[mmaimcal] nutator

[Simon Radford]

Al,

Darrel has delegated to me the task of building a nutator for the first
antenna. Based on several discussions with Jeff Kingsley, here's my
baseline plan.

Comments welcome.

Simon


Optics:
  From Lamb's Memo 246,
  75 cm mirror diameter

Specifications:
  10 ms transition time, 10 Hz repetition rate, +- 1.5' throw,
  mirror moment of inertia < 0.25 kg m^2.

Mechanics:
  Single axis nutator driven by moving coil thrusters,
  moving motors to counterbalance mirror motion,
  mirror and motors mounted on spring pivots,
  stow pins or brakes define reference position when unpowered,
  unobstructed central area.

Control:
  Mirror position sensed by LVDT, 
  analog(?) PID feedback loop,
  shaped input transitions (polynomials, error functions) to avoid
  exciting high frequency resonances.

Design pedigree:
  Base ALMA design on SMA nutator (Cheimets 1994), 
  which is based on Radford, Boynton, & Mechiorri (1990),
  which inherited design features from Payne (1975).

Further design tasks:
  Survey other designs (JCMT, HHT, IRAM 30 m, SEST, IR telescopes),
  evaluate cost/benefit of dual axis nutation or axis rotation.

References:

Optimized Optical Layout for MMA 12 m Antennas
James Lamb, 1999, ALMA Memo 246, 
http://www.alma.nrao.edu/memos/html-memos/abstracts/abs246.html

SMA nutator (note some broken links)
http://ce-www.harvard.edu/projects/chopper/sma_chopper.html

Design and Test of the submilleter array (SMA) chopping subreflector
P. Cheimets, 1994, Proc. SPIE 2200, 347
http://bigtoad.harvard.edu/~cheimets/ce_home_page/projects/chopper/spiepaper.ps

Nutating Subreflector for a Millimeter Wave Telescope,
Radford, S. J. E., Boynton, P. E., & Melchiorri, F., 1990, Rev. Sci.
Instr. 61, 953 
http://www.tuc.nrao.edu/~sradford/papers/nutator.pdf

Switching subreflector for millimeter wave radio astronomy,
Payne, J. M., 1976, Rev. Sci. Instr. 47, 222