ALMA Holography Overview

1.0 Introduction The use of "microwave holography" as the primary method of evaluating the ALMA antennas and setting their panels has been the project's baseline for several years. There is a long history of such measurements in radio astronomy (e.g., [3-5]). We are particularly drawing on experience at the NRAO in holographic measurements of the 12 M Telescope, 140 Ft Telescope, and VLA antennas; at IRAM on the 30 M and the Interferometer; at the CfA on the SMA antennas; and at the JCMT 10m telescope. Early design efforts for ALMA were summarized at a PDR in 1999-Apr. See Holography PDR documents and Holography PDR Summary and Recommendations. The current design is summarized in the Test Interferometer Project Book chapter , which will be updated as needed during the remainder of the development work. Readers are referred to these documents for additional background. Since the PDR, many details of the design of the ground-based holography system have been worked out, and some of these differ from the earlier plan. The transmitter design is completely different, and will now make use of two phase-locked lasers driving a photomixer, rather than a millimeter-wavelength oscillator. Observing on two well-separated frequencies will be supported (both in the 75-110 GHz waveguide band), as will small changes in frequency about each nominal one. The receiver will be locked to the antenna LO system's fixed references, rather than to the transmitter. Cross-correlation will be done in a digital processor, rather than in a vector voltmeter. The design is now being presented for CDR. Following a successful review, we will proceed to detailed design and then to implementation.