Notes for PDR. April 1999.
This supplements the information already given in the MMA Project Book. Please see
There are 2 distinct modes of holography:
1. Single dish mode, to be used in the initial antenna testing and precision panel setting. A frequency of 92.4 GHz will be used; this is a good compromise - about 1 degree of phase measurement precision is needed at ~90 GHz. We will use a terrestrial transmitter mounted on a tower, in the near field about 300m from the telescope. The near-field correction, to be applied during the data analysis, is an important, but manageable, issue. The receivers, including the reference signal system, will be built specially for the holography.
2. Interferometric mode, using astronomical sources. This will allow antenna surface measurements over a range of elevation angles, but because of more limited signal-to-noise ratio, there will be an inevitable tradeoff between precision of surface measurement and less frequent sampling along the surface. This measurement also requires 2 antennas to be fully operational, with correlator and phase-stable LO distribution, but will not require any special receiver hardware. Frequencies of ~86 GHz and ~240 GHz will be used.