ALMA Holography Overview
نویسنده
چکیده
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.
منابع مشابه
Evaluation of the ALMA Prototype Antennas
The ALMA North American and European prototype antennas have been evaluated by a variety of measurement systems to quantify the major performance specifications. Nearfield holography was used to set the reflector surfaces to 17 μm RMS. Pointing and fast switching performance was determined with an optical telescope and by millimeter wavelength radiometry, yielding 2 absolute and 0.6 offset poin...
متن کاملEngineering within the Assembly, Verification and Integration (AIV) Process in ALMA
The Atacama Large Millimeter/submillimeter Array (ALMA) is a joint project between astronomical organizations in Europe, North America, and East Asia, in collaboration with the Republic of Chile. ALMA will consist of at least 54 twelve-meter antennas and 12 seven-meter antennas operating as an interferometer in the millimeter and sub-millimeter wavelength range. It will be located at an altitud...
متن کاملAn overview of compressive sensing techniques applied in holography
In recent years compressive sensing has been successfully introduced in digital holography. Depending on the ability to sparsely represent an object, the compressive sensing paradigm provides an accurate object reconstruction framework, from a relatively small number of encoded signal samples. Digital holography has been proven to be an efficient and physically realizable sensing modality that ...
متن کاملOverview of compressive sensing techniques applied in holography [Invited].
In recent years compressive sensing (CS) has been successfully introduced in digital holography (DH). Depending on the ability to sparsely represent an object, the CS paradigm provides an accurate object reconstruction framework from a relatively small number of encoded signal samples. DH has proven to be an efficient and physically realizable sensing modality that can exploit the benefits of C...
متن کامل