MADEFAST OPTICS Design Decision Tracking

MADEFAST State of the Optics Design on 5 July 1994

This is the state of the design as of the meeting of 5 July 1994.
These are changes/refinements from the state of the design as of the meeting of June 21. The main change to the overall design has simply been the elimination of a lens between the two mirrors. Otherwise, the abstract design is roughly unchanged.

• Decision: The detector (PSD)field of view (FOV) will be targeted at 5 degrees.
  • Task: Determine PSD FOV
  • Status: VALID
  • Rationale: Experiments since 21 June have determined that a smaller FOV would not be optimal.

• Decision: Rather than a packaged "lipstick camera", use the Supercircuits PC-9 Microvideo Cameras
  • Task: Choose Camera
  • Status: VALID
  • Rationale: Lighter, smaller cameras on a board have been found in the catalogs. These enable the camera to be mounted flat on the back of the secondary camera. NOTE: this presumes the use of two mirrors and not the use of a beam splitter.

• Decision: Try for a focal length between 120 and 150 mm rather than 200mm.
  • Task: Determine primary mirror focal length
  • Status: VALID
  • Rationale: Want a more compact seeker design.

• Decision: Modulate the signal.
  • Task: Design Signal Detection
  • Status: VALID
  • Rationale: Signal strength tests indicate that this option, at least considered in the 21 Jun meeting, is necessary to "lock on" to a spot of even moderate intensity.

• Decision: Mount the camera above the gimbal using its own optics rather than mounting both the PSD and CCD at the bottom of the gimbal and have them use the same optics with a beam splitter.
  • Task: Camera Lens Strategy
  • Status: VALID
  • Rationale: 1) Although testing will be done, it is not now thought that the signal intensity will be sufficiently strong to split.
    2) The camera should have a FOV larger than that of the detector, indicating that it should have different optics.

• Decision: A single mirror option has been considered since the 21 Jun meeting but has been put aside for now in favor of double mirrors.
  • Task: Mirror strategy
  • Status: VALID
  • Rationale: The single mirror option would be a simpler construction, but put the PSD where the second mirror would be. This would result in a heavier overall gimbal since counterweights would have to be applied to the bottom of the gimbal. Having the PSD just below the gimbal in the two mirror option eliminates the need for some of that weight and improves the balance and the moment of inertia. However, it was noted that this difference is slight.

    Also, this rationale assumes that the secondary mirror will be machined as the support for the camera in any case, so the same weight is there whether the support also functions as a secondary mirror or not.

• Decision: Eliminate the lens between the double mirrors.
  • Task: Mirror Focal Length Reduction
  • Status: VALID
  • Rationale: A hyperparabaloid secondary mirror (with a fallback of a flat mirror) would acheive the same shorter focal length.

• Task:The issue of encoders versus resolvers is still not decided.

  Issues: The resolvers were used by the previous TI design and the analog output can be resolved as desired. The digital output of the encoders has a resolution limit. However, the limit of one catalog entry is about 300K counts per revolution. And the encoders are small and light.

• Task: Complete demonstration scenario not designed, but considering having the camera board project a bullseye that will center in on the dot projected from the wand.