ngECT Documentation
The ngECT allows the calculation of the estimated sensitivity for continuum and spectral line modes. Specifically:
1. Continuum:
- continuum point source sensitivity in units of µJy/beam
- continuum brightness sensitivity in units of K
2. Line
- line point source sensitivity in units of µJy/beam
- line brightness sensitivity in units of K
Alternatively you can request the calculation of the on-source integration time needed to reach the input image sensitivity.
Note: the ngECT only does natural + outer uv-taper. The capability of including robust values is being developed. This version of the ngECT uses Tsys and aperture efficiency values that are frequency dependent. For the continuum it has the option to use the maximum receiver bandwidth when the receiver bandwidth is smaller than the correlator bandwidth, and calculates the Tsys and efficiency at the band center. At band 6 it tries to center the 20 GHz of instantaneous bandwidth on the desired frequency instead of using the band average. Also, it calculates the line sensitivity and brightness temperature at the desired frequency instead of the band center.
The ngECT uses the following parameters:
| Parameter | Units | Value | Description |
|---|---|---|---|
| c | m/s | 2.998 × 108 | Speed of light |
| kB | J/K | 1.38 × 10−23 | Boltzmann constant |
| ηc | — | 0.98 | Correlator efficiency |
| npol | — | 1 or 2 ("single" or "dual") | Number of polarizations |
| ηQ | — | 0.9625 | Digitizer quantization efficiency |
| Nant | — | [determined based on the subarry selected] | Number of antennas |
| Tsys | K | [determined based on the representative frequency and correlated bandwidth] | System temperature |
| tint | s | [entered by the user or computed to in order to achive the user-requested RMS] | Integration (or observation) time / time on source |
| A | m2 | 254.47 (for the 18m antennas) | Geometric area of single antenna |
| ηA | — | [determined based on the representative frequency and correlated bandwidth] | Aperture (antenna) efficiency |
| bmax | m | [determined based on the subarry selected] | Physical length of longest baseline in subarray |
| SEFD | Jy | [calculated following the relevant equation below] | System Equivalent Flux Density of an antenna |
| ν | [G/M/k]Hz | [entered by the user] | Representative frequency |
| Δν | Hz | [entered by the user] | Correlated bandwidth |
| θ | arcsec/milliarcsec | [entered by the user; optional] | Synthesized beam size (if tapering is requested) |
The theoretical thermal noise or point-source rms expected for an image using natural weighting of the uv data is:
where SEFD is defined as:
The weighted point source sensitivity is defined as:
where ηweight is an inefficiency factor that is estimated through the taperability analysis which takes into account the change in sensitivity due to using an outer uv taper (see more in ngVLA memo #106).
Subarrays
The ngVLA will use multiple array components to cover angular scales ranging from many arcminutes down to fractions of a milli-arcsecond. These components are expected to be flexibly combined into subarrays that can observe in parallel. These subarrays will have different baseline length distributions and numbers of antennas. When performing calculations with the ngECT, users must specify the subarray they intend to use. Some subarray combinations will be more suited for high-resolution imaging and others will be better for recovering extended, low surface brightness emission. For more information on the ngVLA array components and some of their combinations, please see the Array Configuration page and ngVLA memos referenced therein.
Configuration Revisions
The design of the ngVLA array components has evolved with time. The currently planned configuration is Revision F ("RevF"). Views of the RevF antenna locations are available on the Array Configuration page and additional details are available in the "Array Configuration" System-Level documents. The ngECT defaults to using this most recent configuration revision, but users can select an earlier revision (Revision D, see ngVLA memo 92) if they wish to reproduce past calculations. Note that v1.1.x of the ngECT always uses the most recent estimates for the antenna efficiency and system temperatures, so calculations using an older configuration revision may not exactly match calculations done with v1.0 of the ngECT (which used earlier estimates for antenna efficiency and system temperatures).
If you have further questions or encounter an issue with the ngECT, please contact us.
