Dr. Todd R. Hunter Profile

Dr. Todd R. Hunter

Astrophysicist at National Radio Astronomy Observatory

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Publications (8)

Proceedings Article | 13 December 2020 Poster + Paper

ALMA front-end and digitizer technical requirements for enabling the ALMA board's 2030 vision roadmap

Shin'ichiro Asayama, Gie Han Tan, Kamaljeet Saini, John Carpenter, Todd Hunter, Neil Phillips, Hiroshi Nagai, Giorgio Siringo, Nick Whyborn

Proc. SPIE. 11445, Ground-based and Airborne Telescopes VIII

KEYWORDS: Observatories, Telescopes, Astronomy, Interferometers, Software development, Analytical research, Lead

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Proceedings Article | 8 August 2016 Paper

ALMA long baseline phase calibration using phase referencing

Yoshiharu Asaki, Satoki Matsushita, Edward Fomalont, Stuartt Corder, Lars-Åke Nyman, William R. Dent, Neil Philips, Akihiko Hirota, Satoko Takahashi, Baltasar Vila-Vilaro, Bojan Nikolic, Todd Hunter, Anthony Remijan, Catherine Vlahakis

Proc. SPIE. 9906, Ground-based and Airborne Telescopes VI

KEYWORDS: Telescopes, Astronomy, Switching, Interferometers, Calibration, Phase interferometry, Antennas, Spatial resolution, Troposphere, Atmospheric modeling

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The Atacama Large Millimeter/submillimeter Array (ALMA) is the world's largest millimeter/submillimeter telescope and provides unprecedented sensitivities and spatial resolutions. To achieve the highest imaging capabilities, interferometric phase calibration for the long baselines is one of the most important subjects: The longer the baselines, the worse the phase stability becomes because of turbulent motions of the Earth's atmosphere, especially, the water vapor in the troposphere. To overcome this subject, ALMA adopts a phase correction scheme using a Water Vapor Radiometer (WVR) to estimate the amount of water vapor content along the antenna line of sight. An additional technique is phase referencing, in which a science target and a nearby calibrator are observed by turn by quickly changing the antenna pointing. We conducted feasibility studies of the hybrid technique with the WVR phase correction and the antenna Fast Switching (FS) phase referencing (WVR+FS phase correction) for the ALMA 16 km longest baselines in cases that (1) the same observing frequency both for a target and calibrator is used, and (2) higher and lower frequencies for a target and calibrator, respectively, with a typical switching cycle time of 20 s. It was found that the phase correction performance of the hybrid technique is promising where a nearby calibrator is located within roughly 3◦ from a science target, and that the phase correction with 20 s switching cycle time significantly improves the performance with the above separation angle criterion comparing to the 120 s switching cycle time. The currently trial phase calibration method shows the same performance independent of the observing frequencies. This result is especially important for the higher frequency observations because it becomes difficult to find a bright calibrator close to an arbitrary sky position. In the series of our experiments, it is also found that phase errors affecting the image quality come from not only the water vapor content in the lower troposphere but also a large structure of the atmosphere with a typical cell scale of a few tens of kilometers.

Proceedings Article | 19 July 2016 Paper

Analysis of antenna position measurements and weather station network data during the ALMA long baseline campaign of 2015

Todd Hunter, Robert Lucas, Dominique Broguière, Ed Fomalont, William R. Dent, Neil Phillips, David Rabanus, Catherine Vlahakis

Proc. SPIE. 9914, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VIII

KEYWORDS: Observatories, Data modeling, Interferometers, Sensors, Calibration, Oxygen, Humidity, Antennas, Radiometry, Temperature metrology

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In a radio interferometer, the geometrical antenna positions are determined from measurements of the observed delay to each antenna from observations across the sky of many point sources whose positions are known to high accuracy. The determination of accurate antenna positions relies on accurate calibration of the dry and wet delay of the atmosphere above each antenna. For the Atacama Large Millimeter/Submillimeter Array (ALMA), with baseline lengths up to 15 kilometers, the geography of the site forces the height above mean sea level of the more distant antenna pads to be significantly lower than the central array. Thus, both the ground level meteorological values and the total water column can be quite different between antennas in the extended configurations. During 2015, a network of six additional weather stations was installed to monitor pressure, temperature, relative humidity and wind velocity, in order to test whether inclusion of these parameters could improve the repeatability of antenna position determinations in these configurations. We present an analysis of the data obtained during the ALMA Long Baseline Campaign of October through November 2015. The repeatability of antenna position measurements typically degrades as a function of antenna distance. Also, the scatter is more than three times worse in the vertical direction than in the local tangent plane, suggesting that a systematic effect is limiting the measurements. So far we have explored correcting the delay model for deviations from hydrostatic equilibrium in the measured air pressure and separating the partial pressure of water from the total pressure using water vapor radiometer (WVR) data. Correcting for these combined effects still does not provide a good match to the residual position errors in the vertical direction. One hypothesis is that the current model of water vapor may be too simple to fully remove the day-to-day variations in the wet delay. We describe possible new avenues of improvement, which include recalibrating the baseline measurement datasets using the contemporaneous measurements of the water vapor scale height and temperature lapse rate from the oxygen sounder, and applying more accurate measurements of the sky coupling of the WVRs.

Proceedings Article | 12 October 2006 Paper

Photonic mm-wave local oscillator

Robert Kimberk, Todd Hunter, C.-Y. Edward Tong, Raymond Blundell

Proc. SPIE. 6373, Terahertz Physics, Devices, and Systems

KEYWORDS: Astronomy, Oscillators, Extremely high frequency, Modulation, Interferometers, Phase shift keying, Modulators, Antennas, Optical correlators, Temperature metrology

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Proceedings Article | 27 June 2006 Paper

Characterization of instrumental phase stability

D. Kubo, T. Hunter, R. Christensen, P. Yamaguchi

Proc. SPIE. 6275, Millimeter and Submillimeter Detectors and Instrumentation for Astronomy III

KEYWORDS: Astronomy, Oscillators, Calibration, Receivers, Control systems, Antennas, Phase measurement, Analog electronics, Optical correlators, Shape memory alloys

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Proceedings Article | 27 June 2006 Paper

Elevation angle dependence of the SMA antenna focus position

Satoki Matsushita, Masao Saito, Kazushi Sakamoto, Todd Hunter, Nimesh Patel, Tirupati Sridharan, Robert Wilson

Proc. SPIE. 6275, Millimeter and Submillimeter Detectors and Instrumentation for Astronomy III

KEYWORDS: Reflectors, Telescopes, Astronomy, Oscillators, Radio optics, Antennas, Planets, Jupiter, Shape memory alloys, Temperature metrology

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Proceedings Article | 13 December 2002 Paper

Receiver control for the submillimeter array

Todd Hunter, Robert Wilson, Robert Kimberk, P. Leiker, Robert Christensen

Proc. SPIE. 4848, Advanced Telescope and Instrumentation Control Software II

KEYWORDS: Actuators, Oscillators, Calibration, Receivers, Computer programming, Attenuators, Microcontrollers, Antennas, Servomechanisms, Shape memory alloys

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Proceedings Article | 1 June 1994 Paper

Monolithic Si bolometer array for the Caltech Submillimeter Observatory

Ning Wang, Todd Hunter, Dominic Benford, Eugene Serabyn, Thomas Phillips, Samuel Moseley

Proc. SPIE. 2198, Instrumentation in Astronomy VIII

KEYWORDS: Bolometers, Fiber amplifiers, Observatories, Digital signal processing, Optical amplifiers, Cameras, Silicon, Data acquisition, Field effect transistors, Resistors

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Showing 5 of 8 publications

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