W. D. Duncan

SCUBA: A common-user submillimetre camera operating on the James Clerk Maxwell Telescope

SCUBA, the Submillimetre Common-User Bolometer Array, built by the Royal Observatory Edinburgh for the James Clerk Maxwell Telescope, is the most versatile and powerful of a new generation of submillimetre cameras. It combines a sensitive dual-waveband imaging array with a three-band photometer, and is sky-background-limited by the emission from the Mauna Kea atmosphere at all observing wavelengths from 350 μμto 2 mm. The increased sensitivity and array size mean that SCUBA maps close to 10 000 times faster than its single-pixel predecessor (UKT14). SCUBA is a facility instrument, open to the world community of users, and is provided with a high level of user support. We give an overview of the instrument, describe the observing modes, user interface and performance figures on the telescope, and present a sample of the exciting new results that have revolutionized submillimetre astronomy.

Scuba-2: The 10 000 pixel bolometer camera on the james clerk maxwell telescope

SCUBA-2 is an innovative 10000 pixel bolometer camera operating at submillimetre wavelengths on the James Clerk Maxwell Telescope (JCMT). The camera has the capability to carry out wide-field surveys to unprecedented depths, addressing key questions relating to the origins of galaxies, stars and planets. With two imaging arrays working simultaneously in the atmospheric windows at 450 and 850µm, the vast increase in pixel count means that SCUBA-2 maps the sky 100–150 times faster than the previous SCUBA instrument. In this paper we present an overview of the instrument, discuss the physical characteristics of the superconducting detector arrays, outline the observing modes and data acquisition, and present the early performance figures on the telescope. We also showcase the capabilities of the instrument via some early examples of the science SCUBA-2 has already undertaken. In February 2012, SCUBA-2 began a series of unique legacy surveys for the JCMT community. These surveys will take 2.5years and the results are already providing complementary data to the shorter wavelength, shallower, larger-area surveys from Herschel. The SCUBA-2 surveys will also provide a wealth of information for further study with new facilities such as ALMA, and future telescopes such as CCAT and SPICA.

Characterization and reduction of unexplained noise in superconducting transition-edge sensors

The noise in superconducting transition-edge sensors (TESs) commonly exceeds simple theoretical predictions. The reason for this discrepancy is presently unexplained. We have measured the amplitude and frequency dependence of the noise in TES sensors with eight different geometries. In addition, we have measured the dependence of the noise on operating resistance, perpendicular magnetic field, and bath temperature. We find that the unexplained noise contribution is inversely correlated with the temperature width of the superconducting-to-normal transition and is reduced by a perpendicular field and in certain geometries. These results suggest paths to improved sensor performance.

On the atmospheric limitations of ground-based submillimetre astronomy using array receivers

The calibration of ground-based submillimetre observations has always been a difficult process. We discuss how to overcome the limitations imposed by the submillimetre atmosphere. Novel ways to improve line-of-sight opacity estimates are presented, resulting in tight relations between opacities at different wavelengths. The submillimetre camera SCUBA, mounted on the James Clerk Maxwell Telescope (JCMT), is the first large-scale submillimetre array, and as such is ideal for combating the effects of the atmosphere. For example, we find that the off-source pixels are crucial for removing sky noise. Benefiting from several years of SCUBA operation, a data base of deep SCUBA observations has been constructed to help us understand better the nature of sky noise and the effects of the atmosphere on instrument sensitivity. This has revealed several results. First, there is evidence for positive correlations between sky noise and seeing and sky noise and sky opacity. Furthermore, 850-μm and 450-μm sky noise are clearly correlated, suggesting that 450-μm data may be used to correct 850-μm observations for sky noise. Perhaps most important of all: if off-source bolometers are used for sky noise removal, there is no correlation between instrument sensitivity and chop throw, for chop throws out to 180 arcsec. Understanding the effects of submillimetre seeing is also important, and we find that the JCMT beam is not significantly broadened by seeing, nor is there an obvious correlation between seeing and pointing excursions.

14-pixel, multiplexed array of gamma-ray microcalorimeters with 47eV energy resolution at 103keV

The authors present a prototype for a high-energy-resolution, high-count-rate, gamma-ray spectrometer intended for nuclear forensics and international nuclear safeguards. The prototype spectrometer is an array of 14 transition-edge-sensor microcalorimeters with an average energy resolution of 47eV (full width at half maximum) at 103keV. The resolution of the best pixel is 25eV. A cryogenic, time-division multiplexer reads out the array. Several important topics related to microcalorimeter arrays are discussed, including cross-talk, the uniformity of detector bias conditions, fabrication of the arrays, and the multiplexed readout. The measurements and calculations demonstrate that a kilopixel array of high-resolution microcalorimeters is feasible.

Optimized transition-edge x-ray microcalorimeter with 2.4eV energy resolution at 5.9keV

We present measurements from a series of transition-edge x-ray microcalorimeters designed for optimal energy resolution. We used the geometry of the sensors to control their heat capacity and employed additional normal metal features and a perpendicular magnetic field to control the sharpness of the superconducting-to-normal transition. These degrees of control allow an optimal selection of sensor saturation energy and noise. Successive design changes improved the measured energy resolution of the sensors from 4.5eV full width at half maximum at 5.9keVto2.4eV at 5.9keV. Sensors with this energy resolution are well matched to applications in x-ray astrophysics and terrestrial materials analysis.

Array-compatible transition-edge sensor microcalorimeter γ-ray detector with 42eV energy resolution at 103keV

The authors describe a microcalorimeter γ-ray detector with measured energy resolution of 42eV full width at half maximum for 103keV photons. This detector consists of a thermally isolated superconducting transition-edge thermometer and a superconducting bulk tin photon absorber. The absorber is attached with a technique compatible with producing arrays of high-resolution γ-ray detectors. The results of a detailed characterization of the detector, which includes measurements of the complex impedance, detector noise, and time-domain pulse response, suggest that a deeper understanding and optimization of the thermal transport between the absorber and thermometer could significantly improve the energy resolution of future detectors.

A superconductor–insulator–normal metal bolometer with microwave readout suitable for large-format arrays

We demonstrate high bandwidth and low-noise readout of a superconductor–insulator–normal metal–insulator–superconductor hot-electron bolometer element. We measure a noise equivalent temperature of better than 0.6μK∕Hz1∕2 and infer an electrical noise equivalent power of 7×10−17W∕Hz1∕2 for a normal-metal volume of 4.5μm3 at an operating temperature of 270mK. Using a pulsed bias technique we measure a thermal time constant of 1.2μs at 270mK. The microwave readout method we employ is compatible with large-format arrays of 103–105pixels required by future far-infrared space and ground-based observatories.

Time-division multiplexing of high-resolution x-ray microcalorimeters: Four pixels and beyond

We present experimental results from a four-pixel array of transition-edge-sensor, x-ray microcalorimeters read out through a single amplifier channel via a time-division superconducting quantum interference device multiplexer. We map the dependence of the x-ray energy resolution of the microcalorimeters on multiplexer timing parameters. We achieve multiplexed, four-pixel resolution of 6.94±0.05eV full width at half maximum of the MnKα complex near 5.9keV, which is a degradation of only 0.44eV from nonmultiplexed operation. An analysis of straightforward upgrades to the multiplexer predicts that a linear array of 32 of these pixels could be multiplexed with a degradation in resolution of only 0.1eV. These results, the first demonstration of a time-division multiplexer for x-ray detectors, establish a clear path to the instrumentation of a kilopixel microcalorimeter array.

Electrical and optical measurements on the first SCUBA-2 prototype 1280 pixel submillimeter superconducting bolometer array

SCUBA-2 is a submillimeter camera being built for the James Clerk Maxwell Telescope in Hawaii. Bringing CCD style imaging to the submillimeter for the first time, with over 10000 pixels, it will provide a revolutionary improvement in sensitivity and mapping speed. We present results of the first tests on a prototype 1280 pixel SCUBA-2 subarray; the full instrument will be made up of eight such subarrays. The array is made up of transition edge sensor (TES) detectors, with Mo/Cu bilayers as the sensing element. To keep the number of wires reasonable, a multiplexed readout is used. Unlike previous TES arrays, an in-focal plane multiplexer configuration is used, in which the multiplexing elements are located beneath each pixel. To achieve the required performance, the detectors are operated at a temperature of approximately 120 mK. We describe the results of a basic electrical and optical characterization of the array, demonstrating that it is fully operational. Noise measurements were made on several pixels and gave a noise equivalent power below 2.5 x 10(-17) W HZ(-0.5), within the requirements for SCUBA-2. The construction of the testbed used to carry out these measurements is also described.