e-MERLIN / VLBI National Radio Astronomy Facility


e-MERLIN Cycle-4 capabilities


Selected e-MERLIN capabilities are available in Cycle-4.

Note that when complete, e-MERLIN sensitivity and imaging fidelity at C-band will be improved by more than a factor of two by using the full 2-GHz bandwidth. K-band (around 1.2 cm wavelength) and a wider range of correlator configurations will be offered, see The technical capabilities of e-MERLIN

Sensitivity and imaging capability

During Cycle-4 the optimum continuum sensitivity will be achieved in 12 hr on-source, corresponding to about an 18 hr track including calibration, using the Lovell telescope, for targets with declination greater than 20deg.

At C-band, continuum rms values of 7 μJy/beam will be reached by placing the 0.512 GHz bandwidth in the most sensitive part of the band, with the current system (about 5.8-6.3 GHz), assuming minimal losses due to interference giving a useful bandwidth of 0.5 GHz. We will observe using four, adjacent 128-MHz spectral windows.

At L-band, standard continuum observations will be made using eight 64-MHz spectral windows which will be placed contiguously over 0.512 GHz to optimise continuum sensitivity and avoid interference. Typical RFI levels across L-band result in around 10-15% of the total 512MHz band being unusable. Observational tests to date have typically actived sensitivities of 6-10 μJy/beam.

We suggest that continuum observers with no precise frequency requirements just specify L-, C- or K-band, and the optimum range will be chosen at observing time.

Up to four additional spectral windows can be used in spectral line modes within the same 0.512 GHz band, see below.

The nominal Cycle-4 ranges for L- and C-bands are given in Table 1, but observing towards the edges of C-band at this stage will be subject to up to double the noise. The Lovell Telescopes is a University of Manchester instrument and its inclusion is limited within e-MERLIN. For Cycle-4 PATT observations, the University of Manchester has donated up to 10 days of Lovell Telescope time. The e-MERLIN TAC will scientifically assess and allocated Lovell Telescope time to high ranking projects. Please include in your proposal justifications why the Lovell Telescope is needed for your project. The noise levels without Lovell are approximately twice those given for continuum without it.

We give 1σ rms noise levels. We suggest that observers aim for at least 5σ sensitivity for detection experiments although 3σ details are reliable around brighter sources. Dynamic range limitations may affect bright sources (several Jy), especially in spectral line mode.

Table 1: Cycle-4 continuum observing capabilities of e-MERLIN

1.5 GHz
5 GHz
22 GHz
Freq. Range (GHz) 1.23-1.74 4.3-7.5 21-24 C-band range eventually will be extended
Resolution (mas) 150 40 12 Uniform weighting at central frequency
Maximum angular scale (arcsec)1 2.0 0.5 0.1 Natural weighting, good coverage.
Field of View (arcmin) 25-m dishes2 30 7 1.5 To primary beam FWHM.
Field of View (arcmin) including Lovell2 12 5 - To primary beam FWHM.
Continuum sensitivity rms (μJy/bm) in 12 hr on-target, with Lovell 7 7 - Good conditions, target above about Dec. 20 deg.
Continuum sensitivity rms (μJy/bm) in 12 hr on-target, no Lovell 14 13 ~55
Astrometric performance (mas) ~2 ~1 ~1 WRT the ICRF (typical 3-deg target-calibrator separation using VLBA Calibrator Survey)

1Observations shorter than about 6 hr sources should be split up into a few shorter cuts to give good hour angle coverage. This will generally be a better use of instrumental time if several targets nearby on the sky can be interleaved. However, the imaging fidelity for any but the most compact sources will not be as good as for a full track.

2Exact field of view depends on frequency and also on integration time and channel width. For Cycle-4, for less than about 10% smearing, the integration time of 1 sec limits the radius to about 6 arcmin at C-band, 20 arcmin at L-band (greater than the primary beam FWHMs). The default channel widths of 0.25 MHz or 0.0625 MHz, at C- and L-bands, give radii of about 10 and 30 arcmin, respectively.

Spectral capabilities in Cycle-4

Cycle-4 will observe in C-band initially for several months and then change to L-band. It will not be possible to change between bands at short notice until later Cycles. Up to 512 MHz can be observed instantaneously within one band. This can be located flexibly within C-band but there is a calibration overhead. We recommend that, if you need to observe in more than one 512-MHz range (e.g. for spectral lines around 4.8 and 6 GHz), you do this in separate tracks, or if the entire observation is to take place in a single track, frequency changes are made no more often than every couple of hours.

The total bandwidth potentially available is 512 MHz (in future cycles, wider bandwidth will be available at C- and K-bands). The bandwidth is divided by factors of 2n into sub-bands. The data are recorded in spectral windows (spw) which can be the same width as the sub-band, or smaller by a factor of 2n. Spw must not overlap sub-band edges.

The 512-MHz observing band can be placed anywhere in the available band, but once placed, the sub-band locations are fixed. More than one high-resolution spectral window can be placed in the same sub-band if wanted. For maximum sensitivity, the spw should fit within factor-2 divisions of a sub-band. This is illustrated by some possible configurations shown in Figs. 1 and 2.

In Cycle-4, all spw will be divided into 512 channels, each in full polarization.

Spectral resolution

The actual spectral resolution may be broader if Hanning or other smoothing is applied. The sensitivities are given for the optimum part of the band and will be worse towards the band edges, in adverse weather, at low elevation or if interference is present. Bright emission in narrow channels will be more dynamic-range limited. About 10% of edge channels within each band may have lower sensitivity if the spw are not optimally placed.

Table 2: C-band Cycle-4 spectral line configurations.

spw width Channel separationrms 12 hr sensitivity per channel
(MHz) (km/s) (kHz) (km/s) with Lovell no Lovell
    (mJy/bm)(1000 K)(mJy/bm)(1000 K)
1286400 25012.5
643400 125 6.3 0.355.30.669.9
321600 62.53.1 0.507.50.93 14
16 800 31.251.6 0.7111 1.3 20
8 40015.6250.8 1.015 1.9 28
4 2007.71250.4 1.421 2.6 39
2 1003.90630.2 2.030 3.7 56
1 501.95310.1 2.843 5.3 79
0.5 250.97660.05 4.060 7.4112
0.2512.50.48820.03 5.785 11158

Table 3: L-band Cycle-4 spectral line configurations.

spw width Channel separationrms 12 hr sensitivity per channel
(MHz) (km/s) (kHz) (km/s) with Lovell no Lovell
    (mJy/bm)(1000 K)(mJy/bm)(1000 K)
326760 62.513.20.436.40.86 13
163380 31.25 6.60.619.1 1.2 18
8169015.625 3.30.86 13 1.7 26
4 8457.7125 1.7 1.2 18 2.4 36
2 4233.9063 0.8 1.7 26 3.4 52
1 2111.9531 0.4 2.4 36 4.9 73
0.5 1060.9766 0.2 3.4 52 6.9103
0.2552.80.4882 0.1 4.9 73 9.7146

Line rest frequencies

e-MERLIN is suited for imaging thermal lines in absorption against a bright radio source, or maser lines. These are the best-known transitions in the available bands. Those in brackets are probably faint.

Frequencies affected by interference include 1450-1520, 1310-1340 and around 1390 MHz. These frequencies are not always unusable (or others may be affected) but please consult e-MERLIN staff if you want to observe red-shifted lines in these ranges.

Proposal Technical Case

e-MERLIN is queue-scheduled and observing routines are prepared by staff. This means that sources will be observed at the positions and frequencies which you provide now. Please make sure these are accurate as there may not be an opportunity to provide corrections. In exceptional cases (e.g. follow-up of a transient), if you have to give a position which is not better than an arcmin, or hope to provide a better position after the deadline, please indicate this.

e-MERLIN staff will normally choose suitable calibration sources but if you have any preferences (e.g. the same phase reference as other observations) please indicate this.

Observing frequencies

Select the observing band. We recommend that you do not specify the precise frequency for pure continuum observations. The e-MERLIN observing team will pick the most sensitive range at C-band or avoid interference at L-band. If you do need a particular frequency for continuum, please explain why (e.g. for spectral index observations or to combine with other data).