Frequently Asked Questions


The Next-Generation VLA (ngVLA) is a future centimeter-to-millimeter wave interferometer that builds on the legacy of the JVLA, ALMA, and VLBA, as the next major facility in ground-based US radio astronomy. The ngVLA is optimized for observations at wavelengths between the superb performance of ALMA at sub-mm wavelengths, and the future SKA1-MID at longer (decimetric) wavelengths.

 

The ngVLA opens a new window on the Universe through ultra-sensitive imaging of thermal line and continuum emission down to milliarcecond resolution, as well as unprecedented broad band high-angular-resolution continuum polarimetric imaging of non-thermal processes. The ngVLA will perform transformational science covering areas of terrestrial planet formation to the first generation of molecules in the early Universe, as detailed in the recently published ngVLA Science Book.

The current baseline design of the ngVLA consists of 244 dishes of 18m diamter. Of these, 214 are concentrated in the US Southwest and comprise the main array which provides 10x the effective collecting area of the JVLA at 40 GHz. As of mid-2018, 30 ngVLA dishes are distributed at very long baselines up to 9000 kilometers reaching across North America, and are referred to as the Long Baseline Array or LBA. Presently, an unblocked offset-Gregorian geometry is preferred, with its feed arm low, for optimal science and operational performance.

An additional 19 6m dishes will make up a Short Baseline Array (SBA) and will be sensitive to a portion of the larger angular scales undetected by the main array. The SBA may be combined with four 18m (main array) antennas used in total power mode to completely fill in the central hole in the (u, v)-plane left by the 6m dishes.

The ngVLA will nominally operate from 1.2 – 116 GHz (25 – 0.26 cm). Key performance parameters of the ngVLA are available on the Reference Design page.

The US Low Frequency Radio Community is exploring an option for a co-located array that would use the ngVLA infrastructure to cover 20 MHz to 150 MHz. An option to add a prime focus feed to the ngVLA antennas to cover 150 MHz to 800 MHz is also possible. Funding and design for these options (e.g., ngLOBO) is outside the scope of the ngVLA baseline design.

 

 

The resolution of an interferometer is determined by the observed wavelength and its longest baseline. The ngVLA will be designed to operate as two or more subarrays, with each subarray covering a range of baselines to provide a resolution and surface brigthtness sensitivity appropriate for the object under study. With up to ~1000 km baselines, the ngVLA main array will achieve an angular resolution ranging between ~0.5 and 50 mas at 2.6 mm (116 GHz) and 25 cm (1.2 GHz), respectively. Using the full extent of the ngVLA in a configuration including the LBA antennas on continental scales, even higher angular resolution imaging will be possible, achieving ~60 mas and 6 mas at 2.6 mm (116 GHz) and 25cm (1.2 GHz), respectively.

 

Key performance parameters of the ngVLA are available on the Reference Design page.

The ngVLA will be centered at the location of the VLA site on the plains of San Agustin, with additional mid-baseline stations currently spread over greater New Mexico, Arizona, Texas, and Mexico. The ten Long Baseline Array stations are currently located at six VLBA sites plus four existing radio facilities that will take advantage of present infrastructure: Hawaii (2), Washington, California, Iowa, Massachusetts, New Hampshire, Puerto Rico, the US Virgin Islands, and Canada. Nineteen 6m antennas will be distributed in a compact core at the VLA site to form the Short Baseline Array (SBA).

The VLA has been the scientific powerhouse of radio astronomy since its inception in the late 1970s, consisting of 27 movable 25m symmetric antennas, with maximum baselines of 30 km. The VLA underwent a major electronics upgrade, completed in 2011, which provided continuous frequency coverage between 1 – 50 GHz. The ngVLA, by comparison, will consist of of 214 dishes of 18m diameter extending up to ~1000 km baselines and an additional 30 (18m) antennas on scales up to 9,000 km. A compact core of 19 6m antennas will form the Short Baseline Array that will operate in conjunction with the main array as well as independently for short-baseline observations. The ngVLA dishes will not be movable, so different angular scales are achieved by using subarrays, if not the full array. The ngVLA will also extend the operational frequency range from 1.2 – 116 GHz (25 – 026 cm).

ALMA is a radio interferometer optimized to take advantage of the sub-mm (THz) windows that are only accessible at high, dry sites such as the Chajnantor plateau, which is at ~5000 m altitude. While ALMA operates in the 3mm band and above, the ngVLA main array (at an altitude of ~2100 m) can both access the 3mm atmospheric window and provide a factor of ~10 times better sensitivity and baselines that are ~60 times longer than ALMA.

 

As currently envisioned, SKA1-MID will be the premier radio interferometer at decimetric wavelengths, consisting of up to 133 x 15m (+ 64 x 13.5m MeerKAT) dishes with a maximum baseline of up to ~150 km and eventually covering a frequency range spanning 350 MHz (85 cm) to 14 GHz (2 cm). The ngVLA, on the other hand, is being optimized to cover and complement the frequency range above the highest SKA1-MID band, while also achieving much higher angular resolution and sensitivity at cm wavelengths.

The scientific and technical differences between the arrays are apparent, with the ngVLA focusing on cm/mm emission, in particular thermal gas/dust emission on milliarcsecond scales and redshifted molecular line emission. SKA1 is planning to observe at meter through centimeter wavelengths, expanding on the scientific legacy of instruments like WSRT, ATCA, and the VLA.

 

The ngVLA strongly complements SKA1 and ALMA by providing enhanced sensitivity and resolution to pursue new scientific goals and interests in the parameter space bridging those facilities.

Yes.  The project office is currently in the process of exploring international and domestic (academic and industrial) partnerships. International and domestic collaborating institutions have already contributed to community studies and baseline system designs. They also currently participate in project advisory councils. If you are interested in discussing partnership options, please contact our Project Director. We anticipate a name change for the instrument once the project enters into its design and development phase, allowing our international partners to participate in this important activity.

NRAO and its international partners will be building on their ALMA experience and success in developing the ngVLA project. Many projects undergo scoping revisions before and during construction to address planning shortcomings, changes in assumptions or external conditions, new constraints, etc. ALMA was rebaselined in 2005, eventually completing the project within 0.5% of the revised budget estimate. NRAO has a long tradition of excellence in construction of radio astronomy instruments and facilities.

The US government has never been asked to fund the SKA in any significant way; the SKA was unable to gather scientific community support in the Astro2010 Decadal Survey due to uncertainties in the scientific requirements and technical definition of the project at that time. NSF’s withdrawal from supporting overall SKA development in the US was a consequence of funding shortfalls in US science early this decade and a recognition that the project was not broadly supported in the US astronomy community. The fact that US science funding agencies are not engaging with the SKA arises from several factors. Since 2010, the scientific goals of SKA1 have narrowed, and engagement with the broader US community has further declined.

The ngVLA concept has been developed to explore new horizons in cm radio astronomy, synergistic with other existing, under construction, or planned instruments in the US in coming years, and focuses on our PI-driven science interests. The ngVLA can address many of the goals of the Astro2010 Decadal Survey, and will be even more relevant to the science priorities identified for the 2020s in Astro2020.

 

The ngVLA may eventually be considered as a partner with an SKA global program, thereby engaging all parts of the US Radio/Millimeter/Sub-Millimeter community in new opportunities in the 2020s.

 

The ngVLA is being designed as a proposal-driven instrument. The science program will be determined via a competitive peer review process similar to other large PI-driven observatories (e.g., ALMA, VLA, HST, JWST, etc.), adopting best practices for minimizing selection biases. In this way the ngVLA will be a different style of instrument than other facilities on the horizon, such as LSST and SKA1, which have science programs focused on carrying out large surveys.

Building on the current model for ALMA science operations, we anticipate ngVLA users will be delivered Science Ready Data Products (SRDPs; e.g., images and data cubes), rather than raw data, due to the sheer volume of data and computing resources that will be required to reduce data. However, raw visibilities will still be available upon request. Using ALMA and the VLA as test beds, we are building up our expertise and knowledge base of the best methods/practices/deliverables for SRDPs in preparation for the ngVLA.

 

The ngVLA data processing implementation will be based on the CASA package, providing continuity with JVLA and ALMA and continuing to provide a flexible data reduction package for experts in the community. As part of the construction project, CASA will evolve both in the implementation of cutting edge algorithms and the infrastructure needed for broad scale parallelization in order to support the ngVLA.

The Science Ready Data Products (SRDPs) produced by the ngVLA will be available through the NRAO Archive interface, providing a single access point for ALMA and ngVLA data products. After the proprietary period, these SRDPs will be available to the full community, providing a rich archive of images ready for study. Capabilities for inspection and selection prior to download will assist in mitigating download times.

 

Depending on the recommendation for the Astro2020 Decadal Survey, we expect to enter the NSF Major Research Equipment and Facilities Construction (MREFC) design phase in late 2021 and have a final design completed by late 2024. Procurement and construction would then commence in 2025 and should be completed by 2034.

With construction commencing in 2025, we would anticipate a notional Early Science start date in 2028, with full array operations beginning in 2034. Prior to early science, the VLA will likely see a period of reduced capabilities as the ngVLA commissioning efforts ramp up. We are currently starting to develop a transition plan with the community that will bridge VLA and ngVLA operations.

We have instituted an internal cost cap of $1.9B (2018) for construction funds, and a corresponding operational cost cap of $80M (2018) per year (i.e., <3x current VLA + VLBA operations). Of the $1.9B in construction, we anticipate the US to be the majority partner, paying two-thirds to three-quarters of the total cost, with the remaining piece covered by a combination of international and multi-agency partnerships that are actively being pursued. For context, construction costs for the ngVLA are anticipated to be comparable to those for ALMA, allowing for inflation.

The ngVLA project includes a Science Advisory Council (SAC) and Technical Advisory Council (TAC) to guide the science case and technical implementation of the array. To date, the ngVLA Science Working Groups (SWGs) have prepared of order 80 science use cases that were subsequently prioritized by the SAC. These form the highest-level requirements for the system, and have significantly impacted the design concept as it has matured from 2015 through 2018. Examples of community driven changes are the inclusion of 1.2 – 10 GHz frequency coverage, a Short Baseline Array of 6m apertures plus total power antennas, and the Long Baseline Array.

Likewise, the ngVLA TAC has also influenced key design decisions, including the feed design for Band 1 and 2, and the selection of the optical design for the ngVLA antenna reference design.

If you are interested in providing scientific or technological input to help guide the design of the ngVLA, please contact the respective council chairs.

The ngVLA reference design includes an assessment of the technological readiness of each major element in the system. With the exception of the imaging pipeline and the data archive, all ngVLA systems could be built with 2018 technology. The imaging pipeline and archive will require continued advances in processing power and storage technology from 2018 through first science in 2028 in order to achieve our current science goals within the allowable cost.

For other systems, while they can be achieved with present technology, a core goal with ngVLA development is to find solutions that allow for volume manufacturing, improved performance, increased reliability, and reduced lifecycle cost. There is ample scope for advances in technology to improve the value of the array over the course of the design.

Some enabling technologies for ngVLA are discussed on the Technology page. The technologies and approaches developed for ngVLA will have application well beyond the project, enabling many different futures.

Depending on detailed science interests, processing requirements are often set by the lowest observing frequency (i.e., fully imaging large primary beams); as our primary science mission does not go as low as the SKA, the processing challenges are much more tractable. We expect to benefit from additional technological development due to the later delivery of the ngVLA. Even if expected improvements fail to materialize by the beginning of operations, because we store the raw visibility data, reprocessing of data with better algorithms or with increased fidelity can be done when the computational resources become available.

The average total power load is estimated at 3.5MW for the array, central infrastructure, and off-site buildings combined. This is approximately three times the current VLA load. Significant savings are achieved in the design of the antenna electronics, correlator system, and computing cluster when compared to existing facilities. The main power source on the plains is expected to be grid power provided by the local utility company. Green power sources (photovoltaic and wind turbines) have been considered, and are increasingly attractive to reduce operating costs, but are presently outside the scope of construction. Remote ngVLA antennas are being located in part to use local infrastructure (e.g., grid power).

NRAO started the ngVLA project as an internally funded R&D effort in 2015. NSF provided support of order $2M for development in 2015 through 2017. Based on clear community engagement and support in developing the key science goals, NSF reprofiled an additional $11M for 2018 and 2019, which has supported the definition of the ngVLA science case and the development of the ngVLA reference design.  In-kind partner contributions of approximately $1M have also been made over the 2016 to 2018 period.

NRAO is requesting continued funding from the NSF at the ~$8 to 10M/yr level through 2021 when the Astro 2020 Decadal Survey concludes. The design effort would then ramp up through final design in 2024, and progress to construction in 2025.

Yes!  We recognize that the ngVLA will offer a number of opportunities for local and regional engagement, including formal and informal education about radio astronomy in general, and the ngVLA in particular. We also recognize that local communities may have concerns that must be addressed respectfully and cooperatively. NRAO is developing a number of avenues for opening and maintaining dialogues with all interested communities.

Yes!  The NRAO is committed to identifying and developing opportunities for Broader Impact and Broadening Participation through the life of the ngVLA project. The NRAO has invested significant funding for Education and Public Outreach (EPO) exploration and development, and has identified a BI Lead to manage the anticipated multifaceted ngVLA broader impact/broadening participation efforts. An important component of our broadening participation efforts will be substantial research and technical experiences for underrepresented minority students, through new and established community college and university partners. For more information, contact odi@nrao.edu.

Yes, we are planning to operate the ngVLA as an Open Skies observatory, as is the case for all other NRAO facilities, subject to the general principle of reciprocity outlined by the NSF/NASA-sponsored Astronomy & Astrophysics Advisory Committee. Investigators may be awarded ngVLA time based on scientific merit regardless of institutional affiliation.