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Ground-based gamma-ray astronomy has historically implemented two dramatically different techniques. One method employs Imaging Atmospheric Cherenkov Telescope(s) (IACT) that detect the Cherenkov light generated in the atmosphere by extensive air showers. The other method employs particle detectors that directly detect the particles that reach ground level — known as Extensive Air Shower (EAS) arrays. Until recently, the IACT method had been the only technique to yield solid detections of TeV gamma-ray sources. Utilizing water Chernkov technology, Milagro, was the first EAS array to discover new gamma-ray sources and demonstrated the power of and need for an all-sky high duty cycle instrument in the TeV energy regime. The transient nature of many TeV sources, the enormous number of potential sources, and the existence of TeV sources that encompass large angular areas all point to the need for an all-sky, high duty-factor instrument with even greater sensitivity than Milagro. The High Altitude Water Cherenkov (HAWC) Observatory will be over an order of magnitude more sensitive than Milagro. In this paper we will discuss the results from Milagro and the design of the HAWC instrument and its experimental sensitivity.

Array configurations built on a hierarchy of simple elements have excellent properties for cross-correlation imaging interferometers including a smooth distribution of measured Fourier components, high angular resolution, low side lobes, and compact array size. Compared to arrays with a Gaussian distribution of antenna separations, hierarchical arrays (H-arrays) produce beams with higher angular resolution and a tighter concentration of the total power (encircled energy) within a smaller area around the main beam. An attractive feature of H-arrays is their simplicity. The relationships between the Fourier coverage and the array configuration are easy enough to understand that they can be adjusted to achieve different design goals without the need for numerical optimization. H-arrays will be useful for future multi-element interferometers.

We describe the design and performance of the digital servo and motion control system for the 6-meter parabolic antennas of the Submillimeter Array (SMA) on Mauna Kea, Hawaii. The system is divided into three nested layers operating at a different, appropriate bandwidth. (1) A rack-mounted, real-time Unix system runs the position loop which reads the high resolution azimuth and elevation encoders and sends velocity and acceleration commands at 100 Hz to a custom-designed servo control board (SCB). (2) The microcontroller-based SCB reads the motor axis tachometers and implements the velocity loop by sending torque commands to the motor amplifiers at 558 Hz. (3) The motor amplifiers implement the torque loop by monitoring and sending current to the three-phase brushless drive motors at 20 kHz. The velocity loop uses a traditional proportional-integral-derivative (PID) control algorithm, while the position loop uses only a proportional term and implements a command shaper based on the Gauss error function. Calibration factors and software filters are applied to the tachometer feedback prior to the application of the servo gains in the torque computations. All of these parameters are remotely adjustable in the software. The three layers of the control system monitor each other and are capable of shutting down the system safely if a failure or anomaly occurs. The Unix system continuously relays the antenna status to the central observatory computer via reflective memory. In each antenna, a Palm Vx hand controller displays the complete system status and allows full local control of the drives in an intuitive touchscreen user interface. The hand controller can also be connected outside the cabin, a major convenience during the frequent reconfigurations of the interferometer. Excellent tracking performance (~ 0.3′′ rms) is achieved with this system. It has been in reliable operation on 8 antennas for over 10 years and has required minimal maintenance.

Rocket technology, originally developed for military applications, has provided a low-cost observing platform to carry critical and rapid-response scientific investigations for over 70 years. Even with the development of launch vehicles that could put satellites into orbit, high altitude sounding rockets have remained relevant. In addition to science observations, sounding rockets provide a unique technology test platform and a valuable training ground for scientists and engineers. Most importantly, sounding rockets remain the only way to explore the tenuous regions of the Earth’s atmosphere (the upper stratosphere, mesosphere, and lower ionosphere/thermosphere) above balloon altitudes ($\sim$40km) and below satellite orbits ($\sim$160km). They can lift remote sensing telescope payloads with masses up to 400kg to altitudes of 350km providing observing times of up to 6min above the blocking influence of Earth’s atmosphere. Though a number of sounding rocket research programs exist around the world, this article focuses on the NASA Sounding Rocket Program, and particularly on the astrophysical and solar sounding rocket payloads.

Advances in astronomy are intimately linked to advances in digital signal processing (DSP). This special issue is focused upon advances in DSP within radio astronomy. The trend within that community is to use off-the-shelf digital hardware where possible and leverage advances in high performance computing. In particular, graphics processing units (GPUs) and field programmable gate arrays (FPGAs) are being used in place of application-specific circuits (ASICs); high-speed Ethernet and Infiniband are being used for interconnect in place of custom backplanes. Further, to lower hurdles in digital engineering, communities have designed and released general-purpose FPGA-based DSP systems, such as the CASPER ROACH board, ASTRON Uniboard, and CSIRO Redback board. In this introductory paper, we give a brief historical overview, a summary of recent trends, and provide an outlook on future directions.

The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket payload flew for the second time on 2014 December 11. To enable direct Hard X-Ray (HXR) imaging spectroscopy, FOXSI makes use of grazing-incidence replicated focusing optics combined with fine-pitch solid-state detectors. FOXSI’s first flight provided the first HXR focused images of the Sun. For FOXSI’s second flight several updates were made to the instrument including updating the optics and detectors as well as adding a new Solar Aspect and Alignment System (SAAS). This paper provides an overview of these updates as well as a discussion of their measured performance.

X-ray polarimetry has seen a growing interest in recent years. Improvements in detector technology and focusing X-ray optics now enable sensitive astrophysical X-ray polarization measurements. These measurements will provide new insights into the processes at work in accreting black holes, the emission of X-rays from neutron stars and magnetars, and the structure of AGN jets. X-Calibur is a balloon-borne hard X-ray scattering polarimeter. An X-ray mirror with a focal length of 8m focuses X-rays onto the detector, which consists of a plastic scattering element surrounded by Cadmium-Zinc-Telluride detectors, which absorb and record the scattered X-rays. Since X-rays preferentially scatter perpendicular to their polarization direction, the polarization properties of an X-ray beam can be inferred from the azimuthal distribution of scattered X-rays. A close alignment of the X-ray focal spot with the center of the detector is required in order to reduce systematic uncertainties and to maintain a high photon detection efficiency. This places stringent requirements on the mechanical and thermal stability of the telescope structure. During the flight on a stratospheric balloon, X-Calibur makes use of the Wallops Arc-Second Pointer (WASP) to point the telescope at astrophysical sources. In this paper, we describe the design, construction, and test of the telescope structure, as well as its performance during a 25-h flight from Ft. Sumner, New Mexico. The carbon fiber–aluminum composite structure met the requirements set by X-Calibur and its design can easily be adapted for other types of experiments, such as X-ray imaging or spectroscopic telescopes.

As telescope facilities become increasingly more capable, they also become increasingly complex and require additional resources to operate. This is particularly true for the current and future generations of “software defined telescopes” that can support a variety of observing programs simultaneously, either through commensal observations or through support for multiple pointing centers as in the case of dipole arrays or dishes equipped with phased array feeds. At the same time, many current and future facilities are also distributed over large geographic areas, making monitoring and maintenance more difficult and costly. For these reasons, we have developed a new paradigm for telescope operations called the “swarm telescope” that breaks large, single facilities into smaller groups of independent systems that can collaboratively work together to function as a single facility but with much less operational overhead. In this paper, we outline the swarm telescope concept and an example of its implementation at the Long Wavelength Array. We also discuss potential advantages of using this approach for other facilities, in particular the Next Generation Very Large Array.

Modern radio astronomical facilities are able to detect extremely weak electromagnetic signals not only from the universe but also from man-made radio frequency interference of various origins. These range from wanted signals to unwanted out-of-band emission of radio services and applications to electromagnetic interference produced by all kinds of electronic and electric devices. Energy harvesting wind turbines are not only equipped with electric power conversion hardware but also copious amounts of electronics to control and monitor the turbines. A wind turbine in the vicinity of a radio telescope could therefore lead to harmful interference, corrupting the measured astronomical data. Many observatories seek to coordinate placement of new wind farms with wind turbine manufacturers and operators, as well as with the local planning authorities, to avoid such a situation. In our study, we provide examples as well as guidelines for the determination of the separation distances between wind turbines and radio observatories, to enable a benign co-existence for both.

The proposed calculations entail three basic steps. At first, the anticipated maximum emitted power level based on the European EN 550011 (CISPR, 2015) standard, which applies to industrial devices, is determined. Then secondly, the propagation loss along the path to the radio receiver is computed via a model provided by the international telecommunication union. Finally, the received power is compared to the permitted power limit that pertains in the protected radio astronomical observing band under consideration. This procedure may be carried out for each location around a telescope site, in order to obtain a map of potentially problematic wind turbine positions.

The telescope-in-a-box is a small objective ultralight Dobsonian telescope concept that will be accompanied by telescope instrumentation lesson plans. This provides a rich set of opportunities for teaching astronomy, physics and engineering. Our first two prototypes demonstrate that it can be constructed at low cost, while many of the design decisions can lead to lesson plans in telescope instrumentation.

Seismicity induces ground vertical and horizontal displacements that could affect the image quality obtained by telescopes in a similar fashion than atmospheric turbulence. In this work, we study the effect of local seismicity relative to atmospheric turbulence upon the image quality of astronomical observations at El Teide observatory and Roque de los Muchachos observatory, both in the Canary Islands, Spain. Three different aspects of seismicity are studied, namely regional volcanism and seismicity (that is compared with other astronomical sites), seismic noise and possible resonances between seismic noise and the structure of telescopes.

The requirements for excellent image quality of current large and future very large telescopes demand a proper knowledge of atmospheric turbulence. Thus several projects are already pursuing this aim. The precise characterization of the turbulence above a particular site requires long-term monitoring. In this sense, due to the lack of long-term information on turbulence, high-altitude winds (in particular winds at the 200-mbar pressure level, V₂₀₀) have been proposed (Sarazin & Tokovinin 2002,¹ S&T02 hereafter) as a parameter for estimating the total turbulence at a particular site, because records of this parameter exist from several sources. This choice is based on the idea that the greatest source for turbulence generation is related to the highest peak in the vertical wind profile, which is located at the 200-mbar pressure level globally. Moreover, S&T02 found a good correlation between the average velocity of the turbulence, V₀, and V₂₀₀ of the form: V₀= 0.4*V₂₀₀ at the Cerro Pachón and Paranal Observatories in Chile. A linear relationship between V₀ and V₂₀₀ was also found in San Pedro Mártir (Mexico) (Masciadri & Egner 2006,² M&E06 hereafter). We study here the possible connection between V₀ and V₂₀₀ at the Teide Observatory (Spain).

Phase I of the Q/U Imaging ExperimenT (QUIET) measures the Cosmic Microwave Background polarization anisotropy spectrum at angular scales 25 ≲ l ≲ 1000. QUIET has deployed two independent receiver arrays. The 40-GHz array took data between October 2008 and June 2009. The 90-GHz array was deployed in June 2009 and observations are ongoing. Described in this proceeding are the instrument, analysis procedures, and expected science reach for the QUIET Phase I arrays. We also briefly discuss the science forecasts for QUIET Phase II in which arrays roughly an order-of-magnitude larger will be deployed.

Since the beginning of its operations in Summer 2008 the Fermi Gamma-Ray Space Telescope has been monitoring a vast number of high energy astrophysical sources, among which several are commonly thought to be plausible sources of cosmic rays (CRs). In addition to the gamma emission from individual sources like supernova remnants, Fermi has been observing γ rays from the interaction of CRs with the galactic medium, while locally it measures with outstanding accuracy the flux of primary electrons in the Earth orbit up to 1 TeV. After three years of operation the collected information gives novel insights to the long standing puzzle of the OR origin.

The Fermi Large Area Telescope (LAT) has been exploring the high-energy gamma-ray sky since its launch on June 11, 2008. These years of nearly flawless operations allowed a constant improvement of the detector knowledge and, as a consequence, a the periodic update of the events selection and corresponding instrument response parametrization. The main on-going effort is a radical revision of the entire event-level analysis including event reconstruction algorithms in each subsystems. In this work I will review the status of the instrument and its performance and I will also discuss the future prospects for astro-particle physics with the LAT.