ALMA front end amplitude calibration device design and measured performances

Casalta, Joan Manel; Molins, Albert; Bassas, M.; Canchado, Manuel; Creus, E.; Tomàs, Albert

doi:10.1117/12.788756

Cited by 2 publications

(1 citation statement)

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“…Instead, we use an approach developed for single-dish data, modified for our observing technique. Calibration relies on measurements made with the ALMA Calibration Device (Casalta et al, 2008;Yagoubov et al, 2011;van Kempen et al, 2012) located in each antenna: this system is capable of placing two microwave absorbers at different temperatures, the "ambient" (nominally 20 • C) and "hot" (nominally 85 • C) loads, in front of the receiver feed horns of any of the bands. At the beginning and end of the SD observation, five-second measurements of the receiver signal level are made for the following targets:…”

Section: Calibration Measurementsmentioning

confidence: 99%

Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

et al. 2017

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The Atacama Large Millimeter-submillimeter Array (ALMA) radio telescope has commenced science observations of the Sun starting in late 2016. Since the Sun is much larger than the field of view of individual ALMA dishes, the ALMA interferometer is unable to measure the background level of solar emission when observing the solar disk. The absolute temperature scale is a critical measurement for much of ALMA solar science, including the understanding of energy transfer through the solar atmosphere, the properties of prominences, and the study of shock heating in the chromosphere. In order to provide an absolute temperature scale, ALMA solar observing will take advantage of the remarkable fast-scanning capabilities of the ALMA 12 m dishes to make single-dish maps of the full Sun. This article reports on the results of an extensive commissioning effort to optimize the mapping procedure, and it describes the nature of the resulting data. Amplitude calibration is discussed in detail: a path that utilizes the two loads in the ALMA calibration system as well as sky measurements is described and applied to commissioning data. Inspection of a large number of single-dish datasets shows significant variation in the resulting temperatures, and based on the temperature distributions we derive quiet-Sun values at disk center of 7300 K at λ = 3 mm and 5900 K at λ = 1.3 mm. These values have statistical uncertainties of order 100 K, but systematic uncertainties in the temperature scale that may be significantly larger. Example images are presented from two periods with very different levels of solar activity. At a resolution of order 25 ′′ , the 1.3 mm wavelength images show temperatures on the disk that vary over about a 2000 K range. Active regions and plage are amongst the hotter features while a large sunspot umbra shows up as a depression and filament channels are relatively cool. Prominences above the solar limb are a common feature of the single-dish images.

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Section: Calibration Measurementsmentioning

confidence: 99%

Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

et al. 2017

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Tracking ALMA System Temperature with Water Vapor Data at High Frequency

Dent

Wilson

2022

PASP

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As the world-leading submillimeter telescope, the Atacama Large Millimeter/submillimeter Array observatory is now putting more focus on high-frequency observations at Band 7–10 (frequencies from 275 to 950 GHz). However, high-frequency observations often suffer from rapid variations in atmospheric opacity that directly affect the system temperature T sys. Current observations perform discrete atmospheric calibrations (Atm-cals) every few minutes, with typically 10–20 occurring per hour for high frequency observation and each taking 30–40 s. In order to obtain more accurate flux measurements and reduce the number of atmospheric calibrations (Atm-cals), a new method to monitor T sys continuously is proposed using existing data in the measurement set. In this work, we demonstrate the viability of using water vapor radiometer (WVR) data to track the T sys continuously. We find a tight linear correlation between T sys measured using the traditional method and T sys extrapolated based on WVR data with scatter of 0.5%–3%. Although the exact form of the linear relation varies among different data sets and spectral windows, we can use a small number of discrete T sys measurements to fit the linear relation and use this heuristic relationship to derive T sys every 10 s. Furthermore, we successfully reproduce the observed correlation using atmospheric transmission at microwave modeling and demonstrate the viability of a more general method to directly derive the T sys from the modeling. We apply the semi-continuous T sys from heuristic fitting on a few data sets from Band 7 to Band 10 and compare the flux measured using these methods. We find the discrete and continuous T sys methods give us consistent flux measurements with differences up to 5%. Furthermore, this method has significantly reduced the flux uncertainty due to T sys variability for one data set, which has large precipitable water vapor fluctuation, from 10% to 0.7%.

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ALMA front end amplitude calibration device design and measured performances

Cited by 2 publications

References 0 publications

Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Fast-Scan Single-Dish Mapping

Tracking ALMA System Temperature with Water Vapor Data at High Frequency

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