Holography
The purpose of the holography experiment is to assist the alignment of the SMA antenna panels to acheive an overall surface rms of 5-10 microns. The basic requirements for the holography system are the following:
The Smithsonian Submillimeter Array (SMA) will be operating in the frequency range of 180 - 900 Ghz, or the equivalent wavelength range of 1.6 mm - 0.3 mm. In order to have an equivalent wavefront phase error of less than 1/20 of a wavelength for this entire frequency range, a surface rms of about 10 micron is desired, of which 5 micron is allocated for surface measurement and setting.
The microwave holography technique, commonly employed in obtaining the antenna aperture field from the far field measurement, is well suited for the purpose of fast and accurate measurement of the antenna surface, once the initial mechanical alignment of the paraboloid is done. Detailed comparison of the trade-offs of the different surface measurement techniques can be found in the SMA techinical memo No. 50, by Colin Masson (1991).
The holography technique has been employed successfully at most of the radio observatories for reflector surface alignment. The best surface rms obtained so far for the millimeter and submillimeter antennas is about 20 micron. The best repeatability of the measurements is about 5 micron. It appears that the goal of aligning the SMA dishes using the holography technique to a surface rms of about 5 - 10 micron is within practical reach.
The 90 GHz frequency band is selected mainly due to the fact that at the Haystack site, the atmosphere transparency and phase stability is not ideal for measurement at higher frequencies. Of the two holography receivers, one of them will be located on the optical path, near the Cassegrain focus. The second receiver will be positioned inside the chopper control housing, with the horn facing outward. This allows us to do phase coherence holography with just a single antenna, which is useful since at Haystack site, the antenna pads are spaced so close that there are no two clear paths from any two pads to the transmitter tower. The co-alignment of the main and the reference receiver optical paths could also help the phase performance.
The exact frequency for the holography beacon transmitter, 92.4 GHz, is selected such that it is outside the protected astronomical band (which is up to 92 GHz), but still low enough so that the entire 85-93 GHz RF band of the receiver (including the SiO maser frequency of 86 GHz) can be covered by a single ZAX Gunn oscillator, which has a bandwidth of 4 GHz, with an IF frequency of 2.83 GHz.
At 92.4 GHz, 5 micron of surface error, or 10 micron of pathlength error, amounts to about 1 degree of phase error. The corresponding signal-to-noise needed is about 35 dB. Therefore the thermal noise of the instrument should not be a problem for the beacon source we choose to use, which has an output power about 12 mw (see further the calculation in the "subsystem" section).