Active control of ``the Great Pumpkin'' mirror panels Active control of ``the Great Pumpkin'' mirror panels

Active control of ``the Great Pumpkin'' mirror panels

The ``Great Pumpkin'' is radome protected millimeter wavelength capable radioastronomy dish of 13.7 meters diameter, which our club, OH2Z, got for the price of aluminium scrap. (Better to give it a life, than to make beer or soda cans out of it.)

The system has an outher ring of 48 mirror panels with 6 adjustment screws, and an inner ring of 24 mirror panels with 8 adjustment screws. In total there are 480 adjust points.

Operational conditions are:

  • Temperature -30 to +50 C. (Possible extremes, -20 to +30 is most likely real range.)
  • Open air moisture which will condence at cold surfaces.
  • Fully enclosing radome protects system from wind, direct sunshine, and direct rain, but it is not heated (very much), and can leak in the harsh winter in Finland -> effective rain may happen.
  • During construction of the radome the panels are subject to wind load forces, however physical movement limits should do the safety handling in that case, not motor strengths themselves.
  • Condenced water may run off around the system anywhere, get into the most harmfull spot, and then freeze. (Or do whatever causes most damage.)
  • Panel positioning repeatability is needed to be within 0.1 lambda at 100 GHz -- or 0.3 millimeters (0.012 inch).
  • Because the manual panel position adjustment to high accuracy is very cumbersome process, allowing longer movement strokes is of interest, say about 10-15 mm.
  • Maximum forces needed to move the panels include 1/6th part of the mass weight of the panel itself (circa 10-20 kilos total -> 100-200 Newtons), and possible twist (of panel) force.
    All in all this is assumed to be well under 100 Newtons per actuator.
    (This needs an experiment to be constructed to measure the needed forces.)
  • The actuators are not subjected to transversal (sideways) forces, such are handled by slide tubes at the corners of the panels.
  • Preferrably the actuators should be at power-off when not needed to be moved, so they should exhibit hold torque sufficient for that.
  • The 288 motor driver electronics are to be located into cabinets at the center of the dish (rear side) for servicability.
    Possibly into AZ/EL fork, which stands on top of AZ bearings and gears, and has EL bearings and gears at which the mirror itself is attached to.
  • Movement is rare: At powerup a home-seek, then seek to set work position, then at most about 3 mm movement depending on dish orientation, and gravity induced bending of the support structure.
  • A small move (if any) about once a minute is the average need.
  • Movement speed of 1 mm per second is sufficient. Even slower may work.
  • Final position finetuning will be done with so called ``holographic'' radio imaging under computer control.
  • Alternative possible geometry measuring methods include:
    • Radioholography
    • Laser teodolite
    • Ultrasound pulse traversal time (Time resolution better than 1 microsecond!) (Frequency 2-3 MHz ?)

Possible online measurement method: