General Requirements  Although this FSM was not intended fo r a specific program, some  general top-level requirements and performance goals were  established. An angular position accuracy of about 1-2 arcsec was desired to minimize jitter. A clear aperture of 75 mm was chosen  as a good starting point, with the intention that the design be  scalable to larger or smaller apertures. An elevation axis deflection of ±15 deg (mechanical) was chosen to perform ground motion  compensation (GMC) along the flight path, and the azimuth axis  deflection was chosen to be ±60 deg to achieve both off-track pointing and the ability to rotate far en ough to view an on-board  calibration source. A high bandwidth approaching 70-100 Hz was  desired to enable jitter control during pointing and scanning.  The final performance parameters achieved are tabulated in the  section below describing the test results.   Design Description  Mirror Design To meet the aperture and bandwidth goals, the mirror needed to be lightweight and low cost. Mirror  trades included several materials (Be, Al, SiC) and shaft connections (bonded, bolted, or integral). The  final selection was a lightweighted Al mirror to keep costs low, and a bolted connection to facilitate future  design changes.  The flat mirror geometry provides a 75-mm aperture when scanned to a 60-deg optical angle (30-deg  mechanical angle). The flat mirror is elliptical with a 75-mm-by-150-mm clear aperture. Thermal  considerations coupled with the light-weighting requirement for high bandwidth necessitated the open back form shown in Figure 2, which provides a 60% reduction in mass compared with a solid mirror. Total  mirror mass is 0.16 kg corresponding with a 17 kg/m 2 optical surface area mirror density.  Triangular cells were selected over hexagonal or square cells as they are considered to be the optimum  cell geometry (Ref. 2). Triangular cells also work well in providing a uniform distribution of ribs across the  part and structure for mounting features.  The mirror is mounted on four flexure-isolated pads in the center of the mirror back. The mounting pads  require flexure isolation to reduce mounting distortion of the optical surface; however, the stiffness of the  flexures must balance mounting distortion isolation and mirror modal response. Finite element analysis  was used to find the optimum performance balan ce for the scan mirror application.  Mirror Surface Finish The mirror surface is fabricated using single point diamond turning (SPDT) combined with post-polishing.  An RMS surface roughness better than 20 angstroms is obtained utilizing a post-polishing method termed  VQ for “Visible Quality”. This process aids in the  removal of the grooves typical from SPDT.  Table 1. Mirror Optical Performance  Specifications  Description Specification  Mirror Mass 0.16 kg  Operating Temperature 210K Optical Aperture @60 deg Optical Angle  75 mm  RMS Surface Figure  < 1/10 waves HeNe  RMS Surface Roughness < 20 angstroms  Triangular Cell  Open-Back Design Triangular Cell  Open-Back Design Figure 2. Back View of Mirror  Showing Triangular Rib Structure The RMS surface figure of the mounted mirror  was measured to be near 1/12 wave HeNe at  210K. This shows the capability of this mirror to hold good optical figure. Test and analysis  results are discussed in detail later in the paper.  Table 1 summarizes the optical performance specifications. 388