1. Summary of 4 May 2000
In my view, Radford's specification should be the plan. I will remind the ASAC of this at their meeting on 8 May and ask for any specific comments. The ASAC requested calibration accuracy of 1%. Mangum wrote a DRAFT memo which showed that this can be achieved with a system such as that which Berkeley was testing, a dual load system behind the subreflector. In an addendum to Radford's specifications, Radford treated how this might be accommodated. Radford was prepared to construct this nutator by the end of this year, but owing to a shortage of funding, has not proceeded. I believe that the nutator which was costed was priced according to the specs below. Owing to this suspension, Radford's specifications did not get incorporated into the Project Book, since responsibility for the nutator is currently under discussion.

Radford's specification of 1 March was:

  • 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

  • On 6 April, Simon commented on design modifications to accommodate the Berkeley calibration system:

    • ICD 2 shows a 375 mm diameter cylindrical clear volume through the apex structure. [The ICD is, in fact, ambiguous because it gives this diameter at 375 mm on the drawing and 400 mm in a note.]

    In the nutator design, I intend to preserve as much as possible of this clear volume behind the subreflector. My preliminary sketches indicate an internal clear volume of 350 mm (elevation) by 250 mm (cross-elevation). So there should be plenty of room for a calibration system. It would, of course, make sense to integrate the mechanical mounting of the calibration system into the nutator structure.

    The nutating subreflector itself is not yet designed. So we can put in a vertex hole, etc., as necessary.

    It would be a bad idea to nutate the calibration system. This implies a small air gap between the vertex through hole and the calibrator entrance aperture. Is this a problem?

    With the nutator energized, it should be able to hold the mirror position steady, i. e., not nutate, within the pointing specifications. I also have in mind stow pins or something similar that would fix the mirror in a fiducial position. So we might be able to dispense with the fixed subreflector.