MMA HOLOGRAPHY.

Notes for PDR. April 1999.

This supplements the information already given in the MMA Project Book. Please see

http://www.tuc.nrao.edu/~demerson/project_book/chap11/chap11.html

Other Documents:

• Description of planned holography transmitter and receiver system, Time Schedule

  Enlarged Figure 1 of hardware description: Block Diagram
  Enlarged Figure 2: Transmitter Tower
  Enlarged Figure 3: 92.4 GHz Transmitter
  Enlarged Figure 4: Dual Channel Receiver Block Diagram

• Software for the holography system (HTML)   |   Software for the holography system (PDF)
  
• Data analysis for holography, using AIPS++
  
• The Interface Control Document (ICD) describing the antenna-holography interface
  
  
• Notes on photogrammetry for the measurement of antennas
  

Brief Summary of Plans:

Specifications:

• Surface measurement precision = 10 microns.
• Surface sampling = several samples per panel, say 10 cm along the surface.
• Measurement set to be completed within about 1 hour, so that ambient conditions - particularly temperature - will be reasonably constant during the measurements.

Implementation

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.