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SMA Holography

The surface smoothness of the SMA antennas is measured by near-field holography at 232.4GHz using a phase-locked test signal source mounted on the Subaru Telescope building. The complex beam pattern (amplitude & phase) of the antenna under test is measured using on-the-fly mapping with a vector voltmeter back-end. A second antenna in the array provides the phase reference. The resulting beam pattern is Fourier-transformed to produce amplitude and phase distribution (complex illumination) on the aperture. The aperture phase is then corrected for systematic patterns due to the near-field measurements, bore-sight pointing offset and defocus leaving behind phase errors attributable to surface deviations (from a paraboloid). This gives a surface error map from which a list of panel adjustments are derived. The panels are then adjusted accordingly and remeasured. The repeatability of the measurements is 8 microns RMS over timescales of several weeks. Typically 3-4 iterations of this procedure are needed to achieve 15 microns RMS starting from the roughness level of ~60 microns when the antenna is first released from the swing template. The results on antenna 4 are shown below.

Results on Antenna 4

Pictures show the surface error maps for antenna #4. This antenna was adjusted during December 2000 and the surface has been under periodic monitoring measurements since then. The maps show:

(1) the surface before and after adjustment,

(2) short-term repeatability (1 month),

(3) long-term surface stability (7-months), and

(4) average surface since last adjustments (Dec 2000) and the end-to-end repeatabilty over this 7-month time base.

The last two maps exclude the outer most 10-cm of the 300-cm radius dish to avoid edge effects. The average surface rms is 13 micron. The stability of the panel-panel deviations indicate further improvements to the surface are possible. For the inner 275-cm radius region, the surface rms is 12 micron, which is the SMA surface smoothness specification (residual diffraction effects still persist in the outer 25-cm radius region). Finally, the antenna was transported from one pad to another during the 7 months under discussion, implying the surface will not be affected by array reconfigurations.

Swing template mechanism for assembling an SMA dish

(5) Swing template

(1) Surface improvement of Antenna 4

(2) Short-term (1-month) stability

Long term surface stability of Antenna 4

(3) Long-term (7-month) stability

(4) Long-term average and repeatability

Last modified: Thu Mar 4 12:44:49 2004
tksridha@cfa.harvard.edu