Technical Details
The Multispectral Scanner had a 9" fused silica dinner-plate mirror epoxy bonded to three invar tangent bars mounted to base of a Ni/Au brazed Invar frame in a serreuire truss that was arranged with four "Hobbs-Links" (conceived by Dr. Gregg Hobbs) crossing at mid truss. This construct ensured the secondary mirror would simply oscillate about the primary optic axis to maintain focus despite vibration inherent from the 14-inch (360 mm) Be scan mirror. This engineering solution allowed the US to develop LANDSAT at least five years ahead of French SPOT which first used CCD arrays to stare without need for a scanner.
The MSS FPA, or Focal Plane Array consisted of 24 square optical fibers extruded down to .0002"square fiber tips in a 4x6 array to be scanned across the Nimbus spacecraft path in a +/-6 degree scan as the satellite was in a 10:30 polar orbit, hence it had to be launched from Vandenberg AFB. The fiber optic bundle was embedded in a fiber optic plate to be terminated at a relay optic device that transmitted fiber end signal on into six photodiodes and 18 photomultiplier tubes that were arrayed across a 0.30-inch (7.6 mm) thick aluminum tool plate, with sensor weight balanced vs the 9-inch (230 mm) telescope on opposite side. This main plate was assembled on a frame, then attached to the silver-loaded magnesium housing with helicoil fasteners.
Key to MSS success was the scan monitor mounted on the underbelly of the Mg housing. It consisted of a diode source and sensor mounted at ends of four flat mirrors that were tilted so that it took 14 bounces for a beam to reflect length of the three mirrors from source to sender striking Be scan mirror seven times as it reflected seven times off the flat mirrors. It only sensed three positions, both ends of scan and the mid scan, but that was all that was required to determine where MSS was pointed and electronics scanning could be calibrated to display a map.
Read more about this topic: Landsat Program
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