Technical Concepts & Challenges

# Technical Concepts & Challenges

by Davis Murphy last modified Sep 13, 2017

## ngVLA Reference Design Development & Performance Estimates (June 2017)

The predicted performance of the array, based on feedback from the community following the June 2017 ngVLA Science and Technology meeting in Socorro, is summarized below and is described in more detail in ngVLA Memo 17.

The array configuration has a minimum extent of 300 km (E‐W) by 500 km (N‐S), not yet accounting for long‐baseline science cases such as astrometry and geodesy. The array collecting area is distributed to provide high surface brightness sensitivity on a range of angular scales spanning from approximately 1000 to 10 mas. In practice, this means a core with a large faction of the collecting area in a randomized distribution to provide high snapshot imaging fidelity, and arms extending asymmetrically out to 500 km baselines, filling out the (u,v)‐plane with Earth rotation and frequency synthesis.

The present concept is a homogeneous array of 214 18m apertures. The selection of the number of apertures and the antenna diameter is supported by parametric cost estimation. The antenna locations are fixed, with no reconfiguration capability. All antennas are outfitted with front ends that provide access to the atmospheric windows spanning 1.2 GHz to 50.5 GHz and 70 GHz to 116 GHz.

Table 1 provides the current baseline receiver configuration, while Table 2 gives the key imaging performance. While the values are more representative of the current design than those previously reported (e.g., ngVLA Memo 5), the table is by necessity a simplification. As the configuration and our understanding of its performance evolve with design refinements and modeling, most factors in this table can be expected to change as future work on the imaging performance of the array continues.

A sensitivity calculator is currently being developed based on such investigations and will eventually supersede the values included in Table 1.

The implementation of Very Long Baseline Interferometry (VLBI) on the timescale of the ngVLA is being actively investigated to enable science that requires baselines out to transcontinental distances.

Table 1: ngVLA Baseline Receiver Configuration
Band #DewarfL
GHz
fM
GHz
fH
GHz
fH:fLBW GHz
1 A 1.2 2.55 3.9 3.25 2.7
2 B 3.9 8.25 12.6 3.23 8.7
3 B 12.6 16.8 21.0 1.67 8.4
4 B 21.0 28.0 35.0 1.67 14.0
5 B 30.5 40.5 50.5 1.66 20.0
6 B 70.0 93.0 116 1.66 46.0

Table 2: Next Generation VLA Key Performance Metrics
Parameter [units]3
GHz
8
GHz
17 GHz28 GHz41 GHz93 GHzNotes
Band Lower Frequency, fL [GHz] 1.2 3.9 12.6 21.0 30.5 70.0 a
Band Upper Frequency, fH [GHz] 3.9 12.6 21.0 35.0 50.5 116.0 a
Field of View FWHM [arcmin] 19.5 7.3 3.4 2.1 1.4 0.6 b
Aperture Efficiency [%] 0.75 0.75 0.77 0.75 0.72 0.53 b
Effective Area, Aeff, x 103 [m2] 40.8 40.8 41.9 40.8 39.2 28.9 b
System Temp, Tsys [K] 18 22 21 31 48 87 a, f
Max Inst. Bandwidth [GHz] 2.7 8.7 8.4 14.0 20.0 20.0 a
Antenna SEFD [Jy] 265.7 324.8 302.0 457.7 738.2 1817.5 a, b
Resolution of Max. Baseline $\theta_{max}$ [mas]692612752c
Resolution FWHM @ Natural Weighting [mas]130492314104c, d
Continuum rms, 1 hr [μJy/beam] 0.30 0.21 0.20 0.23 0.31 0.76 d
Line Width, 10 km/s [kHz] 100.0 266.7 566.7 933.3 1366.7 3100.0
Line rms, 1 hr, 10 km/s [μJy/beam] 49.9 37.3 23.8 28.1 37.5 61.3 d
Resolution $\theta_{1/2}$ [mas]1000
Continuum rms, 1 hr, Robust [μJy/beam] 0.61 0.45 0.45 0.56 0.77 2.01 e
Line rms 1 hr, 10 km/s Robust [μJy/beam] 100.1 81.7 55.4 68.0 93.2 161.6 e
Brightness Temp (TB) rms continuum, 1 hr, Robust [K] 0.0823 0.0086 0.002 0.0009 0.0006 0.0003 e
TB rms line, 1 hr, 10 km/s, Robust [K] 13.53 1.55 0.23 0.11 0.07 0.02 e
Resolution $\theta_{1/2}$ [mas]100
Continuum rms, 1 hr, Robust [μJy/beam] 0.48 0.36 0.37 0.46 0.64 1.69 e
Line rms 1 hr, 10 km/s Robust [μJy/beam] 79.0 65.9 45.3 56.1 77.4 135.7 e
Brightness Temp (TB) rms continuum, 1 hr, Robust [K] 0.1804 0.0916 0.103 0.0579 0.0338 0.0271 e
TB rms line, 1 hr, 10 km/s, Robust [K] 29.65 16.55 12.49 7.10 4.08 2.18 e
Resolution $\theta_{1/2}$ [mas]10
Continuum rms, 1 hr, Robust [μJy/beam] - - - 0.36 0.51 1.37 e
Line rms 1 hr, 10 km/s Robust [μJy/beam] - - - 44.2 61.5 109.8 e
Brightness Temp (TB) rms continuum, 1 hr, Robust [K] - - - 5.5479 4.4020 4.4272 e
TB rms line, 1 hr, 10 km/s, Robust [K] - - - 679.48 532.52 355.60 e
(a)  6-band 'baseline' receiver configuration.
(b) Reference design concept of 214 18m aperture antennas. Unblocked aperture with 160um surface.
(c) 'South West' Configuration by E. Greisen. Resolution in E-W axis.
(d) Using Natural Weights, dual pol, and all baselines.
(e) Using Weights as described in ngVLA Memo #16, scaled by frequency.
(f) Averaged over the full band. Assumes 6mm PWV and 45 deg. elev. on sky.