ngVLA > Next Generation Very Large Array > Frequently Asked Questions

Frequently Asked Questions

by Davis Murphy last modified Sep 05, 2017

What is the ngVLA?

The next-generation VLA (ngVLA) is a future centimeter-to-millimeter wave interferometer that builds on the legacy of the JVLA and ALMA, 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 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 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.

How big is an ngVLA dish and how many of them will there be?

The current baseline design consists of 214 18m dishes, providing 10x the effective collecting area of the JVLA at 40GHz. Presently, an offset-Gregorian geometry is preferred, with its feed-arm low, similar to the choice for the South African MeerKAT dishes.

What is the frequency range of the ngVLA?

The ngVLA will nominally operate from 1.2 – 116 GHz.

We are also exploring the option for potential commensal observations at lower frequencies that leverage the ngVLA infrastructure (e.g., ngLOBO) that would both cover 5 – 150 MHz with a multi-beam dipole array at ngVLA long-baseline stations and commensal prime focus feeds on the ngVLA antennas covering ~150 – 800MHz).

What is the angular resolution of the ngVLA?

The resolution of an interferometer is determined by the observed wavelength and its longest baseline. With its ~500 km baselines, the ngVLA will achieve an angular resolution ranging between 1 and 100 mas at 2.6 mm (116 GHz) and 25cm (1.2 GHz), respectively. Even higher angular resolution imaging will be possible by combining the ngVLA with existing VLBA antennas into a continental-scale array.

In addition to the main array, we are also exploring the option of using the ngVLA project as a means to initiate an expansion of US VLBI capabilities by replacing existing VLBA antennas/infrastructure with ngVLA technology.

Where will the ngVLA be located?

The ngVLA will be centered at the location of the VLA site on the plains of San Agustin, with additional long-baseline stations spread over greater New Mexico, Texas, and Mexico.

How is the ngVLA different than the VLA?

The VLA has been the workhorse 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 contiguous frequency coverage between 1-50GHz. The ngVLA, by comparison, will consist of 214 18m dishes extending over ~500 km baselines. The ngVLA will extend the operational frequency range from 1.2 - 116GHz.

How is the ngVLA different than ALMA?

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, the ngVLA (at an altitude of ~2100 m) can both access the 3mm atmospheric window and will provide a factor of ~10 times more collecting area and baselines that are ~20x longer than ALMA.

How is the ngVLA different than SKA1-MID?

As currently envisioned, SKA1-MID will be the premier radio interferometer at decametric wavelengths, consisting of up to 133 15m (+ 64 13.5m MeerKAT) dishes with a maximum baseline of ~120 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.

Will the ngVLA have international and/or domestic partners?

Yes. The project office is currently in the process of exploring international and domestic (academic and industrial) partnerships. If 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.

ngVLA vs SKA: [How] Is this different from the SKA? / Is this a competitor to SKA?

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

ngVLA strongly complements SKA1 and ALMA, providing key sensitivity and resolution to pursue new scientific goals and interests while also bridging the capabilities of those facilities.

Re-baselining / De-scoping: What if ngVLA has to re-baseline, as the SKA has? / What makes you think the ngVLA won't have to re-baseline?

NRAO and its international partners will be building upon their ALMA experience and success in developing the ngVLA project definition. Many projects undergo scoping revisions before and during construction to address planning shortcomings, changes in assumptions or external conditions, new constraints etc. NRAO rebaselined ALMA in 2005, eventually completing the project within 0.5% of the revised budget estimate. SKA recently completed a second rebaselining (2017).

The technologies and approaches developed for ngVLA will have application well beyond the project, enabling many different futures.

Funding: Why would the US government financially support this [when they wouldn't support the SKA]? / Is this a US radio community buy-in to the SKA?

The US government has never been asked to fund SKA in any significant way; 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. Since 2010, the scientific goals of SKA1 have narrowed, and engagement with the broader US community has further declined. The fact that US science funding agencies are not engaging with SKA arises from several factors, including funding shortages.

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 focused on our PI-driven science interests. ngVLA can address many of the goals of the ASTRO2010 Decadal Survey, and will be even more relevant (we believe) to the science priorities identified for the 2020s in ASTRO2020.

ngVLA may eventually be considered as an element of an SKA global program, thereby engaging all parts of the US RMS community in new opportunities in the 2020s.

Will I still have to use CASA to reduce my data?

Similar to current ALMA operations, we anticipate ngVLA users will be delivered Science Ready Data Products (SRDPs) 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 for advanced users. Using ALMA and the VLA as a test bed, we are building up our expertise and knowledgebase for understanding 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.

How will I get my ngVLA data?

The Science Ready Data Products (SRDPs) produced by the ngVLA will be available through the NRAOArchive 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 richarchive of images ready for study.Capabilities for inspection andselection prior to download will assist in mitigating download times.

When will construction start?

Depending on the recommendation for the Astro2020 Decadal Survey, we expect to enter the 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.

When will operations start?

With construction commencing in 2025, we would anticipate a notional Early Science start date in 2028, with full array operations beginning in 2034.

What is it going to cost?

We have instituted an internal cost cap of $1.5B (2016) for construction funds, and a corresponding operational cost cap of $75M per year (i.e., <3x current VLA operations). Of the $1.5B in construction, we anticipate the US to be the majority partner, paying roughly half of the total cost (i.e., $750M), with the other half covered by a combination of international and multi-agency partnerships that are actively being pursued.