actuated to index to the next north-south position and the arc at that position is scanned along a reverse path by rotating the inner axle in the opposite direction.
The angular velocity of the LOS equals the rotational velocity of the inner gimbal axis, dΨ/dtmultiplied by 2sin(Θ). The value of 2sin(Θ) decreases on each consecutive arc from north to south, so dΨ/dtmust be increased from arc to arc. The image of the FPA rotates so that its TDI axis always coincides with the scan velocity, and its cross-scan axis (the long axis) is always perpendicular. (Refer to the top arc in Figure 2.)
The second (30°) version offers several advantages over the first (45°) version:
- The angle of reflection is smaller; consequently, the reflection introduces less spurious polarization.
- The major axis of the mirror ellipse can be made shorter, thereby reducing the mass and angular momentum of the mirror.
- Rotation of the image (equivalently, rotation of the projection of the FPA pixels onto the scene) is less.
This work was done by James C. Bremer of Swales Aerospace for Goddard Space Flight Center. GSC-14088